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sustainability
Article
Improvements in Soil Carbon and Nitrogen
Capacities after Shrub Planting to Stabilize Sand
Dunes in China’s Horqin Sandy Land
Yuqiang Li 1,2, *, Yinping Chen 3, *, Xuyang Wang 1,2 , Yayi Niu 1,2 and Jie Lian 1,2
1
2
3
*
Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000,
China; [email protected] (X.W.); [email protected] (Y.N.); [email protected] (J.L.)
Naiman Desertification Research Station, Northwest Institute of Eco-Environment and Resources,
Chinese Academy of Sciences, Tongliao 028300, China
School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
Correspondence: [email protected] (Y.L.); [email protected] (Y.C.); Tel./Fax: +86-931-496-7219 (Y.L.)
Academic Editor: Takuo Nagaike
Received: 24 February 2017; Accepted: 19 April 2017; Published: 23 April 2017
Abstract: Caragana microphylla, a native perennial leguminous shrub, is widely used for desertification
control in China’s Horqin Sandy Land. We investigated the effects of afforestation using C. microphylla
in areas with fixed and active dunes on soil carbon (C) and nitrogen (N) storage in the soil total and
light-fraction (LF) organic matter. Compared to the values in the control areas, soil organic carbon
(SOC) storage to a depth of 100 cm increased by 88%, 74%, and 145% at 9, 15, and 31 years after shrub
planting, respectively; the corresponding values were 68%, 61%, and 195% for total nitrogen (TN)
storage, 109%, 199%, and 202% for LF organic carbon storage, and 203%, 337%, and 342% for LF
nitrogen storage. The soil light-fraction (LF) organic matter contributed significantly to total SOC and
TN storage, despite the low proportion of total soil mass accounted for by the LF dry matter. Thus,
afforestation using C. microphylla was an effective way to sequester C and to restore degraded soils,
but the process was slow; it would take more than 100 years to fully restore SOC storage in active
dunes through afforestation with C. microphylla in the Horqin Sandy Land.
Keywords: desertification control; carbon sequestration; shrub plantation; semiarid ecosystem
1. Introduction
The increase in atmospheric carbon dioxide (CO2 ) concentrations from 280 ppm in pre-industrial
times to 391 ppm in 2011 has been attributed to numerous anthropogenic activities, including
unsustainable land use, that have resulted in an increased severity and extent of soil degradation and
desertification, and this increase has contributed greatly to global warming [1,2]. Arid and semiarid
regions comprise 47.2% of the world’s land and more than two-thirds of these fragile ecosystems have
undergone degradation and desertification, primarily due to overgrazing, other unsustainable human
activities, and climate change [3]. Lal [4] estimated that global desertification led to a total loss of
19 to 29 Pg·C from the plant-soil continuum. However, it is increasingly accepted that restoration
of vegetation cover in the world’s arid and semiarid regions could create a high potential C sink
because of the vast area of this land combined with widespread degradation and desertification in
these areas [5,6].
Many agroforestry management practices can be employed to sequester C and counteract land
degradation. Afforestation (artificial planting of trees or shrubs) is one of the most effective ways
because of the durability and large mass of the woody stems, combined with the ongoing contribution
of organic matter to the soil. Afforestation is usually successful above the isohyet of 200 mm mean
Sustainability 2017, 9, 662; doi:10.3390/su9040662
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Sustainability 2017, 9, 662
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200 mm mean annual rainfall and, occasionally, at a lower annual rainfall, if the introduced or
reintroduced species is appropriate (i.e., does not require more water than the environment can
annual
rainfall
and,
at adegradation
lower annualhave
rainfall,
introducedor
or strongly
reintroduced
species[7].
is
provide)
and if
theoccasionally,
causes of the
beenif the
discontinued
mitigated
appropriate
(i.e.,
does
not
require
more
water
than
the
environment
can
provide)
and
if
the
causes
of
the
Numerous researchers have investigated the benefits associated with afforestation to sequester C
degradation
have
been discontinued
or strongly mitigated
[7].semiarid
Numerous
researchers
have
investigated
and to restore
degraded
soils and ecosystems
in arid and
regions
[5,8–12].
However,
this
the
benefits
associated
with
afforestation
to
sequester
C
and
to
restore
degraded
soils
and
restoration ability cannot be assumed, since some researchers have reported negativeecosystems
or neutral
in
arid and
semiarid regions
[5,8–12].
However,
this restoration
ability
cannot be some
assumed,
since
effects
of afforestation
on the
soil C content
at some
sites [13–15].
In addition,
doubt
hassome
been
researchers
have
reported
negative
or
neutral
effects
of
afforestation
on
the
soil
C
content
at
some
expressed over the effectiveness of afforestation for combating desertification and controlling dust
sites
[13–15].
addition,
some doubt
over the effectiveness of afforestation for
storms
in the In
arid
and semiarid
regionshas
of been
Chinaexpressed
[16].
combating
desertification
and controlling
dustof
storms
in theChina
arid and
semiarid
of China
[16].to
Historically,
the Horqin
Sandy Land
northern
(42°41′N
toregions
45°45′N,
118°35′E
◦ 410 N to 45◦ 450 N, 118◦ 350 E to 123◦ 300 E;
Historically,
the
Horqin
Sandy
Land
of
northern
China
(42
123°30′E; 180 m to 650 m asl) was a grassland with many lakes and lush vegetation dominated by
180
m to 650
m asl)
was a along
grassland
manyscattered
lakes andwoody
lush vegetation
dominatedalmost
by palatable
palatable
grass
species,
withwith
sparsely
species. However,
80% ofgrass
this
species,
along
with
sparsely
scattered
woody
species.
However,
almost
80%
of
this
region
has
suffered
region has suffered from desertification since the 1950s, primarily due to the region’s fragile
from
desertification
1950s,change
primarily
to the region’s
fragile ecology
combined
with
ecology
combined since
with the
climate
anddue
inappropriate
anthropogenic
activities
such
as
climate
change
and
inappropriate
anthropogenic
activities
such
as
overgrazing,
excessive
cultivation
of
overgrazing, excessive cultivation of marginal farmland, firewood harvesting, and excessive
marginal
farmland,
firewood
harvesting,
and excessive
groundwater
[17].
The landscape
groundwater
withdrawal
[17].
The landscape
became dominated
bywithdrawal
active dunes
(Figure
1a) by the
became
dominated
by
active
dunes
(Figure
1a)
by
the
late
1970s,
and
local
people
live
in
poverty
late 1970s, and local people live in poverty due to the severe environmental degradation
indue
the
to
the severe
degradation
region.
Zhou
al. [18]
estimated
that the system
total C and
N
region.
Zhouenvironmental
et al. [18] estimated
that in
thethetotal
C and
N et
losses
from
the plant-soil
of the
2 , amounted to
losses
from
the
plant-soil
system
of
the
Horqin
Sandy
Land,
which
covers
30,152.7
km
2
Horqin Sandy Land, which covers 30,152.7 km , amounted to 107.53 and 9.97 Mt, respectively,
107.53
and 9.97
comparedvalues
with the
original
valuesthat
due occurred
to the desertification
compared
withMt,
therespectively,
original grassland
due
to thegrassland
desertification
in the last
that
occurred
in
the
last
century.
A
range
of
restoration
practices
have
been
widely
implemented
to
century. A range of restoration practices have been widely implemented to control the desertification,
control
the
desertification,
including
the
establishment
of
straw
checkerboards
to
protect
exposed
including the establishment of straw checkerboards to protect exposed soil against wind, the
soil
against wind,
establishment
of grazing
exclosures
to protect
surviving
vegetation
against
establishment
of the
grazing
exclosures
to protect
surviving
vegetation
against
livestock,
and
livestock,
and
afforestation
using
indigenous
and
introduced
tree
and
shrub
species.
These
efforts
afforestation using indigenous and introduced tree and shrub species. These efforts have
have
significantly
reversed
desertification
and improved
the regional
ecological
environment
significantly
reversed
desertification
and improved
the regional
ecological
environment
[19]. [19].
The
The
establishment
of grazing
exclosures
and
theimplementation
implementationofofafforestation
afforestation using
using an
an introduced
establishment
of grazing
exclosures
and
the
introduced
tree
tree species
species (Mongolian
(Mongolian pine,
pine, Pinus
Pinus sylvestris
sylvestris var.
var. mongolica
mongolica Litv.)
Litv.) in
in areas
areas with
with active
active dunes
dunes in
in the
the
region
have
had
significant
positive
effects
through
the
vegetation’s
ability
to
sequester
CO
[17,20].
region have had significant positive effects through the vegetation’s ability to sequester CO22 [17,20].
Figure 1.1. Photographs
Photographs of (a) aa typical
typical area
area of
of active
active dunes;
dunes; (b)
(b) afforestation
afforestation of
of active
active dunes
dunes using
using
Figure
Caraganamicrophylla;
microphylla;(c)
(c)a aplantation
plantation
that
was
subjected
to livestock
grazing;
a plantation
Caragana
that
was
subjected
to livestock
grazing;
and and
(d) a(d)
plantation
that
that underwent
pruning.
underwent
pruning.
Sustainability 2017, 9, 662
3 of 17
Caragana microphylla, a native perennial leguminous shrub, has performed well and is widely
used for stabilization of active dunes in the Horqin Sandy Land (Figure 1b). Previous studies
have investigated the effects of C. microphylla plantations on the spatial variability of soil nutrients
and microbiological properties [21] and on the diversity and composition of native soil bacterial
communities [22]. However, we found only one published study of the accumulation of organic C and
nitrogen (N) in soils following the establishment of an age sequence of C. microphylla plantations in
desertified areas [23]. Our previous study [24] also investigated the effects of C. microphylla plantations
in sandy land on C storage in the soil light-fraction (LF) organic matter, which is a plant-like and
dynamic fraction of the soil organic matter that reflects short-term shifts in soil organic matter storage
and turnover and that has been regarded as an early indicator of management-induced changes in soil
quality [25,26]. However, previous research only sampled soil to a depth of 20 cm [23] or 5 cm [24].
This limitation made it difficult to compare the results with those of other researchers, who have
typically sampled to a depth of 100 cm, which is a common threshold used for meta-analyses [27].
Therefore, the objectives of the current research were to investigate changes in soil organic C
(SOC) and total N (TN) storage to a depth of 100 cm, and to investigate the contribution of the soil LF
organic matter to SOC and TN accumulation following afforestation using C. microphylla in areas of
the Horqin Sandy Land with fixed and active dunes. The present results, combined with the results
of our previous studies [17,20], can provide the basic data required to comprehensively estimate the
potential C and N sequestration in soils that would result from the restoration of severely desertified
sandy land through tree and shrub planting as well as grazing exclosures.
2. Materials and Methods
2.1. Study Area
Our study was conducted in the Naiman Banner (a county-level division) in China’s Inner
Mongolia Autonomous Region, one of the most seriously desertified regions in the southern part of the
Horqin Sandy Land. In 1985, the Chinese Academy of Sciences established the Naiman Desertification
Research Station (42◦ 550 5200 N, 120◦ 410 5600 E, 377 m asl), which is 13 km from the urban center of the
Naiman Banner. The study area is characterized by sand dunes that alternate with gently undulating
interdunal lowlands. The region has a continental semiarid monsoon temperate climate regime, with
a mean annual precipitation of 366 mm, of which 70% falls from June to August, a mean annual
potential evaporation of 1935 mm, and a mean annual air temperature of 6.8 ◦ C. Mean monthly
temperatures range from a minimum of −13.2 ◦ C in January to a maximum of 23.5 ◦ C in July. The mean
wind speed is 4.3 m·s−1 , with occasional occurrences of gales ≥20 m·s−1 in winter and spring, when
the vegetation cover is lowest and the soil is driest [17]. The area’s soils are derived from alluvial and
aeolian sediments, and have been classified as Cambic Arenosols [28], with a coarse texture (medium
to coarse sand) and a loose structure.
2.2. Site Selection and Location
The present study was conducted in April 2015. As study stands, we selected 9-, 15-, and
31-year-old C. microphylla plantations that had been established using seedlings. The 9-year-old
plantations were established in areas with fixed dunes, whereas the 15- and 31-year-old plantations
were established in areas with active dunes. Fixed dunes have better vegetation and soil environments
than active dunes. The vegetation cover is generally 50–70% in fixed dunes, versus less than 10%
in active dunes. Fixed and active dunes can be classified as having light and severe desertification,
respectively, based on the Zhu and Chen [29] classification criteria. For the 9-year-old plantation,
the spacing within and between the rows was 0.5 m × 1.5 m, with an average shrub height of 58 cm
and an average crown diameter of 61 cm × 65 cm at the time of our study. For the 15-year-old
plantation, the spacing within and between the rows was 1.0 m × 2.0 m, with an average shrub height
of 134 cm and an average crown diameter of 149 cm × 137 cm at the time of our study. For the
Sustainability 2017, 9, 662
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31-year-old plantation, the spacing within and between the rows was 1.0 m × 2.0 m, with an average
shrub height of 117 cm and an average crown diameter of 120 cm × 108 cm at the time of our study.
The 9-year-old plantation was protected from livestock grazing, but the 15- and 31-year-old plantations
underwent continuous grazing at an unquantified intensity (Figure 1c). All investigated plantations
were pruned every 3 to 4 years to facilitate regeneration, but the biomass was removed from the sites
to provide firewood for local residents (Figure 1d). Because no continuous long-term observational
data was available for each plantation since its establishment, our approach does not represent a strict
examination of the changes in a given plot over time; rather, it represents a “space for time” approach.
We selected three stands (replicates) in each plantation age class. In addition, we selected three
sand dune sites in neighboring areas to use as the control for each plantation age class. For the
9-year-old plantations, which were established in an area of fixed dunes, the controls were provided
by three sites with fixed dunes (as described in Section 2.2). The older plantations were established in
areas with active dunes. The distances among the plantations ranged from 20 to 82 km and the 15and 31-year-old plantations were established in areas with active dunes, about 62 km apart. Therefore,
for each of the two older plantations, we used three sites with active dunes as the control (i.e., a total
of six sites with active dunes). For each age class, we sampled the soil in three 30 m × 30 m plots at
the plantation sites and three 30 m × 30 m plots at the control sites. Thus, we established a total of
18 plots in the present study. All plots faced south, and had a slope <20◦ . Although we did not control
rigorously for the effect of topographic variations, we chose relatively flat sites along the midslope of
each dune to provide some basic control over the effects of topography. However, in future research,
it will be necessary to control more rigorously for topographic effects.
2.3. Soil Sampling
Each 30 m × 30 m plot was divided into 36 subplots (each 5 m × 5 m) and every subplot was
numbered. We then randomly selected six subplots for soil sampling using simple random sampling.
In each of the three plantation types, we selected two sampling locations: one under the shrub canopy
and one in the soil between the rows of shrubs. At the control sites, samples were collected from six
random subplots within each overall plot. After carefully removing large surface plant debris by hand,
soil samples were collected using a soil auger (2.5 cm in diameter) from five layers: 0 to 10, 10 to 20, 20 to
40, 40 to 60, and 60 to 100 cm below the surface. At each location, 10 soil samples were collected and then
mixed to provide a single composite sample for each soil layer. Therefore, in every plot, we obtained a
total of 60 composite samples (30 under the canopy and 30 between the rows) for the five soil layers,
versus only 30 composite samples at the control sites (i.e., because there were no shrubs at these sites).
Soil samples under the shrub canopies were collected as close to the center of the shrub as possible.
To determine soil bulk density, three additional soil cores were collected from each layer at each
location using a soil auger equipped with a stainless-steel cylinder (5.5 cm in diameter and 4.2 cm
in height). The method was described in detail in our previous study [30]. To avoid bias in the bulk
density that would have resulted from the presence of roots, we rejected any sampling location where
obvious roots were encountered, and replaced that sample with a sample from a different location.
2.4. Laboratory Analyses
Soil samples were air-dried and hand-sieved through a 2 mm mesh to remove roots and other
coarse debris. A portion of each soil sample was then ground to pass through a 0.25 mm mesh
for determination of the SOC and TN concentrations. The remaining portion was stored at room
temperature for determination of the LF organic matter content and the particle size distribution.
A subsample of the air-dried soil was weighed and dried at 105 ◦ C for 24 h to determine the
gravimetric water content, which refers to the soil hygroscopic water and is used for calculating the dry
soil mass that we used to calculate the content of LF organic matter. Soil particle sizes were analyzed
by the wet sieving method using sodium hexametaphosphate as the dispersing agent [31]. Each soil
sample was separated into three fractions using nested sieves with openings of 2, 0.1, and 0.05 mm:
Sustainability 2017, 9, 662
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these defined the coarse sand (2 to 0.1 mm), fine sand (0.1 to 0.05 mm), and silt + clay (<0.05 mm)
fractions. The SOC concentration was determined using the Walkley-Black dichromate oxidation
procedure [32]. The soil TN concentration was determined using the Kjeldahl procedure [33].
The LF organic matter was extracted from the soil using aqueous NaI solution at a density of
1.8 g·cm−3 according to the procedure described by Murage et al. [25]. The LF dry matter content
was expressed as a percentage of the total oven-dry soil mass. To obtain enough LF for our analyses,
we used three to six subsamples (i.e., 60 to 120 g) from each soil sample. We determined a single
value of the LF organic C (LFOC) and N (LFN) concentrations for all five layers combined at a given
location. The organic C and N concentrations in the LF organic matter were determined using the
same procedures that were used for the bulk soil.
2.5. Data Analyses
The C and N storage per unit area in the bulk soil (the bulk elemental storage, BES, in kg·ha−1 )
and in the LF soil organic matter (the light-fraction elemental storage, LFES, in kg·ha−1 ) to a depth of
100 cm were estimated using the following equations:
5
BES = 100 ∑ ( ECi × BDi × Ti )
(1)
i =1
5
LFES =
∑ ( DMi × ECLFi × BDi × Ti )
(2)
i =1
where ECi represents the elemental concentration in the bulk soil (SOC or TN, in g·kg−1 ) in layer i,
BDi represents the soil bulk density (in g·cm−3 ) in that layer, Ti (i = 1, 2, . . . , 5) represents the thickness
of the soil sampling layer i (in cm), 100 represents a unit conversion factor, DMi represents the LF
dry matter content as a proportion of the total soil mass (%), and ECLFi represents the LFOC or LFN
concentration (in g·kg−1 dry LF organic matter).
Data were tested for normality using the Shapiro-Wilk test (with significance at p < 0.05), and
homogeneity of variance was confirmed using Levene’s test (with significance at p < 0.05) before
further statistical testing. No transformations were required because the data met the assumptions
of normality and homogeneity of variance. The measured soil variables and the resultant C and N
storage were analyzed using one-way ANOVA to identify significant differences among the treatments.
When the ANOVA results were significant, we compared means using the least-significant-difference
(LSD) test. Correlations between parameters were calculated using Pearson’s correlation coefficient
(r). The statistical analysis was performed using version 13.5 of the SPSS software (SPSS, Chicago, IL, USA).
3. Results
3.1. Changes in Soil Particle-Size Distributions and Bulk Density
The soil particle-size distributions for the three plantations (Table 1) generally indicated that the
proportion of coarse sand decreased significantly and the proportions of fine sand and silt + clay
increased significantly after the establishment of plantations in areas with sand dunes. Within each
plantation age class, the proportion of coarse sand under the shrub canopy was generally lower than
that between the rows; in contrast, the proportions of fine sand and silt + clay were higher under the
shrub canopy, although the differences were generally not significant. Compared to the control areas,
the mean values under the canopy and between the rows for coarse sand to a depth of 100 cm decreased
by 4%, 4%, and 10%, respectively, in the 9-, 15-, and 31-year-old plantations; the corresponding values
for fine sand increased by 26%, 43%, and 215%, whereas those for silt + clay increased by 161%, 65%,
and 294%. (These values refer to the columns labeled “Mean” in Table 1.) Except for fine sand in the
9-year-old plantation, each of the particle-size fractions for the combined layer from 0 to 100 cm differed
significantly (p < 0.05) between the plantation and the corresponding control area within each age class.
Sustainability 2017, 9, 662
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Table 1. Changes in the soil particle-size distribution, bulk density, light-fraction (LF) organic carbon (LFOC) concentration, and light-fraction nitrogen (LFN)
concentration after afforestation of fixed and active dunes with Caragana microphylla. (Values are mean ± SE.) The column labeled “Mean” represents the average of
the values under the canopy and between the rows.
Soil Properties
Layer
(cm)
9-Year
15-Year
31-Year
Under
Canopy
Between
Rows
Mean
Fixed Dunes
Under
Canopy
Between
Rows
Mean
Active
Dunes
Under
Canopy
Between
Rows
Mean
Active
Dunes
Coarse sand
(2.0–0.1 mm, %)
0201310
10–20
20–40
40–60
60–100
0–100
71.7 ± 4.3a
86.6 ± 3.5a
93.8 ± 0.7a
95.3 ± 0.6a
95.3 ± 0.7a
88.6 ± 1.8a
79.6 ± 2.7ab
92.0 ± 1.1ab
94.5 ± 0.4a
95.3 ± 0.3a
94.5 ± 0.7a
91.2 ± 0.7ab
75.7 ± 2.7ab
89.3 ± 1.8ab
94.2 ± 0.4a
95.3 ± 0.4a
94.9 ± 0.6a
89.9 ± 1.0a
84.4 ± 2.3b
94.3 ± 1.0b
97.7 ± 0.4b
97.1 ± 0.9b
95.3 ± 0.7a
93.7 ± 0.9b
84.2 ± 0.8a
86.9 ± 1.4a
87.6 ± 0.9a
88.2 ± 0.4a
87.4 ± 1.0a
86.8 ± 0.4a
87.5 ± 1.1bc
88.3 ± 1.2ab
89.4 ± 0.3ab
89.3 ± 0.4ab
90.1 ± 0.9a
88.9 ± 0.3a
85.8 ± 0.6ab
87.6 ± 0.8ab
88.5 ± 0.4ab
88.7 ± 0.4ab
88.8 ± 0.7a
87.9 ± 0.2a
89.9 ± 1.4c
90.8 ± 1.7b
91.9 ± 2.1b
92.2 ± 2.1b
93.7 ± 1.8b
91.7 ± 1.5b
72.7 ± 2.8a
86.3 ± 1.3a
88.7 ± 0.9a
91.2 ± 0.9a
91.5 ± 0.7a
86.1 ± 1.0a
77.5 ± 1.7a
90.1 ± 0.9b
90.3 ± 0.9a
90.5 ± 0.9a
91.2 ± 0.7a
87.9 ± 0.6a
75.1±1.7a
88.2 ± 0.9ab
89.5 ± 0.7a
90.8 ± 0.8a
91.4 ± 0.5a
87.0 ± 0.7a
94.8 ± 0.8b
96.8 ± 0.4c
97.0 ± 0.3b
96.3 ± 0.3b
96.6 ± 0.4b
96.3 ± 0.3b
Fine sand
(0.10–0.05 mm, %)
0–10
10–20
20–40
40–60
60–100
0–100
12.5 ± 2.0a
6.5 ± 1.4a
4.2 ± 0.4a
3.3 ± 0.6a
3.6 ± 0.5a
6.0 ± 0.9a
8.7 ± 1.1a
4.1 ± 0.5a
3.5 ± 0.4a
3.7 ± 0.3a
3.9 ± 0.5a
4.8 ± 0.4a
10.6 ± 1.3a
5.3 ± 0.7a
3.9 ± 0.3a
3.5 ± 0.4a
3.7 ± 0.4a
5.4 ± 0.5a
10.7 ± 1.5a
4.1 ± 0.8a
1.5 ± 0.3b
2.0 ± 0.6b
3.1 ± 0.4a
4.3 ± 0.5a
9.3 ± 0.3a
8.7 ± 0.7a
9.8 ± 0.8a
9.6 ± 0.5a
10.8 ± 0.8a
9.6 ± 0.3a
9.1 ± 0.6a
9.5 ± 0.7a
8.6 ± 0.5ab
9.2 ± 0.4a
8.1 ± 0.8ab
8.9 ± 0.2a
9.2 ± 0.4a
9.1 ± 0.5a
9.2 ± 0.4ab
9.4 ± 0.4a
9.5 ± 0.7a
9.3 ± 0.3a
7.6 ± 1.1b
7.1 ± 1.4a
6.7 ± 1.8b
6.1 ± 1.6b
5.3 ± 1.6bc
6.5 ± 1.2b
10.9 ± 1.1a
6.6 ± 0.6a
5.7 ± 0.4a
5.3± 0.4aa
5.1 ± 0.4a
6.7 ± 0.5a
8.0 ± 0.7b
5.4 ± 0.4a
5.8 ± 0.5a
5.8 ± 0.4a
4.9 ± 0.3a
6.0 ± 0.3a
9.4 ± 0.9ab
6.0 ± 0.4a
5.7 ± 0.4a
5.6 ± 0.4a
5.0 ± 0.2a
6.3 ± 0.4a
2.8 ± 0.6c
1.7 ± 0.2b
1.6 ± 0.2b
2.2 ± 0.3b
1.9 ± 0.3b
2.0 ± 0.2b
Silt + clay
(<0.05 mm, %)
0–10
10–20
20–40
40–60
60–100
0–100
15.8 ± 2.3a
6.9 ± 2.1a
1.9 ± 0.3a
1.4 ± 0.2a
1.1 ± 0.3a
5.4 ± 0.9a
11.7 ± 1.7a
3.8 ± 0.8ab
2.0 ± 0.2a
1.0 ± 0.2ab
1.6 ± 0.2a
4.0 ± 0.4a
13.7 ± 1.4a
5.4 ± 1.1a
2.0 ± 0.1a
1.2 ± 0.1ab
1.4 ± 0.2a
4.7 ± 0.5a
4.5 ± 0.9b
1.3 ± 0.3b
0.8 ± 0.2b
0.8 ± 0.3b
1.6 ± 0.6a
1.8 ± 0.4b
6.5 ± 0.9a
4.4 ± 0.9a
2.6 ± 0.6a
2.2 ± 0.2a
1.8 ± 0.4a
3.5 ± 0.4a
3.4±0.6bc
2.1±0.7b
2.0 ± 0.2ab
1.5 ± 0.1a
1.7 ± 0.2a
2.1 ± 0.2bc
4.9 ± 0.4ab
3.3 ± 0.5ab
2.3 ± 0.3ab
1.9 ± 0.1a
1.8 ± 0.2a
2.8 ± 0.1ab
2.5 ± 0.5c
2.1 ± 0.4b
1.4±0.4b
1.7 ± 0.7a
1.0 ± 0.3a
1.7 ± 0.3c
16.5 ± 1.7a
7.2 ± 0.8a
5.6 ± 0.5a
3.6 ± 0.5a
3.4 ± 0.4a
7.2 ± 0.6a
14.5 ± 1.2a
4.4 ± 0.6b
3.9 ± 0.5b
3.7 ± 0.5a
3.8 ± 0.5a
6.1 ± 0.4a
15.5 ± 1.0a
5.8 ± 0.5ab
4.8 ± 0.4ab
3.6 ± 0.5a
3.6 ± 0.3a
6.7 ± 0.4a
2.4 ± 0.3b
1.5 ± 0.3c
1.5 ± 0.2c
1.5 ± 0.2b
1.5 ± 0.3b
1.7 ± 0.2b
0–10
10–20
20–40
40–60
60–100
0–100
0–100
0–100
1.46 ± 0.03a
1.51 ± 0.03a
1.52 ± 0.03a
1.52 ± 0.03a
1.52 ± 0.03a
1.50 ± 0.03a
182.4 ± 2.8a
13.3 ± 0.3a
1.49 ± 0.03a
1.52 ± 0.03a
1.53 ± 0.03a
1.57 ± 0.05a
1.54 ± 0.03a
1.53 ± 0.03a
155.3 ± 5.8bd
10.0 ± 0.5b
1.47 ± 0.03a
1.51 ± 0.03a
1.52 ± 0.03a
1.54 ± 0.04a
1.53 ± 0.03a
1.52 ± 0.03a
168.9 ± 3.6c
11.7 ± 0.3c
1.50 ± 0.01a
1.54 ± 0.02a
1.56 ± 0.01a
1.56 ± 0.02a
1.58 ± 0.01a
1.55 ± 0.01a
150.4 ± 3.9d
7.2 ± 0.2d
1.47 ± 0.03a
1.49 ± 0.01a
1.52 ± 0.01a
1.53 ± 0.03a
1.56 ± 0.01a
1.51 ± 0.01a
204.8 ± 11.3a
12.7 ± 0.5a
1.56 ± 0.01bc
1.56 ± 0.02b
1.54 ± 0.01a
1.55 ± 0.02a
1.55 ± 0.01a
1.55 ± 0.01ab
183.3 ± 13.3a
11.7 ± 0.7a
1.51 ± 0.01ab
1.52 ± 0.01ab
1.53 ± 0.01a
1.54 ± 0.02a
1.56 ± 0.01a
1.53 ± 0.01ab
194.0 ± 11.8a
12.2 ± 0.6a
1.60 ± 0.03c
1.55 ± 0.02b
1.54 ± 0.03a
1.56 ± 0.02a
1.55 ± 0.01a
1.56 ± 0.02b
149.4 ± 4.0b
6.4 ± 0.4b
1.41 ± 0.05a
1.53 ± 0.04a
1.52 ± 0.03a
1.50 ± 0.04a
1.53 ± 0.06a
1.50 ± 0.04a
184.9 ± 7.5a
12.2 ± 0.8a
1.53 ± 0.04bc
1.53 ± 0.02a
1.54 ± 0.03a
1.55 ± 0.02a
1.55 ± 0.02a
1.54 ± 0.03ab
172.7 ± 6.8a
11.7 ± 0.3a
1.47 ± 0.01ab
1.53 ± 0.01a
1.53 ± 0.03a
1.53 ± 0.03a
1.54 ± 0.03a
1.52 ± 0.02ab
178.8 ± 5.5a
12.0 ± 0.4a
1.62 ± 0.02c
1.61 ± 0.02a
1.60 ± 0.01a
1.58 ± 0.01a
1.57 ± 0.01a
1.60 ± 0.01b
149.7 ± 2.4b
6.8 ± 0.5b
Bulk density
(g·cm−3 )
LFOC (g·kg−1 LF)
LFN (g·kg−1 LF )
Values of a parameter for a given plantation age labeled with different letters differ significantly (p < 0.05, ANOVA followed by a least-significant-difference test).
Sustainability 2017, 9, 662
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Soil
bulk
densitydecreased
decreasedfollowing
following the
the establishment
establishment of
thethe
Soil
bulk
density
ofplantations
plantations(Table
(Table1).1).However,
However,
differences
between
plantations
corresponding
control
areas
were
only
significant
<
differences
between
thethe
plantations
andand
thethe
corresponding
control
areas
were
only
significant
(p <(p0.05)
0.05)
in
the
upper
10
cm
of
the
soil
for
the
15and
31-year
age
classes.
No
significant
difference
was
in the upper 10 cm of the soil for the 15- and 31-year age classes. No significant difference was detected
for the layer
combined
0 tobetween
100 cm between
the plantation
and control
The under
soil
fordetected
the combined
fromlayer
0 to from
100 cm
the plantation
and control
areas.areas.
The soil
under the shrub canopy generally had a lower bulk density than that between the rows, but the
the shrub canopy generally had a lower bulk density than that between the rows, but the differences
differences were only significant (p < 0.05) in the layers from 0 to 10 cm and from 10 to 20 cm in the
were only significant (p < 0.05) in the layers from 0 to 10 cm and from 10 to 20 cm in the 15-year-old
15-year-old plantation, and from 0 to 10 cm in the 31-year-old plantation.
plantation, and from 0 to 10 cm in the 31-year-old plantation.
3.2. Changes in SOC and TN Concentrations
3.2. Changes in SOC and TN Concentrations
The shrub plantations showed increased SOC (Figure 2) and TN (Figure 3) concentrations. At
The shrub plantations showed increased SOC (Figure 2) and TN (Figure 3) concentrations. At all
all three plantation ages, SOC and TN in the top 20 cm of the soil were significantly higher than in
three plantation ages, SOC and TN in the top 20 cm of the soil were significantly higher than in the
the corresponding control dunes; in the 31-year age class, the differences were significant at all
corresponding control dunes; in the 31-year age class, the differences were significant at all depths.
depths. The mean SOC concentrations (for the values under the shrub canopy and between the
Therows)
meantoSOC
concentrations
the9-,values
under
the shrubplantations
canopy andincreased
betweento
the2.5,
rows)
a depth
a depth
of 100 cm (for
in the
15-, and
31-year-old
2.1, to
and
3.0
of 100
cm
in
the
9-,
15-,
and
31-year-old
plantations
increased
to
2.5,
2.1,
and
3.0
times
the
values
times the values for the corresponding control area (the fixed and active dunes). The correspondingfor
theTN
corresponding
control
areato(the
and3.6active
Thevalue.
corresponding
TNconcentrations
concentrations
concentrations
increased
2.4, fixed
2.0, and
timesdunes).
the control
SOC and TN
increased
to
2.4,
2.0,
and
3.6
times
the
control
value.
SOC
and
TN
concentrations
decreased
decreased rapidly with increasing depth for each age class; the 9-year-old plantations had the rapidly
most
with
increasing
depth for each age class; the 9-year-old plantations had the most rapid decrease.
rapid
decrease.
-1
SOC concentration (g kg )
0
2
4
6
8
Soil depth (cm)
0-10
10
*
10-20
*
X Data
Under canopy
Mid-row
Mean
Fixed dunes
ns
20-40
ns
40-60
9-year
ns
60-100
-1
SOC concentration (g kg )
0
2
4
Soil depth (cm)
0-10
6
8
10
*
10-20
*
Under canopy
Mid-row
Mean
Active dunes
ns
20-40
40-60
ns
60-100
ns
15-year
-1
SOC concentration (g kg )
0
2
4
Soil depth (cm)
0-10
6
8
10
*
10-20
*
20-40
*
40-60
*
60-100
*
Under canopy
Mid-row
Mean
Active dunes
31-year
Figure 2. Changes in the soil organic C (SOC) concentration in plots with sand dunes (fixed and
Figure 2. Changes in the soil organic C (SOC) concentration in plots with sand dunes (fixed and active
active dunes) and in the 9-, 15-, and 31-year-old shrub plantations. Values represent means ±
dunes) and in the 9-, 15-, and 31-year-old shrub plantations. Values represent means ± standard errors
standard errors (SE) for sample positions under the shrub canopy, between the rows (“mid-row”),
(SE) for sample positions under the shrub canopy, between the rows (“mid-row”), and for the average
and for the average of these two positions (“Mean”). Points labeled with * differed significantly
of these two positions (“Mean”). Points labeled with * differed significantly between the sand dunes
between the sand dunes and the mean (p < 0.05); points labeled with ns did not differ significantly.
and the mean (p < 0.05); points labeled with ns did not differ significantly.
Sustainability 2017, 9, 662
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Figure
3. Changes
in the
totalNN(TN)
(TN)concentration
concentration in
sand
dunes
(fixed
and and
active
Figure
3. Changes
in the
soilsoil
total
inplots
plotswith
with
sand
dunes
(fixed
active
dunes)
and
in
the
9-,
15-,
and
31-year-old
shrub
plantations.
Values
represent
means
±
standard
dunes) and in the 9-, 15-, and 31-year-old shrub plantations. Values represent means ± standard errors
errors
(SE) for
sample under
positions
shrub canopy,
between
the (“mid-row”),
rows (“mid-row”),
for the
(SE) for
sample
positions
theunder
shrubthe
canopy,
between
the rows
and and
for the
average
average
of
these
two
positions
(“Mean”).
Points
labeled
with
*
differed
significantly
between
of these two positions (“Mean”). Points labeled with * differed significantly between the sandthe
dunes
sand dunes and the mean (p < 0.05); points labeled with ns did not differ significantly.
and the mean (p < 0.05); points labeled with ns did not differ significantly.
SOC and TN concentrations under the canopy were significantly higher than those between
SOC
andinTN
concentrations
under
the
were
significantly
higher
than those
between
the rows
each
layer of the upper
40 cm
of canopy
the soil in
the 9and 15-year-old
plantations,
but were
significantly
in the
eachupper
layer 40
of the
cminofthe
the9-soil
in 15-year-old
the 31-year-old
plantation.but
Forwere
the rows
in each higher
layer of
cm upper
of the60
soil
and
plantations,
both SOC
and TN
concentrations,
a weak
observed
the samples
obtained
significantly
higher
in each
layer of the
upperdifference
60 cm ofwas
the soil
in thebetween
31-year-old
plantation.
For both
SOC and TN concentrations, a weak difference was observed between the samples obtained between
the rows and the samples obtained in the control areas within each plantation. There was no significant
Sustainability 2017, 9, 662
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9 of 17
between the rows and the samples obtained in the control areas within each plantation. There was
no significant difference in SOC and TN for the combined layer from 0 to 100 cm for the samples
obtained between
the rows
and
sampleslayer
fromfrom
active
dunes
in the
difference
in SOC and
TN for
thethe
combined
0 to
100 cm
for 15-year-old
the samplesplantation.
obtained between
the rows and the samples from active dunes in the 15-year-old plantation.
3.3. Changes in Soil LF Organic Matter
3.3. Changes in Soil LF Organic Matter
The trends for the LF dry matter content (Figure 4) were similar to those for the SOC and TN
The trends(Figures
for the LF
dry3):
matter
content
4) were
to those forofthe
SOC and and
TN
concentrations
2 and
LF dry
matter(Figure
increased
aftersimilar
the establishment
plantations
concentrations
(Figures
2
and
3):
LF
dry
matter
increased
after
the
establishment
of
plantations
and
decreased with increasing depth in the soil. The content differed significantly in the top 20 cm of the
decreased
withthe
increasing
depth
the soil.areas
The content
differed
thedry
top 20
cm ofonly
the
soil between
plantation
andincontrol
in all three
agesignificantly
classes. TheinLF
matter
soil
betweenfor
the
and control
all three
classes.
LF dry matter
only
accounted
accounted
a plantation
small proportion
of theareas
totalinsoil
mass age
(from
0.033The
to 1.005%),
but was
significantly
for
a small proportion
totalSOC
soil mass
(from 0.033
was significantly
with
correlated
with both of
thethetotal
concentration
(r to
= 1.005%),
0.976, p but
< 0.001)
and the TNcorrelated
concentration
both
the total
concentration (r = 0.976, p < 0.001) and the TN concentration (r = 0.971, p < 0.001).
(r = 0.971,
p < SOC
0.001).
LF dry matter (% of total soil mass)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Soil depth (cm)
0-10
1.4
*
10-20
*
X Data
Under canopy
Mid-row
Mean
Fixed dunes
ns
20-40
40-60
ns
60-100
ns
9-year
LF dry matter (% of total soil mass)
0.0
0.2
0.4
Soil depth (cm)
0-10
0.6
0.8
1.0
1.2
1.4
*
10-20
*
Under canopy
Mid-row
Mean
Active dunes
ns
20-40
40-60
ns
60-100
ns
15-year
LF dry matter (% of total soil mass)
0.0
0.2
0.4
Soil depth (cm)
0-10
*
ns
20-40
60-100
0.8
1.0
1.2
1.4
*
10-20
40-60
0.6
Under canopy
Mid-row
Mean
Active dunes
ns
ns
31-year
Figure 4.
4. Changes
Changesininthe
thesoil
soillight-fraction
light-fraction
(LF)
dry
matter
content
in plots
dunes
(fixed
Figure
(LF)
dry
matter
content
in plots
withwith
sandsand
dunes
(fixed
and
and active
dunes)
and 31-year-old
shrub plantations.
Values represent
means ±
active
dunes)
and inand
the in
9-, the
15-, 9-,
and15-,
31-year-old
shrub plantations.
Values represent
means ± standard
standard
(SE) for
sampleunder
positions
undercanopy,
the shrub
canopy,
the rows (“mid-row”),
errors
(SE)errors
for sample
positions
the shrub
between
thebetween
rows (“mid-row”),
and for the
and for of
the
average
of these (“Mean”).
two positions
(“Mean”).
Points
labeledsignificantly
with * differed
significantly
average
these
two positions
Points
labeled with
* differed
between
the sand
between
the
sand
dunes
meanlabeled
(p < 0.05);
withsignificantly.
ns did not differ significantly.
dunes
and
the
mean
(p < and
0.05);the
points
withpoints
ns didlabeled
not differ
Sustainability 2017, 9, 662
10 of 17
The LF dry matter content for the combined layer from 0 to 100 cm was significantly higher under
the shrub canopy than between the rows within each plantation, but there was no significant difference
between the values obtained between the rows and the respective values obtained in the control areas
in the 9- and 15-year-old plantations. The LF dry matter had much higher C and N concentrations
than the bulk soil in the controls in all three age classes (Table 1). In addition, the LFOC and LFN
concentrations in each plantation were significantly greater than those in the control areas.
3.4. Changes in Soil C and N Storage and Accumulation Rates
The total soil storage of SOC and TN were estimated using Equation (1) and the soil storage of
LFOC and LFN were estimated using Equation (2) for the individual soil layers and for the combined
layer from 0 to 100 cm (Table 2). Compared to the values in the control areas, the SOC storage to
a depth of 100 cm increased by 9033, 5185, and 11,148 kg·ha−1 (by 88%, 74%, and 145%, respectively) 9,
15, and 31 years after shrub planting. The corresponding increases were 866, 496, and 1462 kg·ha−1
(by 68%, 61%, and 195%) for TN storage, 2108, 2361, and 2648 kg·ha−1 (by 109%, 199%, and 202%) for
LFOC storage, and 187, 172, and 205 kg·ha−1 (by 203%, 337%, and 342%) for LFN storage.
Table 2. Changes in the storage of soil organic carbon (SOC), total nitrogen (TN), light-fraction
(LF) organic carbon (LFOC), and light-fraction nitrogen (LFN) to a depth of 100 cm following the
afforestation of fixed dunes and active dunes with Caragana microphylla. (Values are mean ± SE based
on the mean of the storage under the shrub canopy and between the rows.) SOC and TN values were
calculated using Equation (1); LFOC and LFN values were calculated using Equation (2).
Layer (cm)
9-Year
31-Year
Fixed Dunes
Plantation
Active Dunes
Plantation
Active Dunes
2279 ± 183b
1240 ± 77b
2004 ± 128b
1786 ± 67a
2918 ± 210a
10,227 ± 357b
3000 ± 179a
1920 ± 191a
2378 ± 247a
1837 ± 120a
3039 ± 169a
12,174 ± 643a
966 ± 102b
936 ± 60b
1636 ± 102b
1137 ± 175b
2314 ± 313b
6989 ± 413b
4908 ± 391a
2651 ± 283a
4243 ± 202a
2386 ± 182a
4665 ± 443a
18,853 ± 908a
1017 ± 133b
948 ± 164b
1720 ± 273b
1461 ± 308b
2559 ± 115b
7705 ± 896b
TN storage (kg·ha−1 )
0–10
925 ± 111a
10–20
364 ± 40a
20–40
339 ± 18a
40–60
186 ± 5a
60–100
319 ± 18a
Subtotal
2133 ± 141a
274 ± 22b
142 ± 17b
248 ± 12b
211 ± 14a
392 ± 21a
1267 ± 45b
321 ± 11a
219 ± 28a
266 ± 37a
190 ± 26a
319 ± 23a
1315 ± 97a
119 ± 9b
90 ± 7b
160 ± 19b
141 ± 13a
309 ± 39a
819 ± 66b
590 ± 41a
325 ± 30a
465 ± 15a
274 ± 20a
556 ± 50a
2210 ± 105a
104 ± 12b
102 ± 13b
172 ± 24b
139 ± 25b
231 ± 31b
748 ± 90b
LFOC storage (kg·ha−1 )
0–10
1734 ± 343a
10–20
686 ± 116a
20–40
610 ± 47a
40–60
342 ± 30a
60–100
663 ± 76a
Subtotal
4035 ± 446a
673 ± 77b
342 ± 59b
395 ± 62b
206 ± 34b
311 ± 65b
1927 ± 246b
633 ± 85a
463 ± 72a
738 ± 126a
573 ± 74a
1139 ± 130a
3546 ± 300a
199 ± 37b
147 ± 46b
256 ± 66b
164 ± 51b
419 ± 156b
1185 ± 223b
930 ± 58a
653 ± 70a
982 ± 78a
507 ± 73a
885 ± 67a
3957 ± 148a
181 ± 65b
165 ± 49b
321 ± 115b
193 ± 69b
449 ± 185b
1309 ± 432b
LFN storage (kg·ha−1 )
0–10
120 ± 24a
10–20
47 ± 8a
20–40
42 ± 3a
40–60
24 ± 2a
60–100
46 ± 5a
Subtotal
279 ± 31a
32 ± 4b
16 ± 3b
19 ± 3b
10 ± 2b
15 ± 3b
92 ± 12b
40 ± 5a
29 ± 5a
46 ± 8a
36 ± 5a
72 ± 8a
223 ± 19a
9 ± 2b
6 ± 2b
11 ± 3b
7 ± 2b
18 ± 7b
51 ± 10b
62 ± 4a
44± 5a
66 ± 5a
34 ± 5a
59 ± 5a
265 ± 10a
8 ± 3b
8 ± 2b
15 ± 5b
9 ± 3b
20 ± 8b
60 ± 20b
SOC storage
0–10
10–20
20–40
40–60
60–100
Subtotal
Plantation
15-Year
(kg·ha−1 )
8406 ± 1068a
3291 ± 478a
2849 ± 338a
1753 ±146a
2961 ± 182a
19,260 ± 1867a
Values of a parameter for a given plantation age labeled with different letters differ significantly (p < 0.05, ANOVA
followed by a least-significant difference test).
The greatest increases in SOC and TN storage after the establishment of plantations were found
in the upper 20 cm of the soil (Table 2). In the layer from 0 to 10 cm, SOC storage in the 9-, 15-, and
31-year-old plantations increased to 3.7, 3.1, and 4.8 times the values in the corresponding control
Sustainability 2017, 9, 662
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areas, respectively, versus 3.4, 2.7, and 5.7 times for TN storage. In the layer from 10 to 20 cm, the
corresponding ratios were 2.7, 2.1, and 2.8 times for SOC storage and 2.6, 2.4, and 3.2 times for TN
storage. However, the corresponding SOC storage for the combined layer from 20 to 100 cm increased
to 1.1, 1.5, and 2.0 times the values in the corresponding control areas, versus increases of 1.0 (i.e., no
increase), 1.3, and 2.4 times for TN storage.
The ratio of LFOC storage to total SOC storage and the ratio of LFN storage to TN storage
increased with increasing time after plantation establishment. Compared with the control values, the
LFOC/SOC storage ratio to a depth of 100 cm increased from 19% to 21%, from 17% to 29%, and
from 17% to 21%, respectively, 9, 15, and 31 years after shrub planting. The corresponding ratios for
LFN/TN storage increased from 7% to 13%, 6% to 17%, and 8% to 12%. LFOC accounted for 23%,
46%, and 24% of the total increase in SOC storage to a depth of 100 cm, respectively, in the 9-, 15-,
and 31-year-old plantations; the corresponding LFN contributions were 22%, 35%, and 14% of the TN
storage increase.
By 9, 15, and 31 years after shrub planting, the SOC accumulation to a depth of 100 cm (the
increase compared with the value in the corresponding control area, divided by the time since
plantation establishment) was 1004, 346, and 360 kg·C·ha−1 ·yr−1 , respectively, versus 96, 33, and
47 kg·N·ha−1 ·yr−1 for TN accumulation, 234, 157, and 85 kg·C·ha−1 ·yr−1 for LFOC accumulation, and
21, 11, and 7 kg·N·ha−1 ·yr−1 for LFN accumulation. The SOC accumulation rates for the combined
layers from 0 to 20 cm in the 9-, 15-, and 31-year-old plantations were 909, 201, and 180 kg·C·ha−1 ·yr−1 ,
respectively, versus 97, 22, and 23 kg·N·ha−1 ·yr−1 for TN accumulation. That is, 91, 58, and 50% of the
total increase in SOC storage occurred in the upper 20 cm of the soil profile, respectively, in the 9-, 15-,
and 31-year-old plantations, versus 100%, 67%, and 49% of the total increase in TN storage. This means
that LF organic matter contributed strongly to C and N storage in the top 20 cm of the soil and that the
soil profile below 20 cm showed increasing SOC and TN accumulation with increasing plantation age.
4. Discussion
4.1. Challenges for Afforestation in Arid and Semiarid Ecosystems
Researchers have investigated the effects of afforestation on soil carbon and nitrogen storage,
and especially on SOC storage, because of the roles of this storage in determining soil quality and in
the carbon cycle of arid and semiarid areas [34–37]. In the present study, we quantified the effects of
the conversion of sand dunes to shrub plantations in a semiarid region of northern China. We found
accumulation of 1004, 346, and 360 kg·C·ha−1 ·yr−1 for SOC to a depth of 100 cm, respectively, 9, 15,
and 31 years after planting, versus 96, 33, and 47 kg·N·ha−1 ·yr−1 for TN. In China’s semiarid Shanxi
Province, the conversion of degraded wastelands to Pinus tabulaeformis plantations increased SOC to a
depth of 100 cm by 383 kg·C·ha−1 ·yr−1 , but decreased the corresponding TN by 6.5 kg·N·ha−1 ·yr−1
24 years after planting, versus decreases of 1098 kg·C·ha−1 ·yr−1 for SOC and 119 kg·N·ha−1 ·yr−1
for TN 8 years after planting [36]. In semiarid Patagonia, the SOC in afforested systems 15 years
after planting did not differ from that in adjacent degraded steppes [5]. During the period from 5 to
29 years after afforestation of Australian pastures, SOC did not change substantially, but TN decreased
greatly in the plantations [35]. In an arid grazing region in Chile, SOC decreased by 32% 2 years after
establishing shrub plantations [34]. In New Zealand, the afforestation of grassland had no net effect on
SOC even 20 years after planting [38].
In contrast with these previous studies, our results suggested that the planting of shrubs in
areas with sand dunes has high potential for increasing carbon and nitrogen sequestration in the
Horqin Sandy Land. One possible explanation for these differences may relate to differences in the
nitrogen requirements among the plantation species. Another relates to the interaction between soil
and climatic conditions, since many factors (e.g., soil type, temperature, and moisture content; soil
microbial populations) will affect rates of organic matter decomposition and loss from the soil (e.g., as
CO2 or CH4 for carbon; as nitrogen oxides for nitrogen). It is also important to note that afforestation
Sustainability 2017, 9, 662
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microbial populations) will affect rates of organic matter decomposition and loss from the soil (e.g.,
projects oriented toward C sequestration face multiple risks due to their longevity, since this makes
as CO2 or CH4 for carbon; as nitrogen oxides for nitrogen). It is also important to note that
them vulnerable to unsustainable management practices (e.g., failing to prevent illegal harvesting
afforestation projects oriented toward C sequestration face multiple risks due to their longevity,
of the wood) and to climate change; in addition, none of these studies (including the present study)
since this makes them vulnerable to unsustainable management practices (e.g., failing to prevent
investigated
the effects
ofwood)
vegetation
succession
over none
long of
periods,
particularly
in the
illegal harvesting
of the
and tocommunity
climate change;
in addition,
these studies
(including
context
of
the
biodiversity
loss
that
may
result
from
monoculture
plantings
[39].
the present study) investigated the effects of vegetation community succession over long periods,
Our studyinarea
in the Horqin
Sandy Landloss
is one
theresult
key areas
implementation
of [39].
China’s
particularly
the context
of the biodiversity
thatof
may
from for
monoculture
plantings
Three-Norths
Shelter
Forest
Program.
The
sustainability
of
the
plantations
in
this
region
is
now
being
Our study area in the Horqin Sandy Land is one of the key areas for implementation of China’s
questioned,
sinceShelter
the benefits
withsustainability
afforestation of
of the
desertified
areasinhave
Three-Norths
Forestassociated
Program. The
plantations
this been
regiondecreasing
is now
over
time,
particularly
in tree
Trees in
millions
of hectares
of plantations
the Horqin
being
questioned,
since
the plantations.
benefits associated
with
afforestation
of desertified
areasinhave
been
Sandy
Land, most
of which
represent
afforestation
with
Populus
spp.,of
are
now dying
(Figure 5a,b)
decreasing
over time,
particularly
in tree
plantations.
Trees
in millions
hectares
of plantations
in
Horqin Sandy
most of by
which
represent
afforestation
with
Populus spp.,
are nowprogram
dying
at the
an increasing
rate,Land,
as reported
China
Central
Television’s
“Half-Hour
Economy”
5a,b) at anchannel
increasing
rate, as reported
by worse,
China Central
Television’s
“Half-Hour
Economy”
on (Figure
their economics
(CCTV-2)
[40]. Even
a dominant
indigenous
tree species
of the
program
on their
economics
channelmacrocarpa,
(CCTV-2) has
[40].also
Even
worse,
a dominant
tree5c).
sandy
grassland
of the
region, Ulmus
begun
dying
in recent indigenous
years (Figure
species
of
the
sandy
grassland
of
the
region,
Ulmus
macrocarpa,
has
also
begun
dying
in
recent
years
Researchers [16,19] believe that the tree deaths have resulted partly from warmer and drier weather
(Figure 5c).
Researchers
[16,19]
believe
the treeour
deaths
have
resulted
from
warmer
and
conditions
linked
to climate
change.
Forthat
example,
study
area
in thepartly
Naiman
Banner
showed
drier
linked toand
climate
change.
For example,
our study
area
the Naiman of
◦ C weather
a 0.9
increase conditions
in air temperature
an 81.4
mm decrease
in rainfall
based
on in
a comparison
Banner
showed
a
0.9
°C
increase
in
air
temperature
and
an
81.4
mm
decrease
in
rainfall
based would
on a
mean values from 2000 to 2009 with the average from 1959 to 1999 [41]. This climate change
comparison of mean values from 2000 to 2009 with the average from 1959 to 1999 [41]. This climate
greatly decrease the water availability to trees while simultaneously increasing their transpiration,
change would greatly decrease the water availability to trees while simultaneously increasing their
and could therefore partly explain the tree deaths from 2000 to 2010. The climate change would also
transpiration, and could therefore partly explain the tree deaths from 2000 to 2010. The climate
have increased the already excessive withdrawal of groundwater to support agricultural production
change would also have increased the already excessive withdrawal of groundwater to support
(Figure 5d), which would further reduce the availability of groundwater resources to support tree
agricultural production (Figure 5d), which would further reduce the availability of groundwater
growth.
The mean annual depth to groundwater in the study area was 2.2 m from 1979 to 1997, but
resources to support tree growth. The mean annual depth to groundwater in the study area was 2.2
increased
7.6 to
m1997,
from but
2005
to 2012 [42].
depth
even
to 13
m ateven
some
sites mentioned
m fromto
1979
increased
to 7.6The
m from
2005
to reached
2012 [42].12The
depth
reached
12 to 13
in the
report
[40]. in the CCTV-2 report [40].
m atCCTV-2
some sites
mentioned
Figure
5. Photographsofof(a)
(a)dying
dyingPopulus
Populus spp.
spp. in
(b)(b)
dying
Populus
spp.spp.
in a in
Figure
5. Photographs
inaafarmland
farmlandshelterbelt;
shelterbelt;
dying
Populus
road
shelterbelt;
(c)
dying
Ulmus
macrocarpa
in
a
National
Nature
Reserve;
(d)
excessive
withdrawal
a road shelterbelt; (c) dying Ulmus macrocarpa in a National Nature Reserve; (d) excessive withdrawal
of groundwater
supportagricultural
agriculturalproduction;
production; (e)
(e) drying
of groundwater
toto
support
drying of
of the
theMolimiao
Molimiaoreservoir
reservoir(formerly
(formerly
Asia’s largest desert reservoir); (f) drying of the Shelihu reservoir (formerly Asia’s second-largest
Asia’s largest desert reservoir); (f) drying of the Shelihu reservoir (formerly Asia’s second-largest desert
desert reservoir); (g) formation of a soil crust 7 years after afforestation in areas with active dunes
reservoir); (g) formation of a soil crust 7 years after afforestation in areas with active dunes that were
that were protected from livestock grazing; and (h,i) reactivation of the soil surface as a result of
protected from livestock grazing; and (h,i) reactivation of the soil surface as a result of heavy grazing
heavy grazing more than 10 years after afforestation.
more than 10 years after afforestation.
Sustainability 2017, 9, 662
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The Molimiao reservoir (43◦ 310 43.000 N, 121◦ 460 48.200 E, Figure 5e) and the Shelihu reservoir
120◦ 330 30.100 E, Figure 5f), claimed to be the largest and second-largest desert reservoirs
in Asia, with capacities of 1.92 × 108 and 1.28 × 108 m3 , respectively, are both located in the Horqin
Sandy Land and have played important roles in supporting local development. However, the Molimiao
reservoir dried up completely in 2003 and the Shelihu reservoir dried up completely in 2010 due to
a combination of climate change with unsustainable water use. Most areas served by these reservoirs are
currently used to cultivate corn. The decrease in natural water availability (e.g., rainfall and recharge of
soil water) and the increase in irrigated agriculture have created a vicious cycle in which environmental
protection has been sacrificed to achieve economic development in the Horqin Sandy Land.
The urgent need for a more sustainable trade-off between protecting and improving the local
environment and developing the local economy is becoming increasingly acute in the world’s arid
and semiarid regions. These regions are generally associated with poverty because of their limited
endowments of biological and ecological resources [39,43]. Afforestation of these lands is problematic
when it reduces the availability of arable land for crops or pastures, thereby endangering food security.
Therefore, vandalism of these lands (often by felling trees and shrubs for their wood and replacing
them with farmland) is threatening the sustainability of the plantations. In our study area, the net
income could increase to RMB 15,000 (US $2175)·ha−1 ·yr−1 if land with moderate nutrient conditions
were cultivated with corn. (This assumes that sufficient water is available to support this activity,
which is not necessarily possible in the study area.) In contrast, the net income from land planted
with Populus spp. is RMB 36,000 (US $5219)·ha−1 (based on harvesting of 60 m3 of timber) over
a 15-year rotation period, representing an annual income of only RMB 2400 (US $348)·ha−1 ·yr−1 (i.e.,
only 16% of the agricultural income). Because afforestation projects have such a low yield, farmers
sometimes destroy plantations so they can farm the land. To mitigate this problem, the current
ecological compensation payments system should be improved to give farmers a reason to protect
plantations; for example, carbon offset credits should be accounted for in China, and the resulting
credits transferred to residents of afforestation areas to compensate them for the loss of arable land.
Livestock grazing is another critical factor that influences the effectiveness of afforestation. In arid
and semiarid regions, soil crusts that commonly formed once active dunes have been stabilized through
re-vegetation [44]. These soil crusts perform essential ecosystem services, including enhancement
of soil stability and fertility [45], protection of the soil from wind erosion [46], and fixation of C and
N [47,48]. Figure 5g shows the soil crust that formed in an area of active dunes 7 years after afforestation
using Populus spp. in the Horqin Sandy Land because the land was protected from livestock grazing.
However, these soil crusts are fragile and vulnerable to anthropogenic and natural disturbances, and
are easily destroyed by livestock [44]. Figure 5h,i show how the soil surface can become reactivated
(i.e., mobile) as a result of continuous grazing, even over 10 years after tree and shrub establishment.
In the present study, the 9-year-old plantation that was protected from grazing showed significantly
greater SOC and TN accumulation rates in the uppermost soil layer (0 to 10 cm) than the rates in the
15- and 31-year-old plantations, both of which were disturbed by continuous grazing. For example, the
SOC accumulations to a depth of 10 cm were 681, 136, and 126 kg·C·ha−1 ·yr−1 , respectively, in the 9-,
15-, and 31-year-old plantations. The 9-year-old plantation was established in areas with fixed dunes,
whereas the 15- and 31-year-old plantations were established in areas with active dunes. The fixed
dunes had a greater nutrient content than the active dunes, so the difference in the initial quality of
the desertified land partially explains the large differences in SOC accumulation rates between the
9-year-old plantation and the older plantations. However, the large increase in SOC in the 9-year-old
plantation compared with its control areas supports our belief that protecting plantations from grazing
is crucial to achieve a high SOC accumulation in the upper soil layers.
Identifying suitable species is very important for afforestation of degraded lands in arid and
semiarid regions [49]. The most challenging aspect of such afforestation projects is the choice between
indigenous and exotic species. Often, the advantages associated with exotic species (e.g., higher
growth rates, better wood quality, or markets) outweighs the hidden and diffuse benefits offered by
(42◦ 450 15.300 N,
Sustainability 2017, 9, 662
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indigenous species, such as biodiversity conservation and the use of plants that are better adapted
to the afforestation sites [39]. In the present study area, fast-growing exotic broadleaved Populus
species were widely planted for rapid establishment of shelterbelts. However, recent research has
suggested that these species are not an optimal choice for the region because they have low water-use
efficiency, creating an acute contradiction between the increased water demand created by these
trees and the decreasing water supply that has resulted from the background of climate change
and unsustainable human use of the water resources [16,19]. Our current work confirmed that
C. microphylla, an indigenous shrub species, performed well in the Horqin Sandy Land in terms of its
ability to improve soil properties and combat desertification, and in terms of its ability to survive for
at least 31 years under local environmental conditions. Shrub species have many advantages, such
as extensive root systems, lower water consumption than trees, fast growth, high yield, and strong
reproduction [50]. Therefore, such shrubs are highly adaptable and represent a better choice than many
tree species for afforestation in arid and semi-arid regions. Although previous research suggested that
a mean annual rainfall of 200 mm may be sufficient to sustain tree plantations [7], this value may be
optimistic for many of the species that have been used in afforestation. Further research will be needed
to determine the minimum rainfall required to support each afforestation species. This will allow
managers to choose species that are most likely to survive in a given area. In addition, researchers
should determine the potential of other life forms, such as desert steppe grasses, to determine whether
they may be a better alternative than woody vegetation.
4.2. Carbon Sequestration Potential of Desertification Control in China’s Horqin Sandy Land
To sequester C and restore degraded land in arid and semiarid regions, afforestation and
exclosures that protect an area from humans and livestock grazing are two of the most widely suggested
options [5,7]. In China’s Horqin Sandy Land, we have investigated the positive effects of grazing
exclosures and tree planting (using an introduced species of Mongolian pine) in areas with active
dunes on the vegetation characteristics and soil properties of afforestation sites [17,20]. We confirmed
that the exclosures and afforestation can control desertification in the region.
One of our previous studies showed that SOC storage to a depth of 100 cm increased by
566 kg·C·ha−1 ·yr−1 during a 25-year grazing exclosure [20], versus 205 kg·C·ha−1 ·yr−1 during a
28-year afforestation period using Mongolian pine [17]; both studies were established in areas with
active dunes. In the present study, the 31-year-old plantation established in areas with active dunes
was closest to the ages in the two previous studies, and showed an increase of 360 kg·C·ha−1 ·yr−1
in SOC storage. The existing areas of non-desertified grassland in the Horqin Sandy Land have
48,779 kg·C·ha−1 of SOC storage to a depth of 100 cm [18]. Based on an average SOC storage of
6688 kg C ha−1 in active dunes (the average of the values measured in the two previous studies and in
the present study) and the abovementioned rate of SOC increase, it would take 74 years to fully restore
the SOC storage of the active dunes through grazing exclosures, versus 117 years through afforestation
using the shrub species C. microphylla and 205 years through afforestation using Mongolian pine.
However, the results for the present study represent a conservative estimate, since branches of the
shrubs in our study plantations were harvested every 3 to 4 years and the two older plantations were
subjected to continuous browsing by livestock. Although we did not quantify the biomass removals
by these processes, retaining the organic matter on the sites would likely accelerate the accumulation
of organic matter, thereby shortening these times.
5. Conclusions
In the semiarid Horqin Sandy Land of northern China, the establishment of shrub plantations
using C. microphylla in areas with fixed and active dunes led to significant increases in C and N storage
in the soil. The greatest increases occurred in the top 20 cm of the soil profile. The soil LF organic
matter was an important contributor to soil C and N sequestration, and its magnitude was strongly and
significantly correlated with the total soil organic C and N concentrations. To increase the sustainability
Sustainability 2017, 9, 662
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of artificial tree and shrub planting, it will be necessary to encourage local residents to preserve
plantations. One possibility would be to improve ecological compensation mechanisms, perhaps by
accounting for carbon offset credits. Afforestation is an effective option to sequester carbon and to
restore degraded soils, but this process was very slow. Our results suggest that it would take more than
100 years to fully restore the SOC storage of the active dunes through afforestation with C. microphylla
in the Horqin Sandy Land. However, if grazing and harvesting of firewood were prohibited in the
plantations, recovery is likely to be faster.
Acknowledgments: This research was supported by the One Hundred Person Project of the Chinese Academy
of Sciences (Y551821), the National Key Research and Development Program of China (2016YFC0500901), and
the National Natural Science Foundation of China (grants 31640012, 31560161, 31260089, and 31400392). We are
grateful to the anonymous reviewers for their constructive comments.
Author Contributions: All co-authors assisted with manuscript writing. Yuqiang Li and Yinping Chen conceived
and designed the experiment, and wrote this paper. Xuyang Wang, Yayi Niu, and Jie Lian were responsible for the
field investigation, soil sampling, and laboratory analyses.
Conflicts of Interest: The authors declare no conflict of interest.
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