For Review OnlyRapid soil water recovery after conversion of introduced
peashrub and alfalfa to natural grassland on northern China’s Loess Plateau
Journal: Canadian Journal of Soil Science
Manuscript ID CJSS-2020-0010.R1
Manuscript Type: Article
Date Submitted by the Author: 28-Apr-2020
Complete List of Authors: Cao, Ruixue; Institute of Geographic Sciences and Natural Resources Research Chinese Academy of SciencesPei, Yanwu; Northwest Agriculture and Forestry UniversityJia, Xiaoxu; Institute of Geographic Sciences and Natural Resources Research CASHuang, Laiming; Institute of Geographic Sciences and Natural Resources Research Chinese Academy of Sciences,
Keywords: soil moisture, soil desiccation, soil water recovery, thinning, China’s Loess Plateau
Is the invited manuscript for consideration in a Special
Issue?:Not applicable (regular submission)
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Rapid soil water recovery after conversion of introduced peashrub and
alfalfa to natural grassland on northern China’s Loess Plateau
Ruixue Cao1, 2, Yanwu Pei3, Xiaoxu Jia1, 3, 4, Laiming Huang1, 3, 4*
1 Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic
Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101,
China.
2 State Key Joint Laboratory of Environmental Simulation and Pollution Control, School of
Environment, Beijing Normal University, Beijing, 100875, China.
3 State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest
Agriculture and Forestry University, Yangling 712100, China.
4 College of Resources and Environment, University of Chinese Academy of Sciences,
Beijing 100190, China.
* Corresponding author:
Dr. Laiming Huang
Key Laboratory of Ecosystem Network Observation and Modeling,
Institute of Geographic Sciences and Natural Resources Research,
Chinese Academy of Sciences, Beijing 100101, China.
Email: [email protected]
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Abstract: To evaluate the potential of soil water recovery after thinning, in-situ soil water
content in the 0–500 cm soil profile under thinned (50–100%) and un-thinned peashrub and
alfalfa plots and a nearby natural grassland in Liudaogou watershed in China’s Loess Plateau
(CLP) was measured monthly during 2015–2017 growing season using a neutron probe. At
the start of experiment, the profile soil water storage (SWS0–500 cm) under introduced peashrub
and alfalfa was respectively 18.8% and 12.2% lower than that under natural grassland. This
showed that there was higher water consumption by planted vegetation, compared with native
grass. After thinning, SWS0–500 cm in thinned peashrub and alfalfa plots was significantly
higher than that in un-thinned plots due to decrease in both interception and transpiration. The
increase in SWS0–500 cm in the 100% thinned peashrub plot (159.9–216.1 mm) was much
higher than that in 50% thinned peashrub (39.1–169.8 mm) and 100% thinned alfalfa (20.3–
118.1 mm) plots. This indicated that the extent of soil water recovery varied with thinning
intensity and vegetation type. At the end of the third growing season, soil water restoration
frontier in the thinned peashrub and alfalfa plots (>300 cm) was much greater than that in the
un-thinned plots (<180 cm). It also indicated that with thinning, soil water (<300 cm) can
recover rapidly following two successive wet years. The results suggested that concerns about
soil desiccation and the potential impact on long-term sustainability of restored ecosystems on
CLP were resolvable.
Keywords: soil moisture; desiccation; soil water recovery; thinning; China’s Loess Plateau
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Introduction
Because of high erodibility of loess deposits and intensive human disturbance, China’s
Loess Plateau (CLP) is one of the most severely eroded regions in the world (Stolte et al.
2005; Zhu et al. 2019). In order to control soil erosion and improve ecosystem services on the
plateau, a series of restoration measures including “Grain for Green Project” and “Natural
Forest Protection”, have been initiated by the Chinese government since 1990s. Croplands
were converted to artificial forests, shrubs and grasslands using a large variety of introduced
plant species. Vegetation cover on CLP has increased dramatically from 6.5% in the 1970s to
59.6% in 2013 (Chen et al. 2015). With the implementation of vegetation restoration, annual
runoff in the Yellow River has decreased significantly over the past decades (Wang et al.
2015). Sediment concentration in 12 main sub-catchments in CLP region along the Yellow
River Basin declined by 21% due to massive afforestation in 1998–2010 (Wang et al. 2015).
Although the planted forests, shrubs and grass have decreased surface runoff by increasing
infiltration and soil water holding capacity, severe soil water depletion and negative water
balance has occurred as a result of increased transpiration and soil water consumption (He et
al. 2003; Jia et al. 2017; Huang et al. 2019). This has resulted in the formation of dry soil
layers (DSL) across the plateau (Wang et al. 2010; Wang et al. 2012b; Wang et al. 2018a), in
turn endangering the health and services of the restored ecosystems.
There is wide DSL formation and distribution (Huang et al. 2019) in artificial
ecosystems such as black locust (Robinia pseudoacacia), peashrub (Caragana Korshinskii)
and alfalfa (Medicago sativa) on northern CLP (Li et al. 2008; Wang et al. 2012b; Jia and
Shao 2013; Jia et al. 2015; Guo et al. 2018). The occurrence of DSL is as a result of the
improper introduction of exotic plant species and/or high-density planting, accelerating soil
desiccation and degrading ecosystem function (Huang et al. 2019). Feng et al. (2016) found
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that revegetation in CLP region is approaching a sustainable water limit. To recover lost soil
water in DSL and alleviate negative effects of soil desiccation on restored ecosystems, various
regulation measures have been used in the region. This includes thinning of artificial forests,
conversion of exotic trees or shrubs to native grass and introduction of more water-saving
plant species (Huang and Gallichand 2006; Fu et al. 2012; Wang et al. 2012a). Studies
estimate that the duration of soil water recovery varies within 4.4–8.4 years (average of 7.3
years) in the upper 0–300 cm soil layer and 6.5–19.5 years (average of 13.7 years) in the 0–
1000 cm soil layer after conversion of 30-year-old apple orchard to winter wheat (Huang and
Gallichand 2006). Soil water recovery time not only varies with soil depth, but also with land
use conversion mode. Liu et al. (2008) reported that while soil water in DSL under
10-year-old alfalfa grassland could recover after 18 years of alfalfa-crop rotation, the recovery
time of soil water in 200–300 cm DSL under cropland was less than 10 years (3.1–9.8 years),
depending on cropping intensity (Liu et al. 2010). These results suggest that soil water
recovery time is site-dependent because of variations in soil desiccation degree, differences in
inter-annual rainfall and changes in soil characteristics. Although soil water recovery from
desiccation has been extensively studied in CLP region, most studies have been based on
model simulations. Uncertainties of model simulations could limit our understanding of soil
water recovery and deep soil water recharge. Given the importance of soil water to the
sustainability of restored ecosystems in arid and semi-arid regions of CLP, in-situ
observations of soil water dynamics in response to thinning or land use conversion are
needed. This can lead to accurate evaluation of soil water recovery potential and
quantification of the extent of deep soil water recharge on the recovery process.
Precipitation is basically the only source of soil water in sloping lands on CLP because
groundwater levels are generally 20–300 m below land surface (Li and Huang 2008; Qiao et
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al. 2017). The amount of precipitation influences water recharge into deep soil layers and
maximum infiltration depth of rainwater. Studies show that infiltration depth of rainwater is
mostly limited to the top 100 cm soil layer on CLP in both normal and dry years (Wang et al.
2010; Liu and Shao 2016). In wet years, however, infiltration depth of rainwater can exceed
200–300 cm and infiltrated water below this depth is significant (Liu et al. 2010). Here, we
hypothesized that high rainfall in wet years can quickly replenish soil water after conversion
of introduced peashrub/alfalfa to natural grassland in northern CLP region. We tested this
hypothesis by in-situ monitoring of soil water dynamics in response to thinning or land use
conversion in a semi-arid region of CLP during 2015–2017 growing seasons. The objective of
the study was to quantify the extent of soil water recovery after thinning or land use
conversion. The results of the study will guide informed decision on future vegetation
restoration towards sustainable water management.
Materials and methods
Study area
This study was conducted at Shenmu Erosion and Environment Research Station, which
is located in Liudaogou watershed in northern CLP region (38°46′–38°51′N, 110°21′–
110°23′E) (Fig. 1). This region is characterized by semi-arid continental climate, with mean
annual rainfall of 421 mm (1961–2014) and mean annual air temperature of 8.4 °C
(http://www.nmic.cn/). Most of the rainfall (77.4%) occurs from June to October. The lowest
(–9.7 °C) and highest (23.7 °C) air temperatures generally occur in January and July,
respectively. The mean annual potential evapotranspiration can reach 785 mm. The elevation
of the studied watershed varies within 1094–1274 m and there are many deep gullies in the
watershed because of severe wind and water erosion. To control severe erosion in the region,
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vegetation has been widely planted over the past decades. The vegetation type is
predominately perennial, including purple alfalfa (Medicago sativa), korshinsk peashrub
(Caragana korshinskii), apricot trees (Prunus armeniaca) and Salix psammophila.
Abandoned croplands are recovered by native grasses such as bunge needlegrass (Stipa
bungeana), Agropyron cristatum and Artemisia scoparia. Soil is formed from the loess
deposits of low-fertility and loose-structure. The main soil type is Aridic Calcisols according
to the World Reference Base for Soil Resources (FAO, 2006); or Haplocalcids according to
Keys to Soil Taxonomy (Soil Survey Staff, 2010). More detailed information about the
investigated watershed is given by Mao et al. (2018).
Experimental design and soil water measurement
To assess soil water deficit under artificially planted vegetation, we determined the mean
soil water content (SWC) in the 0–500 cm soil profile in original peashrub (28-year) and
alfalfa (25-year) fields before thinning. The data collected were then compared with mean
SWC of nearby natural grassland (14-year). To determine soil water recovery after thinning or
land use conversion, the selected alfalfa field (20 m × 25 m) was divided into two plots (A1
and A2) and the peashrub field (60 m × 25 m) divided into three plots (P1, P2 and P3) (Fig.
1b). A1 and A2 were plots of original alfalfa (control) and total (100%) thinning of alfalfa
(i.e., conversion of alfalfa to grass for natural succession). P1, P2 and P3 were plots of
original peashrub (control), partial (50%) thinning of peashrub and total (100%) thinning of
peashrub (i.e., conversion of peashrub to grass for natural succession). The dominant
vegetation species in the study area after land use conversion was native herbaceous plants.
The natural recovered vegetation in the 100% thinned peashrub plot was dominated by
Pennisetum flaccidum, followed by Solaum septemlobum, Chenopodium glaucum and Rumex
trisetifer. The main species in the 100% thinned alfalfa plot were Artemisia capillaries, Carex
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giraldiana, Chenopodium aristatum L. and Setaria viridis (L.) Beauv. The changes in
vegetation cover after thinning were measured by photographic method using unmanned
aerial vehicle (DJI Phantom 4 Pro) (Fig.1c).
Three aluminum neutron-probe access tubes (4 cm in diameter and 520 cm in depth)
were installed at 2-m intervals along the center line of each plot to measure SWC. Before
thinning, initial SWC in peashrub field, alfalfa field and the nearby natural grassland was
measured using a calibrated neutron probe (CNC 503DR Hydroprobe, Beijing Super Power
Company, Beijing China) on May 6th, 2015. Then volumetric SWC in the 0–500 cm profile in
each plot was measured monthly during the May-October growing seasons of 2015–2017.
SWC was measured at 10 cm interval in the top 100 cm soil layer and 20 cm interval below.
The routinely calibrated and fitted piecewise linear equation for neutron probe device in this
study was:
d ≤ 100 cm, θ = 73.30 CR + 3.9565 (n = 7, R2 = 0.8996, p < 0.001) (1)
d > 100 cm, θ = 60.09 CR + 1.8995 (n = 55, R2 = 0.7578, p < 0.001) (2)
where θ is volumetric SWC (%) and CR is slow-neutron counting rate at a given soil depth d
(cm). The slow-neutron counting rates were computed as ratios of the slow-neutron counts at
a specific depth of soil to the standard count obtained with the probe in its shield (i.e. 660 in
this study).
Soil water storage (SWS) (mm) was calculated as follows:
(3)𝑆𝑊𝑆 = 10∑𝑛𝑖 = 1𝜃𝑖∆𝑧
where n is the total number of soil layers; is average soil water content (cm3/cm3) in layer 𝜃𝑖
i; and is the measured interval depth (cm).∆𝑧
Evapotranspiration (ET) for continuous analysis was determined from soil water balance
method as:
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ET= (4)𝑃 - ∆𝑆 - 𝐷 - 𝑅
where P is the precipitation (mm), which was monitored in the meteorological station at
Shenmu Erosion and Environment Research Station; is the change in SWS in the profile ∆𝑆
for the analyzed soil depth (mm); D is the percolation below the measured depth (mm); and R
is surface runoff (mm). In this study, no significant runoff was observed in the plot due to the
gentle slope (<5°) and relatively high infiltration rate of loess soils during the experimental
period.
Statistical analysis
A one-way analysis of variance (ANOVA) was used to determine the differences in
SWC between different growing seasons. Then the paired sample test was used to determine
the difference in SWC before and after thinning. The analyses and statistics were done in the
Statistical Package for Social Sciences (SPSS 24.0 for Windows). The figures were drawn
using Origin Pro 9.0.
Results and discussions
Initial soil water depletion in original peashrub and alfalfa field
Vertical distributions of mean SWC in the 0–500 cm soil profile in peashrub and alfalfa
fields before thinning were compared with those of natural grassland (Fig. 2). As shown in
Fig. 2, SWC in the top 0–30 cm soil layer in peashrub (0.092–0.118 cm3/cm3) and alfalfa
(0.091–0.185) fields was consistently higher than that in natural grassland field (0.058–0.079)
(p < 0.01). In contrast, SWC in the soil layer below the 100 cm depth was generally lower in
peashrub (0.082–0.141) and alfalfa (0.095–0.128) fields than in grassland (0.121–0.152) field
(p < 0.05) (Fig. 2). This indicated that artificial plants consumed more water than native grass
in the deeper soil layer (>100 cm), attributed to higher evapotranspiration and deeper roots of
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exotic peashrub and alfalfa than those of indigenous grass (Cheng et al. 2009; Ridley et al.
2001; Mao et al. 2018). The much lower SWC in the surface layer (0–30 cm) in natural
grassland, as compared with the original alfalfa and peashrub field (Fig. 2), was mainly due to
lower vegetation cover and higher evaporation in natural grassland. In addition, higher sand
content in surface soils of natural grassland (57.4%) than those of peashrub (40.2%) and
alfalfa (39.9%) field could also result in lower soil water holding capacity and SWC in the
surface soils of grassland. Published studies also noted variations in soil water profile
distribution under different vegetation covers in the investigated watershed (Liu and Shao
2014). The mean SWC in the 0–500 cm soil profile in peashrub and alfalfa field was 0.112
and 0.114 cm3/cm3, respectively, which was significantly different from that in natural
grassland (0.133 cm3/cm3) (p < 0.01). SWS in 0–500 cm soil profile in original peashrub
(533.28 mm) and alfalfa (576.66 mm) fields was respectively 18.8% and 12.2% lower than
that in natural grassland (656.61 mm), suggesting a more rapid decline in soil water under
planted vegetation (peashrub and alfalfa) compared with native grass. Higher SWS has also
been observed in grassland than in forestland in other studies in Liudaogou watershed, and
ascribed to lower evapotranspiration in grassland than in forestland (Jia and Shao, 2013).
Soil water dynamics in thinned peashrub and alfalfa plots
The dynamic change in SWC in peashrub and alfalfa plots under different thinning
treatments during the 2015–2017 growing seasons is plotted in Fig. 3. SWC at different
depths in both peashrub and alfalfa plots under different thinning treatments increased with
time, attributed to the increase in rainfall during the study period (2015 with 453 mm, 2016
with 605 mm and 2017 with 607 mm) (Fig. 3). Compared with profile average SWC at the
start of the experiment, the final averaged SWC in the 0–500 cm soil profile under the 100%
thinned peashrub (0.194 cm3/cm3), 50% thinned peashrub (0.165), 100% thinned alfalfa
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(0.172), and then under un-thinned peashrub (0.128) and alfalfa (0.139) plots increased
respectively by 73.2%, 47.3%, 50.8%, 14.2% and 21.9%. The increase in SWC in thinned
peashrub and alfalfa plots was greater than that in the corresponding un-thinned plots (Fig. 3).
This suggested that thinning management in the peashrub and alfalfa plots can reduce soil
desiccation and accelerate soil water recovery during wet years. Zhu et al. (2017) also noted
that deep soil water deficit can be mitigated by thinning of Picea crassifolia plantation in
northwestern China.
SWC not only changes with time, but also with soil depth (Fig. 3). Based on the
coefficient of variation (CV), SWC was divided into three layers — active (0–100 cm with
CV = 25–40%), sub-active (100–400 cm with CV = 10–25%) and stable (400–500 cm with
CV < 10%) layers. SWS in the three layers in the peashrub and alfalfa plots under different
thinning treatments were calculated at the end of each growing season in 2015–2017 and the
results given in Table 1. As compared with un-thinned plots, there was significant increase in
SWS in the active (0–100 cm), sub-active (100–400 cm) and stable (400–500 cm) layers in
thinned peashrub and alfalfa plots at the end of each growing season (Table 1). The increase
in SWS in the thinned plots was attributed to the decreasing interception and transpiration
(Bréda et al. 1995). The increase in SWS in the 0–500 cm soil profile in the 100% thinned
peashrub plot was 159.9–216.1 mm relative to that in un-thinned plot in 2015–2017, which
was much higher than that in the 50% thinned peashrub plot (39.1–169.8 mm) and 100%
thinned alfalfa plot (20.3–118.1 mm). This suggests that the extent of soil water recovery not
only varies with thinning intensity, but also with vegetation type (Gebhardt et al. 2014).
Inter-annual variability in rainfall amount could also influence the extent of soil water
recovery. Other factors affecting soil water recovery after thinning include soil texture,
regional environmental conditions, and the growth and regeneration of understory plants (Pan
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et al. 2015). SWS in different soil layers also increased with time in both thinned and
un-thinned peashrub and alfalfa plots (Table 1). This was because of the sharp increase in
rainfall during the study period — i.e., rainfall increased from 453 mm in 2015 to >600 mm in
2016 and 2017. As a result, there were significant differences in SWS among the different
growing seasons in 2015–2017 in the active (0–100 cm) and sub-active (100–400 cm) layers
in all the plots (Table 1). This suggested that the wetting front (the maximum depth of
precipitation infiltration) associated with 600 mm of accumulated precipitation could reach
the 400-cm depth of soil layer. Liu et al. (2010) showed that the wetting front reached a depth
of 5 m in the southern CLP region in a record wet year with 954 mm of precipitation. In this
study, however, SWS did not differ significantly in the stable (400–500 cm) soil layer at the
end of the growing seasons, except in the 100% thinned peashrub plot (Table 1). This
indicated that SWS below 400 cm in the 50% thinned peashrub, 100% thinned alfalfa and the
two un-thinned plots were not affected by the variations in precipitation. The possible
explanation was that i) these plots maintained a balance between water consumption and
replenishment below the 400 cm soil layer; and/or ii) soil water below the 400 cm layer was
not replenishable by precipitation or absorbable by plant roots in the plots.
Deep soil water recharge in peashrub and alfalfa plots under thinning
SWC in natural grassland was used as the reference value to estimate soil water deficit
and recharge in peashrub and alfalfa plots under the thinning treatments during the study
period. Soil water restoration frontier is defined as the minimum depth of soil water recovery
where SWC in the peashrub and alfalfa plots exceeds that in natural grassland plot (i.e.,
Y-intercept in Fig. 4). Soil water conservation at the end of each growing season during
2015–2017 is the increase in soil water in the wet layer of a treatment over that in natural
grassland plot (Table 2). This is equal to the area between SWC deficit line and Y-axis in Fig.
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4. Both soil water restoration frontier and soil water conservation in the plots increased with
time, ranging from 30 cm to 500 cm and from 11.9 mm to 325.5 mm, respectively (Table 2).
At the end of the growing season in 2017, soil water restoration frontier in the two un-thinned
plots (120 cm and 180 cm) was (<200 cm) relatively shallow (Table 2 and Fig. 4). This is
consistent with reported levels of rainfall infiltration depth in artificial peashrub and alfalfa
fields in northern CLP region (Chen et al. 2008a 2008b; Wang et al. 2011b; Fu et al. 2013).
After three years of thinning, the thinned peashrub and alfalfa plots had much deeper soil
water restoration frontier compared with un-thinned plots (Table 2 and Fig. 4). Soil water
restoration frontiers in the 50% thinned peashrub, 100% thinned peashrub and 100% thinned
alfalfa plots were respectively 320, 500 and 300 cm at the end of 2017 growing season (Table
2 and Fig. 4). This indicated that there was soil water recovery in the 500 cm soil profile after
conversion of peashrub to natural grassland in the third growing season. Wang et al. (2008)
showed that soil water in the 0–500 cm soil profile can recover in the third year after
conversion of alfalfa to cropland. In this study, however, soil water below the 300 cm soil
depth after the conversion of alfalfa to grassland and in 50% thinned peashrub plot did not
fully recover at the end of third growing season (Table 2 and Fig. 4). Differences in soil water
recovery can be related to site-specific conditions, including local climate, vegetation type
and soil texture. Studies show that vertical replenishment of SWC at plot scale are primarily
controlled by vegetation type and soil properties (Jia and Shao 2013; Huang et al. 2019).
Although the effects of rotation, plant density regulation and land use conversion on
SWS have been extensively studied (Aase and Pikul 2000; Li and Huang 2008; Wang et al.
2008; Laik et al. 2014), the timescale of soil water recovery under different management
conditions remain unclear. Our results showed that the timescale of soil water replenishment
in the 0–300 cm soil layer in thinned peashrub and alfalfa plots was less than three years. This
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is because soil water restoration frontier reached the depth of 300 cm at the end of the
experiment (Table 2). However, it could take longer to recover deeper (>300 cm) SWC in the
50% thinned peashrub and 100% thinned alfalfa plots. Long-term field experiments are
therefore needed to test water recovery time in deeper soil layers under thinning management.
Using both in-situ observation and model simulation data, Wang et al. (2012a) showed that it
take 2, 6, 11, 13 and 18 years to recover SWC respectively in the 200–300 cm, 300–400 cm,
400–500 cm, 500–600 cm and 600–800 cm soil layers after conversion of alfalfa pasture to
cropland in CLP region. This should ease concerns about soil desiccation and the potential
impacts on long-term sustainability of restored ecosystems in CLP region. Soil water can
recover in a relatively short time by thinning or switch from one land use into the other under
successive wet years. In dry years with low precipitation, however, the rate and degree of soil
water recovery requires further study and long-term monitoring is needed to address this
problem.
Precipitation-driven soil water restoration
Considering the buried depth (50–200 m) of groundwater in Liudaogou watershed (Qiao
et al. 2018), compensation and recovery of soil water depend primarily on precipitation in wet
years (Zhao et al. 2016). Liu and Shao (2016) divided precipitation years into three types in
Liudaogou watershed using the domestic common division standard (i.e., Pwet > Pgrowing average
+ 0.33δ; Pdry < Pgrowing average – 0.33δ, where Pwet is the amount of precipitation during growing
season in wet year (mm); Pdry is the amount of precipitation in dry year (mm); Pgrowing average is
the mean precipitation for many years during growing season (mm); δ is the mean square
error). The mean precipitation during growing season was 386.0 mm and the mean square
error was 107.4 mm calculated from the precipitation data for 1971 to 2014. According to the
divided method of precipitation years, during the growing season, the amount of precipitation
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in wet year is greater than 421.4 mm, the year with precipitation less than 350.6 mm is
considered as a dry year, and in a normal year the precipitation is 350.6–421.4 mm. In this
study, Pgrowing was 385, 497 and 559 mm in 2015, 2016 and 2017, respectively. Thus, 2015
was a normal year and 2016 and 2017 were wet years. Increase in SWS in the 0–500 cm soil
profile at the end of the growing season relative to the start (△SWS0-500cm) in wet years of
2016 and 2017 were calculated and presented in Table 3. Evapotranspiration (ET) in Table 3
was obtained from the water balance. △SWS0-500cm in peashrub plots under different thinning
managements was 60.8–98.6 mm in 2016 and 11.4–149.5 mm in 2017. Higher △SWS0-500cm
was observed in thinned peashrub plots than in un-thinned plot in both 2016 and 2017 (Table
3). Similarly, △SWS0-500cm was higher in the 100% thinned alfalfa plot (81.9–121.8 mm) than
the un-thinned (3.6–80.1 mm) plot (Table 3). The ratio of △SWS0-500cm to Pgrowing
(△SWS0-500cm : Pgrowing) was also higher in thinned peashrub and alfalfa plots than in
un-thinned plots, with 2.0–26.7% in peashrub plots and 0.7–21.8% in alfalfa plots (Table 3).
The results suggested that in wet years, the contribution of precipitation to soil water
restoration increased after thinning management. The significant difference in △SWS0-500cm
and (△SWS0-500cm : Pgrowing) among different thinning treatments in 2017 was in sharp contrast
to the similarities of △SWS0-500cm and (△SWS0-500cm : Pgrowing) among different thinning
treatments in 2016 (Table 3). This was attributed to the higher hysteresis effect in peashrub
and alfalfa plots than in converted natural grasslands in response to the increase in
precipitation (Lauenroth and Sala 1992; Wiegand et al. 2004). Because soil water restoration
frontier in the 100% thinned peashrub plot extended down to 500 cm in 2017 (Table 2), water
conserved in the soil percolated further downward through matrix flow to a depth greater than
500 cm (Tan et al. 2017). Thus, the results in Table 3 could have underestimated the ratio of
△SWS0-500cm to Pgrowing (△SWS0-500cm : Pgrowing) and overestimated ET in the 100% thinned
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peashrub plot in 2017. Despite the possible underestimation, the proportion of △SWS0-500cm to
Pgrowing reached 26.7% in the 100% thinned peashrub plot (i.e., conversion of peashrub to
natural grassland) and 21.8% in the 100% thinned alfalfa (i.e., conversion of alfalfa to natural
grassland) plots. This was much higher than the reported proportion of soil water conservation
to precipitation (8%) after conversion of apple orchard to cropland (Huang and Gallichand
2006). One possible explanation is that the coarser soil texture (sandy loam vs. silty clay
loam) and the higher rainfall (606 mm vs. 545 mm) in our study than in the study by Huang
and Gallichand (2006) was responsible for the discrepancy. Thus, coarse loess soil, combined
with high rainfall in wet years, makes thinning management more efficient in soil water
restoration in the study area.
Implications for future vegetation reconstruction
Studies show that both climate and land use affect regional soil water distribution and
dynamics in northern CLP region (Wang et al. 2010b; Jia et al. 2015; Huang et al. 2019).
Given increasing drought frequency under climate warming, land use management is an
effective approach to regulating soil water deficit in the short term. Improper introduction of
high water-demanding plants and excessive afforestation (e.g., over-planting) on CLP
increasingly threatens the local ecosystem health. As a result, introduced exotic trees, shrubs
and grass with high water consumption should be replaced with more water-saving native
species or only planted at low density (Xia and Shao 2008; Liu and Shao 2015; Huang et al.
2019). Our results showed that soil water in DSL was replenishable by moderate thinning
(50% thinning) or by the conversion of peashrub/alfalfa to natural grassland (100% thinning)
in successive wet years. In contrast, the DSL in unthinned peashrub and alfalfa plots were not
fully recovered in spite of successive wet years. Nevertheless, the rate and degree of soil
water recovery by thinning is largely controlled by rainfall amount. It should be noted that
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soil water recovery by thinning in dry years would require a longer time than the present
study. Sohn et al. (2016) conducted a meta-analysis and found that soil drought was mitigated
by thinning forests. Increasing evidence shows that low stand densities promote vigor of
individual trees due to increased SWC availability (Gebhardt et al. 2014; Zhu et al. 2017).
Recharge of soil water after thinning is related to reduction in both interception and
transpiration due to reduced leaf-area index in thinned forest stands, compared with
un-thinned forest stands (Bréda et al. 1995). Furthermore, Prima et al. (2017) showed that
increase in water infiltration in a thinned Mediterranean oak forest reduced surface runoff and
enhanced soil water restoration. Despite widespread recognition of soil drought-mitigation
through thinning, the degree and timescale of soil water recovery vary with thinning intensity
and vegetation type (Huang and Gallichand 2006; Liu et al. 2010; Wang et al. 2012a;
Gebhardt et al. 2014). This was confirmed by our study, which showed differences in the
increase in SWS0–500 cm under 100% thinned peashrub (159.9–216.1 mm), 50% thinned
peashrub (39.1–169.8 mm) and 100% thinned alfalfa (20.3–118.1 mm) plots. There is still a
challenge to develop optimal thinning intensity for different vegetation types to maintain
sustainable vegetated ecosystem. Recent works on determining the threshold of vegetation
productivity (Feng et al. 2016), equilibrium vegetation cover (Zhang et al. 2018), regional
water resources development boundary (Wang et al. 2018b) and soil water carrying capacity
for vegetation (Jia et al. 2019) provide quantitative guides for thinning management in
excessively revegetated regions. In addition, soil water restoration after thinning is a dynamic
process that is affected by rainfall amount, plant growth and management practices. Thus,
long-term study is needed to elucidate the mechanisms controlling the dynamic changes of
soil water storage with and without thinning. Such knowledge will provide a basis for
management strategies that promote sustainable water use for vegetation restoration and
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ecosystem development.
Conclusions
To evaluate the impact of thinning on soil water recharge, artificially planted peashrub
and alfalfa in high density were partially thinned or fully converted into natural grassland.
The initial profile soil water storage (SWS0–500 cm) under the introduced peashrub (533.28
mm) and alfalfa (576.66 mm) was respectively 18.8% and 12.2% lower than that under
natural grassland (656.61 mm). It demonstrated that water consumption by planted vegetation
was higher than that by native grass. After thinning, SWS0–500 cm in thinned peashrub and
alfalfa plots was significantly higher than that in un-thinned plots (the control) due to
decreased interception and transpiration. The increase in SWS0–500 cm in the 100% thinned
peashrub plot (159.9–216.1 mm) was much higher than that in the 50% thinned peashrub
(39.1–169.8 mm) and the 100% thinned alfalfa (20.3–118.1 mm) plots. This suggested that
the extent of soil water recovery varied with both thinning intensity and vegetation type. At
the end of third growing season, soil water restoration frontier (minimum depth of soil water
recovery) in the thinned peashrub and alfalfa plots (>300 cm) was much higher than that in
un-thinned plots (<180 cm). It then indicated that soil water (<300 cm) can recover rapidly
following two successive wet years (with annual rainfall 42.5% higher than the long-term
average) by thinning. This should ease concerns about soil desiccation and its potential
impacts on long-term sustainability of restored ecosystems in CLP region. Further studies of
the mechanism and timescale of deep soil water restoration (>300 cm) under different
management practices (e.g., thinning, land use conversion and crop rotation) are needed to
guide sustainable ecological construction in the CLP region study area.
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Acknowledgements
This study was supported by the Second Tibetan Plateau Scientific Expedition and
Research Program (STEP, Grant No. 2019QZKK0306) and projects from the National
Natural Science Foundation of China (41601221), the Ministry of Science and Technology of
China (2016YFC0501605), Chinese Academy of Sciences (XDA23070202), the Youth
Innovation Promotion Association of Chinese Academy of Sciences (2019052), State Key
Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and
Water Conservation, CAS & MWR (A314021402-2010) and Bingwei Outstanding Young
Talent Project from the Institute of Geographical Sciences and Natural Resources Research
(2017RC203). We greatly thank the anonymous reviewers for their detailed and constructive
comments.
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923–935. doi:10.1002/2017JG004038.
Table 1 Mean (±SE) soil water storage (mm) in peashrub and alfalfa plots under different
thinning treatments at the end of each growing seasons in 2015–2017 in northern China’s
Loess Plateau region.
Note: Uppercase letters in Table 1 indicate significant differences across different thinning treatments in the same year (p < 0.05); lowercase
letters in Table 1 indicate significant differences in the same treatment across different time periods (p < 0.05).
Peashrub AlfalfaDepth
(cm)
Node
time 100% thinning 50% thinning control 100% thinning control
2015/10/25 100.90±3.28Aa 95.90±4.78Aa 85.60±2.54Ba 108.20±0.77Ca 94.30±1.4Ba
2016/10/23 146.90±6.03Ab 140.0±10.38Ab 109.40±4.87Bb 146.70±8.85Ab 140.10±2.51Ab
0-100 cm
2017/11/05 162.70±2.11Ab 163.60±4.58Ac 142.20±5.68Bc 176.70±1.37Cc 170.90±3.26ACc
2015/10/25 311.80±10.75Aa 291.50±6.31Ba 237.40±4.22Ca 330.00±0.58Da 322.60±7.49Aa
2016/10/23 413.50±2.17Ab 369.20±7.77Bb 309.20±3.97Cb 388.10±3.68Db 379.80±0.9Db
100-400 cm
2017/12/05 619.90±20.03Ac 510.00±12.41Bc 331.30±17.71Cc 516.80±22.83Bc 412.60±5.12Dc
2015/10/25 186.00±41.21Aa 90.50±1.05Ba 115.80±7.74Ca 120.20±4.09Ca 109.30±1.03Da
2016/10/23 172.10±41.92Ab 94.70±1.74Ba 117.00±8.49Ca 120.70±1.16Ca 115.30±3.24Ca
400-500 cm
2017/11/05 199.50±49.53Ac 91.70±2.68Ba 122.10±6.54Cab 126.00±4.36Cab 117.90±2.46Ca
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For Review OnlyTable 2 Soil water restoration frontier and conservation in peashrub and alfalfa plots under
different treatments, compared with natural grassland during study period in northern China’s
Loess Plateau region.
Peashrub AlfalfaNode time
100% thinning 50% thinning control 100% thinning control
2015-10-25 50 30 30 30 40
2016-10-23 240 180 40 180 220
soil water restoration frontier a
(cm)
2017-11-05 500 320 120 300 180
2015-10-25 18.5 18.9 15.5 11.9 22.6
2016-10-23 100.7 54.2 19.4 85.2 69.9
soil water conservation b
(mm)
2017-11-05 325.5 184.6 48.2 217.2 112.8
a Soil water restoration frontier refers to the first depth in the soil profile of where soil water content exceeds that in natural grassland.
b soil water conservation refers to soil water increase in the wet layers in different treatments relative to that in nature grassland.
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For Review OnlyTable 3 Contribution of precipitation to annual soil water conservation
Pgrowing a △SWS0-500cm
b ET c △SWS0-500cm:PgrowingTreatments Wet Years
(mm) (mm) (mm) (%)
2016 497 98.6 398.4 19.8100% thinned peashrub
2017 559 149.5 409.5 26.7
2016 497 92.9 404.1 18.750% thinned peashrub
2017 559 77.7 481.3 13.9
2016 497 60.8 436.2 12.2Un-thinned peashurb
2017 559 11.4 547.6 2.0
2016 497 81.9 415.1 16.5100% thinned alfalfa
2017 559 121.8 437.2 21.8
2016 497 80.1 416.9 16.1Un-thinned alfalfa
2017 559 3.6 555.4 0.7
a the precipitation during the growing season.
b the increase of soil water storage in the 0–500 cm soil profile at the end of growing season relative to the start of the season.
c the evapotranspiration calculated based on water balance.
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Fig. 1 Location of the study area in the northern China’s Loess Plateau region (a) and
schematic depiction of the experimental design (b). A1 and A2 are respectively plots of
original alfalfa (control) and total (100%) thinning of alfalfa (i.e., conversion of alfalfa to
grass for natural succession). P1, P2, and P3 are respectively plots of original peashrub
(control), 50% thinning of peashrub, and total (100%) thinning of peashrub (i.e., conversion
of peashrub to grass for natural succession). Change in vegetation cover after thinning was
measured by photographic method using unmanned aerial vehicle (DJI Phantom 4 Pro) (c).
Figure (a) was created using ArcMap version 10.5.0.
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0
100
200
300
400
500
0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20Soil water content (cm3/cm3)
Soil
dept
h (c
m)
FallowPeashrubAlfalfa
Native grasses
Fig. 2 Vertical distribution of soil water content in original peashrub and alfalfa fields before
thinning and in a nearby natural grassland on northern China’s Loess Plateau.
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For Review Only2015
-6-9
2015
-9-9
2015
-12-9
2016
-3-9
2016
-6-9
2016
-9-9
2016
-12-9
2017
-3-9
2017
-6-9
2017
-9-9
2017
-12-9
100
200
300
400
500100% thinning peashrub
100
200
300
400
500
Soil
dept
h (c
m)
Soil
dept
h (c
m)
Soil
dept
h (c
m)
Soil
dept
h (c
m)
Soil
dept
h (c
m)
50% thinning peashrub
100
200
300
400
500control peashrub
100
200
300
400
500100% thinning alfalfa
100
200
300
400
500
0.06 0.09 0.13 0.16 0.20 0.24 0.27
Soil water content (cm3/cm3)
control alfalfa
Time
0
50
100
Prec
ipita
tion
(mm
)
Precipitation
-30
-15
0
15
30
Sola
r rad
iatio
n(M
J·m
-2)
Ave
rage
Air
Tem
pret
ure
(℃)
Average Air Tempreture Solar radiation
Fig. 3 Dynamic change in soil water content in peashrub and alfalfa plots under different
thinning treatments during the experimental period on northern China’s Loess Plateau.
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0
200
400
-0.05 0.00 0.05 0.10 0.150
200
400
-0.05 0.00 0.05 0.10 0.150
200
400
-0.05 0.00 0.05 0.10 0.15
0
200
400
-0.05 0.00 0.05 0.10 0.150
200
400
-0.05 0.00 0.05 0.10 0.15
100% Thinning50% Thinning
Difference in soil water content relative to natural grassland (cm3/cm3)
Alfa
lfaSo
il de
pth
(cm
)
Difference in soil water content relative to natural grassland (cm3/cm3)
Control group
Peas
hrub
Soil
dept
h (c
m)
Control group
2015-10-252016-10-32017-12-5Initial deficit
100% Thinning
Fig. 4 Difference in soil water content in peashrub and alfalfa plots relative to that in natural
grassland at the end of the 2015–2017growing seasons in northern China’s Loess Plateau
region.
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