for review only · for review only abstract: to evaluate the potential of soil water recovery after...

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)

https://mc.manuscriptcentral.com/cjss-pubs

Canadian Journal of Soil Science

For Review Only

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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mailto:[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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http://www.nmic.cn/

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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

Page 15 of 31



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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.

Page 31 of 31



for review only · for review only abstract: to evaluate the potential of soil water recovery after...

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