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Geoderma

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Changes in soil physical and chemical properties after short drought stress insemi-humid forests

Qingyin Zhanga, Mingan Shaoa,b,c, Xiaoxu Jiab,c,⁎, Xiaorong Weia

a State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, Northwest A&F University, Yangling 712100, Chinab Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographical Sciences and Natural Resource Research, Chinese Academy of Sciences, Beijing100101, Chinac College of Resources and Environment, University of Chinese Academy of Sciences, Beijing, 100190, China

A R T I C L E I N F O

Handling Editor: Junhong Bai

Keywords:Soil propertySoil moistureShort droughtAggregateBlack locustLoess plateau

A B S T R A C T

The Loess Plateau region of China is under increasing water shortage due to declining soil water content afterreforestation. Thus, assessing the changes in soils properties in reforested lands is critical for sustainable re-storation of vegetation. We conducted a continuous rainfall manipulation experiment from June 2015 toNovember 2016 using mature black locust (Robinia pseudoacacia) on southern Loess Plateau of China. The soilhydrological properties, aggregate distribution and aggregate organic carbon (OC) and total nitrogen (N) con-centrations were measured. Two years of drought stress caused a significant decline in saturated hydraulicconductivity and total porosity, but an increase bulk density in the 0–10 cm soil layer. The short drought stresssignificantly decreased soil aggregate stability due to a decline in macro-aggregate mass. Drought stress sig-nificantly decreased total soil OC concentration and macro- and micro-OC or N concentrations, but had nosignificant effect on total N concentration after two years of drought. Furthermore, the influence of short droughton soil hydrological properties, aggregate distribution and aggregate OC and total N concentrations was mainlyevident at the depth of 10 cm. Our results indicated that short drought has the potential to damage soil prop-erties. The results of this study could provide more insight into the sustainability of afforestation in semi-humidareas of China's Loess Plateau.

1. Introduction

Soil moisture is an indispensable component of the terrestrial eco-system and it is related to different hydrological processes and ecolo-gical functions in different soil layers (Deng et al., 2016; Yang et al.,2012). The north Loess Plateau region of China is under increasingwater shortage (Deng et al., 2016, 2017; Liu et al., 2015; Wang et al.,2011) due to declining soil water. During the last 30 years, dramaticclimate changes have occurred over the drylands in north China, wheresoil moisture is a significant component of the overall terrestrial waterresources (Nemani et al., 2003). Therefore, knowledge of soil propertiesand soil water processes is critical for not only effective management ofwater resources, also successful restoration of vegetation in the semi-humid area (Ge et al., 2019; Deng et al., 2016; Wu et al., 2016; Zhangand Shangguan, 2016).

Restoration of vegetation is a useful way of preventing soil de-gradation and soil loss by erosion (Clemente et al., 2004; Requena et al.,

2001). Because of numerous ecological benefits, restoration of vegeta-tion is being promoted globally (Malagnoux, 2007). This process isoften characterized by land-use change from native croplands to plan-tation farms of fast-growing exotic tree species (Qiu et al., 2010; Weiet al., 2009). For example, afforestation of croplands usually increasessoil OC and nutrient content (Hazlett et al., 2007; Qiu et al., 2012; Weiet al., 2013); thereby improving soil structure due to residue accumu-lation under planted plant canopy (Menyailo et al., 2002; Wei et al.,2013). Residue decomposition contributes immensely to soil OC andnutrient (N, P) pool through the incorporation of C and associatednutrients into underlying soils (Chen and Xu, 2005; Kong et al., 2019;Mendham et al., 2002; Zhao et al., 2019). Cao et al. (2007) observed adramatic increase in soil OC and available N in afforested lands on theLoess Plateau. Several other studies have reported changes in soilphysical properties after vegetation restoration, including change in soilbulk density (Breuer et al., 2006; Zhang et al., 2013), saturated soilhydraulic conductivity (Unger, 2001; Wu et al., 2013; Zhang et al.,

https://doi.org/10.1016/j.geoderma.2018.11.051Received 25 January 2018; Received in revised form 17 October 2018; Accepted 30 November 2018

⁎ Corresponding author at: Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographical Sciences and Natural Resource Research,Chinese Academy of Sciences, Beijing 100101, China.

E-mail address: jiaxx@igsnrr.ac.cn (X. Jia).

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2013) and soil infiltration rate (Wu et al., 2016; Zhao et al., 2019). Forexample, change in soil hydraulic conductivity (Unger, 2001) has amajor effect on infiltration (Schwartz et al., 2003) because root activityand aggregate stability improve after conversion of croplands tograsslands. Qiu et al. (2011) reported that less rainfall infiltrates deepersoil layers as the period of time after reconversion increases and thatnatural steppes have the shallowest infiltration depth (10 cm or less).However, Zhang et al. (2010) reported that soil water content increasessignificantly with time after afforestation. These differences suggestthat changes in soil properties exhibit diverse effects during successionsfollowing vegetation restoration.

However, description of the effects of afforestation on soil proper-ties should consider the effect of precipitation pulses (Mantovani et al.,2015; Yahdjian and Sala, 2010). As precipitation in arid regions usuallyoccurs as discrete events, interspersed by dry spells, soils experiencewet-dry cycles with large biological consequences (Austin et al., 2004).During dry periods, both C and N turnover slows down, which couldlead to microbial death and reduced plant metabolism (Yahdjian et al.,2006). When soils rewet, C and N mineralization is stimulated; in-creasing soil microbial biomass and reducing labile organic matter pool(Austin et al., 2004). Also, the effect of land-use change on soil C, N andchemical properties differ markedly with mean annual precipitation(Rhoades, 2007). Thus, soil water conditions could regulate the effectsof afforestation on soil properties.

Black locust (Robinia pseudoacacia) is considered a promising treefor reforestation due to its fast growth and high ability to fix atmo-spheric N, and has therefore been planted on>70,000 ha of the LoessPlateau since the 1950s (Guo et al., 2005). Black locust can greatlyimprove soil properties by enhancing soil structure, soil quality, soil OCsequestration (Ussiri et al., 2006) and soil N content (Lopez et al.,2014). Black locust is observed to induce significant changes in soilphysic-chemical, nutritional and biological properties on China's LoessPlateau (Xu and Liu, 2004; Xue et al., 2007). In the plateau region,water shortage during summer could limit growth of afforested plan-tations (Deng et al., 2016, 2017; Jia et al., 2017). Also there has beendramatic climate change over the last 30 years in the drylands in northChina, where soil moisture is a significant component of the overallterrestrial water resources (Nemani et al., 2003). Hence, to deepen ourunderstanding of soil properties under drought stress in the forestecosystem, there is the need for research into the effects of droughtstress on soil properties in the semi-humid forest lands.

We hypothesize that short droughts could reduce hydrologicalproperties and aggregate OC and total N in the 0–20 soil layer. To testour hypothesis, we conducted a short drought experiment where wetested hydrological properties and aggregate OC and total N in surfacesoil layer under mature forest on the Loess Plateau. In the experiment,we sought to assess the effects of short drought stress on selected soilproperties under afforestation with deciduous tree species (Robiniapseudoacacia) in temperate climate.

2. Materials and methods

2.1. Study area

The study was conducted at Yehe National Forestry Center of FufengCounty, Shaanxi Province, China (34.55°N, 107.90°E; 1080m a.s.l.),which is located on the southern Loess Plateau. Yehe has a temperateclimate with mean annual precipitation of 592mm, 70% of which oc-curs during the growing season from June to September. The meanannual temperature is 11.5 °C, with a range of−2 °C in January to 26 °Cin July. Forest cover in the area is 2980 ha and the soil is mainly GleyicPhaeozems (World Reference Base for Soil Resources), with followingsoil texture: 11% sand, 20% clay and 69% silt (Zhang et al., 2018),which was determined by the laser-diffraction method (Yang et al.,2015). Throughout the region, Robinia pseudoacacia (Linn.) and Pinustabuliformis (Carr.) communities dominate artificial afforestation for-ests. Stipa bungeana (Trin.) and Artemisia argyi (H.) are the dominantplants of the understory vegetation, covering some 80–90% of theground surface.

2.2. Experimental design and sampling

We selected a 10-year-old black locust stand in the south of China'sLoess Plateau following an initial reconnaissance in March 2015. A(randomized complete block design) experiment design was used, withtreatments excluding rainfall and control (Zhang et al., 2018). Withinthe selected study site, three 41× 20m area blocks were randomlyselected. The blocks were at least 500m apart and with similar soil typeand topography. The physical and chemical properties of the initial soilconditions for the 0–20 soil layers are given in Tables 1 and 2. Eachblock had two 20×20m area plots, including one control and onedrought treatment.

In November 2016, soil samples were taken at five points withineach plot, including from the four corners and the center of each site.Disturbed samples were taken for each plot at two layer increments(0–10 and 10–20 cm) using a 5.0 cm diameter auger; which sampleswere used to measure aggregate size distribution and soil OC. Visiblepieces of large roots and stones were removed. Undisturbed soil coreswere collected (3 replicates) using a soil bulk sampler with a diameterof 5.0 cm, height of 5.0 cm and a stainless steel cutting ring for each ofthe two soil layers.

Aboveground biomass (AGB) measurements for understory vegeta-tion were taken using 1m×1m quadrat once a month, for the periodfrom April to November 2016. Roots (belowground biomass, BGB) ofthe understory vegetation were extracted manually at the 0–10 and10–20 cm soil layers once a month, also for the period from April toNovember 2016. Three sampling points were randomly selected in eachplot using a 20 cm×20 cm quadrat and the collected roots washed inflowing water. All the biomass was then oven-dried at 70 °C to a

Table 1Morphological and gleyic properties of the 0–20 cm soil layer under the initial soil conditions — data adapter after Yao et al. (1992).

Soil depth Soil horizon Soil colour Soil structure Soil consistency Pedogenic forms Cation exchange capacity Base saturation pH

0–10 cm Plow layer Grayish brown Granular Loose Redeposited loess 14.7 mg/100 g 96.1% 8.410–20 cm Plow sole Grayish brown Blocky structure Compaction Redeposited loess 15.2 mg/100 g 96.7% 8.5

Table 2Physical and chemical properties of the 0–20 cm soil layer under initial soil conditions in May 2015. Values represent mean ± SE for sample size of n=3.

Soil layer (cm) Plot Macro-aggregate (%) Micro-aggregate (%) Silt+ clay (%) Total porosity (%) Ks (mmmin−1) BD (cm−3) SOC (g kg−1) TN (g kg−1)

0–10 Drought 49.8 ± 2.8 42.6 ± 4.2 7.6 ± 0.2 47.2 ± 2.9 1.21 ± 0.2 1.18 ± 0.1 14.5 ± 1.2 0.85 ± 0.1Control 48.5 ± 5.2 43.6 ± 2.5 7.9 ± 0.4 48.9 ± 3.5 1.15 ± 0.1 1.20 ± 0.1 14.9 ± 1.5 0.81 ± 0.2

10–20 Drought 48.1 ± 3.1 43.4 ± 1.8 8.5 ± 0.6 41.8 ± 4.5 0.82 ± 0.1 1.39 ± 0.2 9.13 ± 1.1 0.54 ± 0.1Control 47.9 ± 2.7 43.5 ± 3.6 8.6 ± 0.5 41.1 ± 1.9 0.87 ± 0.2 1.41 ± 0.1 8.74 ± 0.8 0.62 ± 0.1

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constant weight and weighed to determine AGB and BGB, respectively.

2.3. Rainfall and soil water content

Precipitation outside the forest canopy was measured using thetipping bucket rain gauge (CS700-L; Campbell Scientific, Logan, UT,USA), with resolution of 0.2mm. Annual precipitation (815mm) waslarger than the long-term average by ~2 SD or more in 2015, but an-nual precipitation (556mm) was smaller than the long-term average in2016 (Fig. 1). Soil volumetric water content (VWC) was measured using8 electric conduction sensors (EC-5, Decagon, USA) installed around thetrees at 10 and 20 cm depth soil layers in the control and droughttreatments during June 2015 to November 2016 (Zhang et al., 2018).

2.4. Soil physical and chemical properties

Saturated hydraulic conductivity (Ks) was determined using theconstant head method (Kanwar et al., 1989) and then oven-dried at105 °C before weighing for soil BD measurement. Soil total porosity wascalculated using soil particle density of 2.65 g cm−3 and BD.

The distribution of the aggregates by wet sieving through 0.25 and0.053mm sieves as explained by Kemper and Rosenau (1986). Here,the bulk soil samples were air-dried and sieved and fractionated into5mm, 3mm, 2mm, 1mm, 0.5 mm and 0.25mm classes. Similarly, 50 gof dry sieved aggregates were placed on a set of five stacking sieveswith 5mm, 2.0 mm, 1.0 mm, 0.5 mm and 0.25mm openings. The ag-gregates were presoaked by capillarity at 0 kpa in distilled water for30min before oscillated vertically in water for 1min at a movement ofapproximately 6 cm at 30 cycles/min, using 4 cm amplitude in a me-chanical agitator. The residue on the stacked sieves was transferred to aglass pan. The soil plus water that passed through the sieve were pouredonto 0.053mm sieve and the process repeated. The stable aggregatesthat remained on each sieve were oven-dried at 50 °C for 24 h andweighed. We mixed the>0.25mm soil aggregates to form the macro-aggregate (> 0.25mm). Therefore, we were able to derive macro-ag-gregate (> 0.25mm), micro-aggregate (0.25–0.053mm) andsilt + clay fractions (< 0.053mm). Mean weight diameter (MWD),geometric mean diameter (GMD) and fraction dimension (Dm) werecalculated using weight proportion and mean diameter of each sizeclass (Kemper and Rosenau, 1986; Tyler and Wheatcraft, 1992).

A sub-sample of air-dried, undisturbed soil from each subplot wasground and passed through a 0.25mm sieve to measure total soil OCconcentration. The OC concentrations of both the total soil and ag-gregate fractions were analyzed using a VARIO EL III CHON analyzer(Elementar, Germany) at the Testing and Analysis Center of NorthwestUniversity, China. Soil total nitrogen (N) concentrations of both thetotal soil and aggregate fractions were determined using the Kjeldahlmethod (Jackson, 1973).

2.5. Statistical analysis

The effect of drought on the physical and chemical properties of thesoils was evaluated using one-way ANOVA, followed by least significantdifference tests for each soil layer (p < 0.05). A correlation matrix wasbuilt to identify the relationships among soil macro-aggregate content,micro-aggregate content, silt + clay content, total porosity, BD, Ks, SOCand TN. All statistical analyses were done in SPSS ver. 18.0 (SPSS Inc.,Chicago, IL, USA).

3. Results

3.1. Bulk density, total porosity and hydraulic conductivity

Bulk density, total porosity and hydraulic conductivity of the soilchanged significantly under drought stress (Table 3). In the surface soillayer (0–10 cm), bulk density decreased from 1.29 g cm−3 to1.18 g cm−3 after two years of drought, which was significant atp < 0.05. Total porosity and saturated hydraulic conductivity weresignificantly lower under drought treatment than the control for the0–10 cm soil layer (p < 0.05). No significant changes were observed inbulk density, total porosity and hydraulic conductivity in the under-lying soil layers. Also, there were significant differences between the0–10 cm and 10–20 cm soil layers for the three soil hydrologicalproperties (p < 0.05). The results illustrated that drought stress dete-riorated soil hydrological properties. While bulk density increased, totalporosity and hydraulic conductivity decreased under drought stress.

3.2. Aggregate distribution and stability

The water-stable aggregate size distribution of the forest soil was

Fig. 1. Temporal variations in measured values of precipitation at the study site (a) and in the 0–1m soil volumetric water content (b) under drought stress andcontrol treatments in June 2015 to November 2016. The two gray dotted lines indicate measurement periods of initial and 2-year drought, respectively.

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dominated by macro-aggregate and micro-aggregate fractions under inthe treatments and layers (Fig. 2). For the 0–10 cm soil layer, theamount of macro-aggregate fraction under drought stress was sig-nificantly lower than in the control. However, the amount of the micro-aggregate fraction under drought stress was significantly higher than inthe control (p < 0.05, Fig. 2a). Silt+ clay fraction did not changesignificantly with drought stress. Unlike the 0–10 cm soil layer, therewere no significant effects of drought stress on aggregate fraction in the10–20 cm soil layer (Fig. 2b).

MWD of soil aggregates in the 0–10 cm soil layer under controltreatment was significantly higher than under drought stress(p < 0.05), but not in the 10–20 cm soil layer (Fig. 3a). Meanwhile,GMD and Dm of soil aggregates were not significantly affected by

drought stress for both the two soil layers (Fig. 3b and c).

3.3. Aggregate OC, total N and C/N ratio

In general, the concentration of aggregate OC and N was higherunder the control than under drought stress treatment (Figs. 4 and 5). Inthe surface layer, the aggregate OC decreased with decreasing ag-gregate size as follows: macro-aggregates≈micro-aggregates >silt+ clay. Total N concentration followed the same trend, which wassimilar for the deeper soil layers. In the 0–5 cm soil layer, OC con-centration was significantly lower under drought stress for macro- andmicro-aggregate classes; which was by 25% for macro-aggregates and22% for micro-aggregates. However, drought stress had no significant

Table 3Selected soil physical and chemical properties for the 0–20 cm soil layer after 2-year drought stress. Ks is saturated hydraulic conductivity; BD is soil bulk density;SOC is soil organic carbon; TN is total nitrogen. Values represent mean ± SE for sample size of n= 3. Different lowercase letters represent significant differencesbetween soil layers under the same treatment, and different capital letters represent significant differences between treatments at the same soil layer (p < 0.05).

Soil layer (cm) Plots Total porosity (%) Ks (mmmin−1) BD (cm−3) SOC (g kg−1) TN (g kg−1)

0–10 Drought 40.6 ± 2.1Ba 0.88 ± 1.3Ba 1.29 ± 0.1Aa 12.8 ± 0.9Ba 0.71 ± 0.1AaControl 48.1 ± 1.9Aa 1.26 ± 1.1Aa 1.18 ± 0.1Bb 15.4 ± 0.8Aa 0.78 ± 0.2Aa

10–20 Drought 40.5 ± 2.7Aa 0.83 ± 0.3Aa 1.41 ± 0.2Aa 9.30 ± 1.3Ab 0.55 ± 0.1AbControl 42.7 ± 2.7Ab 0.90 ± 0.5Ab 1.36 ± 0.1Aa 8.92 ± 0.7Ab 0.50 ± 0.1Ab

Fig. 2. Effect of drought on aggregate size distribution at (a)the 0–10 cm and (b) the 10–20 cm soil layers. Size fractionsare macro-aggregate (> 0.25mm), micro-aggregate(0.25–0.053mm) and silt + clay (< 0.053mm). Error barsare standard error of the mean. Different lowercase lettersindicate significant differences between treatments withinaggregate-size class (p < 0.05).

Fig. 3. Effect of drought stress on (a) mean weight diameter (MWD); (b) geometric mean diameter (GMD); and (c) soil fractal dimension (Dm) of wet-sieved soil. Errorbars are standard error of the mean. Different lowercase letters indicate significant differences between treatments within each soil layer (p < 0.05).

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effect on OC of silt + clay (Fig. 4).The aggregate N had a similar trend (Fig. 5). In the 0–10 cm soil

layer, aggregate-based N decreased under drought tress for macro- andmicro-aggregate classes (which was 19% less for macro-aggregates and18% less for micro-aggregates). In the 10–20 cm soil layer, there was nosignificant difference in soil N concentration of each aggregate fraction

between the control and drought stress treatment. To a much less de-gree, drought stress influenced aggregate C/N ratio than the control(Fig. 6).

For the 0–10 cm soil layer, total soil OC concentration was sig-nificantly lower under drought stress than the control (Table 3). In the10–20 cm soil layer, there was no significant difference in total soil OC

Fig. 4. Effect of drought stress on aggregate OC concentra-tion at (a) the 0–10 cm and (b) the 10–20 cm soil layers. Sizefractions are macro-aggregate (> 0.25mm), micro-ag-gregate (0.25–0.053mm) and silt + clay (< 0.053mm).Error bars are standard error of the mean. Different lower-case letters indicate significant differences between treat-ments within aggregate-size class (p < 0.05).

Fig. 5. Effect of drought stress on aggregate N concentrationat (a) the 0–10 cm and (b) the 10–20 cm soil layers. Sizefractions are macro-aggregate (> 0.25mm), micro-ag-gregate (0.25–0.053mm) and silt+ clay (< 0.053mm).Error bars are standard error of the mean. Different lower-case letters indicate significant differences between treat-ments within aggregate-size class (p < 0.05).

Fig. 6. Effect of drought stress on aggregate C:N ratio at (a)the 0–10 cm and (b) the 10–20 cm soil layers. Size fractionsare macro-aggregate (> 0.25mm), micro-aggregate(0.25–0.053mm) and silt+ clay (< 0.053mm). Error barsare standard error of the mean. Different lowercase lettersindicate significant differences between treatments withinaggregate-size class (p < 0.05).

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concentration between the treatments. Total soil N concentration didnot significantly change after two years of drought stress in both soillayers (Table 3). The relationships among the variables showed that OCand N concentrations were both positively correlated with soil hydro-logical properties such as total porosity, Ks and BD (Table 4).

4. Discussion

4.1. Changes in soil hydrological properties

Knowledge on soil hydrological properties is critical for under-standing soil water movement and predicting soil parameters that affectagronomic and environmental operations in the region (Zhang et al.,2006). Our results showed that two years of drought stress had a ne-gative effect on the investigated soil hydrological properties —e.g., itreduced total porosity and hydraulic conductivity but increased bulkdensity in the upper soil layer (Table 3). In our study, drought stressincreased soil bulk density in the 0–10 cm soil layer, which was at-tributed to the lower fine root biomass and porosity of the surface soillayer (Fig. 7 and Table 3). This agreed with previous a study where itwas reported that heavy pedestrian traffic and drought stress negativelyaffect surface soil properties (Yuksek, 2009). For the 10–20 cm soillayer, there was no significant difference in bulk density after two yearsof drought stress, suggesting that short drought had no clear effect onbulk density in deeper soil layers. Generally, mean hydraulic con-ductivity for all the soil samples was significantly higher than reportedvalues (0.05–0.58mmmin−1) for soils in the central and southern re-gions of the Loess Plateau; attributed largely to the differences in soil

parent materials in the regions (Li and Shao, 2006; Wang et al., 2008).Hydraulic conductivity for the control soil in the 0–10 cm soil layer wassignificantly higher than under drought stress (p < 0.05). However,the differences in hydraulic conductivity were not significant for the10–20 cm soil layer (Table 3). This suggested that drought stress limitedhydraulic conductivity in the surface layer. Knowledge of temporalvariability total porosity is fundamental for an accurate understandingof soil processes during vegetation restoration (Bodner et al., 2013). Inour study, total porosity decreased in the 0–10 cm soil layer after twoyears of drought stress (Table 3). The main reason for the decrease inporosity was attributed to the less amount of aboveground biomass inthe soil after the start of drought stress (Fig. 7). This implied that soilorganic matter decreased during this period, in turn reducing soilstructure by compaction and less pore spaces (Wu et al., 2016).

Based on the changes in plant communities (Fig. 7) in this study, thetwo years of drought generally affected the soil properties (Aponteet al., 2010). This could have been driven by changes in organic matterinput quality (Grady and Hart, 2006); suggesting that soil propertieschange in the long run due to changes in plant communities. However,Curiel-Yuste et al. (2014) found no chronic effect of drought on soilmicrobial properties and therefore suggested reductions in rainfall by,for example, 15%. The results can also be explained by the effect ofseasonality because season is a critical factor that generally affects soilproperties, including enzymatic activities or microbial biomass (Bastidaet al., 2008; Lucas-Borja et al., 2012). Soils in the study area weresampled during early winter season, when variations in soil propertieswere about the average values. Nevertheless, the effect of seasonneeded further studies in future research.

4.2. Aggregate size distribution and stability

In our soils, macro-aggregates and micro-aggregates dominatedwater-stable aggregate size distribution in the 0–20 cm soil layers. Thisagreed well with other studies on forestlands on the Loess Plateau(Zhang et al., 2018; Zhu et al., 2017), where silt+ clay fractions werenot always dominant. This was in contrast with cropland soils on theLoess Plateau, where there was usually high degree of micro-aggrega-tion (86%) due to intensive tillage practices (Wei et al., 2013).

The effect of drought stress on aggregate size distribution and sta-bility was evident up to the depth of 10 cm (Figs. 2 and 3). Droughtstress decreased the proportion of macro-aggregates, but increasedmicro-aggregates in the 0–10 cm soil layer, which resulted in 21% re-duction in MWD. The proportion of micro-aggregates increased mainlyat the expense of macro-aggregates. The negative impact of droughtstress on macro-aggregation has been noted in several other studies insemi-arid (Zhang et al., 2012) and sub-tropical (Yao, 2005) soils. Song(2010) reported that macro-aggregate fraction is negatively related todrought degree or drought intensity. Soil aggregate stability indices(MWD) decreases with decreasing macro-aggregate fraction and initialdecline in micro-aggregate fraction (Kabiri et al., 2015). Also poor

Table 4Correlation matrix for macro-aggregate content, micro-aggregate content, silt+ clay content, total porosity, soil bulk density (BD), soil saturated hydraulic con-ductivity (Ks), soil organic carbon (SOC), and total nitrogen (TN).

Variable Macro Micro Clay+ silt Total porosity BD Ks SOC TN

Macro 1Micro −0.79⁎⁎ 1Clay+ silt 0.18 0.24 1Total porosity 0.58⁎⁎ −0.53⁎⁎ 0.15 1BD −0.51⁎⁎ 0.64⁎⁎ −0.08 −0.57⁎⁎ 1Ks 0.48⁎ −0.53⁎⁎ 0.14 0.76⁎⁎ −0.68⁎⁎ 1SOC 0.29 0.18 −0.11 0.49⁎ 0.64⁎⁎ 0.52⁎⁎ 1TN 0.15 0.08 −0.21 0.57⁎⁎ 0.53⁎⁎ 0.67⁎⁎ 0.81⁎⁎ 1

⁎ Indicates statistical significance at p < 0.05.⁎⁎ Indicates statistical significance at p < 0.01.

Fig. 7. Changes in understory vegetation biomass (aboveground and below-ground) after 2-year drought stress in May to November 2016.

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growth of the understory vegetation hindered the development of fungiand other microbes (Ye et al., 2013), preventing the plant form devel-oping root network (Fig. 7) and limiting earthworm population(Mackay and Kladivko, 1985); both of which positively affect soil ag-gregation.

4.3. Effect of drought on aggregate OC and N

In both the control and drought stress treatments, we found thatmacro-aggregate and micro-aggregate OC and N concentration domi-nated the soil OC and N concentration, and were greater than that ofsilt + clay fractions at any soil layer (Figs. 4 and 5). Generally, macro-aggregates contained higher soil OC and N than micro-aggregates inboth treatments. This matched with aggregate hierarchy concept wheremicro-aggregates bind together into macro-aggregates by transienttemporary binding agents (Tisdall and Oades, 1982). This aggregatehierarchy, along with the important role of organic matter in soil ag-gregation, has been observed in temperate forestlands (Kabiri et al.,2015).

The hypothesis was confirmed that short drought stress had sig-nificant effect on soil OC and N concentration in macro-aggregates andmicro-aggregates (Figs. 4 and 5). The negative effect of drought stresson aggregate OC and N agreed with the results form temperate forests(Wright and Hons, 2005). This confirmed that changes in macro-ag-gregate or micro-aggregate OC and N can be used as indicators for landmanagement effects on soil (Six et al., 2002). In this study, two years ofdrought stress significantly reduced aboveground and belowgroundbiomass of understory vegetation (Fig. 7). Generally, nutrient transferbetween soils and plants can stabilize C in days to weeks (He et al.,2006) because of shedding of fresh plant litter on the soil surface andresidue input by understory vegetation (Gale et al., 2000). Conse-quently, the two years of drought stress significantly limited soil ag-gregate OC and N concentration in the 0–10 cm soil layer. Furthermore,drought stress reduced earthworm population and arbuscular mycor-rhizal fungi activity due to decreasing bulk density. This was attributedto enhanced macro-aggregate and micro-aggregate formation and Cstabilization (Gómez-Paccard et al., 2015). We also observed that the10–20 cm soil layer contained smaller proportion of aggregate OC andN than the 0–10 cm soil layer (Figs. 4 and 5). One explanation for this isthat there was higher fine root biomass in the 0–10 cm soil layer (Fig. 7,Cheng et al., 2006). This resulted in greater mass of organic matterincorporated into the surface soil. This result was consistent with ob-servations by Six and Jastrow (2002) who noted that OC concentrationwas more stable in the 10–20 cm than in the 0–10 cm soil layer despiteshort spell of drought stress.

5. Conclusions

Our results showed that short spell of drought stress had a sig-nificant effect on soil physical and chemical properties. Two years ofdrought stress significantly decreased saturated hydraulic conductivityand total porosity, but increased bulk density in the 0–10 cm soil layer.This was probably due to lower fine root biomass and residue input byunderstory vegetation in the surface layer. Drought stress also de-creased macro-aggregate mass, but increased micro-aggregate mass inthe surface layer (0–10 cm); resulting in a decline in soil aggregatestability. Drought stress significantly limited total soil OC concentrationand macro- and micro-aggregate OC or N concentration, but had nosignificant effect on total N concentration in the 2-year experiment. Inaddition, the effect of a short spell of drought stress on soil physical andchemical properties was evident down to 10 cm depth of the soil. Theresults suggested that short-term changes in external environmentcould affect soil hydrological properties and aggregate OC and N, ratherthan total soil N concentration in afforested areas of China's LoessPlateau.

Acknowledgements

This study was supported by the National Natural ScienceFoundation of China (41390461 and 41530854), the InternationalPartnership Program of Chinese Academy of Sciences(161461KYSB20170013), the National Key Project for Research andDevelopment (2016YFC0501605), the Youth Innovation PromotionAssociation of Chinese Academy of Sciences (2017076), and the OpenResearch Fund of State Key Laboratory of Soil Erosion and DrylandFarming on the Loess Plateau (A314021402-1806). We thank the editorand reviewers for the insightful comments and suggestions on thispaper.

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