Under climate change, there is a drastic increase in the frequency of drought events, which significantly impacts the biogeochemical processes in regional soil. However, the specific patterns of how different drought intensities affect the physicochemical and biological properties of soil remain unclear. Therefore, this study is based on the utilization of meta-analysis to investigate the impact patterns of drought, light drought, and extreme drought on crucial physicochemical and biological properties of soil under different conditions (soil depth, rhizosphere, experimental conditions, and land use types). The results indicate that: (1) During extreme drought, the variation in soil water content follows the pattern: surface (0-15cm) soil > shallow (15-45cm) soil, rhizosphere > non-rhizosphere, potted soil > field soil, cropland > grassland > forest. On the other hand, during light drought, the pattern is: surface (0-15cm) soil < shallow (15-45cm) soil, rhizosphere < non-rhizosphere, potted soil < field soil, cropland < forest < grassland. (2) Soil C:N ratio decreases during light drought, while soil AP (available phosphorus) and BG (β-1,4 glucosidase activity) activities decrease or are reduced during extreme drought. In terms of soil chemical properties under different conditions, extreme drought increases pH and MBC:MBN in shallow soil, non-rhizosphere, field soil, grassland, and forestland. Light drought decreases C:N ratio and BG activity in shallow soil, non-rhizosphere, field soil, and forest. (3) Extreme drought and light drought significantly decrease F:B (Fungal:Bacteria) ratio in shallow soil (15-45cm) by 119.6% and 217.4% respectively. They also increase F:B ratio in forestland and non-rhizosphere soil by 161.5% and 91.5% in the case of forestland, and 161.4% and 86.5% in the case of non-rhizosphere soil. Light drought significantly increases soil microbial diversity (Shannon index increase by 2.7%), while extreme drought significantly decreases soil microbial diversity (Shannon index decrease by 7.6%). This study contributes to the understanding of the impact of extreme drought and other drought events on regional soil ecosystems and provides scientific support for identifying the mechanisms through which extreme hydrological events affect soil biogeochemical processes. These findings are important for soil management and ecological conservation.
Article
Effects on major edaphic properties across various contexts: light drought vs. extreme drought
https://doi.org/10.21203/rs.3.rs-3771026/v1
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drought
soil physicochemical and biological properties
soil and land conditions
meta-analysis
Amid the escalating impacts of climate change, the frequency of drought events has surged, as evidenced by (Xu et al., 2019), leading to reduced crop yields (Farooq et al., 2009; Leng and Hall, 2019) and exacerbating the food security crisis(Prasad et al., 2011). Predictions indicate that arid regions could expand from 45% to a staggering 68% of Earth's terrestrial surface by the century's conclusion (Huang et al., 2016). Drought impacts soil physicochemical and biological properties, reducing aboveground and belowground biomass, and affecting carbon sequestration processes in terrestrial ecosystems(Kang et al., 2018; Wu et al., 2018; Zhang et al., 2020). Vegetation growth processes also falter under drought's influence (Fauset et al., 2012; Gupta et al., 2020; Hulshof et al., 2013; Otieno et al., 2010). Furthermore, soil microorganisms, as a vital component of soil biota, play an important role in the transformation of soil nutrients and elements, closely linked to soil physicochemical properties(Mager and Thomas, 2011; Nielsen et al., 2011; Peng et al., 2021), which have a keen ability to recognize changes in the soil microenvironment(Ferran Garcia-Pichel et al., 2013), They play a crucial role in influencing and maintaining biogeochemical cycles and terrestrial ecosystem stability.
Scrutiny of existing literature reveals that drought conditions precipitate marked alterations in soil’s physical, chemical, and biological make-up. Parameters such as soil water content (SWC), the carbon to nitrogen (C:N) ratio and available phosphorus (AP) all undergo a reduction (Dominguez et al., 2017; Monokrousos et al., 2020; Olatunji et al., 2018). Soil nitrogen cycling and plant uptake of nitrogen are both affected by drought(Borken and Matzner, 2009; Leyrer et al., 2022). Additionally, soil carbon and nitrogen, being essential elements for plant growth, are sensitive to changes in soil moisture (Wang et al., 2017). Therefore, numerous researchers have studied the changes in soil carbon and nitrogen cycling under drought conditions(Sardans et al., 2008; Sun et al., 2020; Wu et al., 2012). From the perspective of soil biological properties, drought significantly reduces total microbial biomass and bacterial abundance in the soil, wherease the F:B ratio increases markedly (de Vries et al., 2018; Hu et al., 2019). Soil microorganisms have the ability to enhance their adaptability to cope with environmental changes (Leyrer et al., 2022; Schimel et al., 2007). Under extreme drought conditions, resilient soil microorganisms can survive or even grow rapidly, while less-adapted microorganisms may die off (Allison and Martiny, 2008; Schimel et al., 2007; Yang et al., 2021). Thus, extreme drought reshapes the soil’s physicochemical landscape and disrupts microbial community structures (Gao et al., 2021; Hueso et al., 2012; Parker and Schimel, 2011).
Despite extensive research, the specific response patterns of soil physical, chemical, and biological properties to extreme drought and other drought events remain obscure. To address this research gap, based on meta-analysis, we investigated effects of drought conditions, especially light drought and extreme drought, on soil physicochemical and biological properties. Meanwhile, we analyzed characteristics of key soil physicochemical and biological properties in various soil depths, rhizospheric interactions, experimental setups, and types of land use. Our research underpins the quest to elucidate the mechanisms through which extreme hydrological events influence soil biogeochemical processes. Accordingly, our research aims to: (a) Investigate changes in soil’s physical properties, such as SWC, along with chemical attributes including the carbon to nitrogen (C:N) ratio, available phosphorus (AP), soil pH, β-1,4-glucosidase activity (BG), and MBC:MBN ratio, and biological properties like the fungal to bacterial (F:B) ratio and soil microbial diversity index (Shannon diversity index) under varying drought conditions. (b) Identify the differential responses of soil physicochemical properties to drought intensities across soil layers, regions (rhizosphere vs. non-rhizosphere), experimental methodologies (pot experiments vs. field studies), and land use categories (cropland, forests, and grasslands). (c) Clarify the interrelations among soil’s physical, chemical, and biological properties when subjected to differing levels of drought stress.
2.1 Data collection
Leveraging the key words "drought", "water stress", "water reduction" and "soil physical and chemical properties", "soil water", "soil microorganisms", we embarked on a thorough literature review published from 2000 to 2021 in Web of Science and China National Knowledge Infrastructure. The corresponding data selection criteria are as follows: (1) Inclusion of studies under drought conditions, especially light drought and extreme drought. (2) Selection of studies containing the mean (MD), standard (SD) deviation or error (SE) of the experimental and control groups as well as the sample size. For those that provide standard errors, they can be converted to SD using the formula SD = SE\(\surd \text{n}\). (3) Focus on studies examining soil physical and chemical properties or soil biological properties. (4) Considerations of soil depth at 0–45 cm, similarity of geographical conditions (avoiding as much as possible the influence of external conditions such as vegetation and climate) between drought and control experiments. If drought intensity was not specified in the study, it was generally considered as only drought in our study. If there were multiple sets of experimental data in the study, we just used mean value of these data as input data. If soil physicochemical data missed SD, one- tenth of its mean value (Rose et al., 2014) was used as the SD. Based on these criteria, we collected 720 observations from 81 published papers to explore effects of drought conditions (especially light and extreme drought) on 8 soil physical, chemical and biological properties across different contexts. The process screening is as shown in Fig. 1.
Out of 81 papers, 18 and 26 research publications were carried out in light drought and extreme drought, respectively. The following criteria can be used to illustrated light drought and extreme drought: (1) A field water holding capacity of 65 ± 5% was classified as light drought, and 25 ± 5% was classified as extreme drought. (2) A 50% rainfall reduction was for extreme drought and 25% rainfall reduction was for light drought. (3) The SWC of 60–75% was used to define light drought, and SWC of 20% was used to define extreme drought. If the article directly states the intensity of drought, the author's criteria for classifying the intensity of drought are directly adopted. If the differentiation was not feasible, we grouped it as drought episodes. There, drought conditions included light drought, extreme drought and others. In other words, 81 papers were considered eligible for the analyzing impact of drought conditions on soil properties.
In addition, we set the following criteria for classified data. 1. We divided into topsoil (0-15cm) and subsoil (15-45cm) along soil depth. Topsoil was clearly defined as soil layers of 0-5cm, 0-10cm, 0-20cm and other top soil layer determined. The other soil depths were classified as subsoil. 2. Soil were collected from rhizospheric and bulk soil (non-rhizospheric) among all experiments. 3. The experimental settings included pot and field treatments. 4. The three major land use types were forest, grassland and cropland.
2.2 Data Analysis
The GetData Graph Digitizer software (version 2.26, Russia) was utilized for extracting graphical data from the literature, and OriginPro 8.5 (OriginLab Corporation, Massachusetts, USA) was employed to construct forest plots.
Data were analyzed using the random-effect model in MetaWin 2.1(Rustad et al., 2001). The log response ratio (ln RR) (Hedges et al., 1999) was used to show the effect of different drought intensities on soil physicochemical and biological properties. And we formulated the following ln RR:
where Xe and Xc are the means of a target variable in the experimental treatment and control, respectively. RR with values > 1 shows positive effects of drought conditions on the studied properties and negative effects if values are < 1.
The corresponding variance of lnRR (v) was calculated as follows:
\(\nu {\text{=}}\frac{{{{\left( {{S_{\text{e}}}} \right)}^2}}}{{{N_{\text{e}}}{{({X_e})}^2}}}+\frac{{{{\left( {{S_c}} \right)}^2}}}{{{N_c}{{({X_c})}^2}}}\)
where Se and Sc are the SD of a studied variable in the experimental and control groups, respectively, and Ne and Nc are the replicate sizes of studied variable in the experimental and control groups, respectively.
The average reponse ratio (lnRR+) was calculated as follows:
\(\ln R{R_+}=\frac{{\sum\nolimits_{{i=1}}^{k} {\ln R{R_i}*{\varpi _i}} }}{{\sum\nolimits_{{i=1}}^{k} {{\varpi _i}} }}\)
where lnRRi and ωi are the effect values and weights of i studied variable, respectively. The weight (ωi) was calculated as the reciprocal of the variance (1/VlnRR).
\(95\% CI=\ln {\text{RR}} \pm 1.96\sqrt v\)
Those 95% CIs without overlap with zero indicated a significance level of P < 0.05.
3.1 General Variations of key soil properties to drought episodes
The key soil physicochemical and biological variables exhibited notable differences under conditions of drought (Fig. 2).The variations of the SWC, soil pH, MBC:MBN ratio, and F:B ratio were most pronounced during droughts. Droughts precipitated a notable reduction in the SWC, and a significant augment in soil pH, MBC:MBN ratio and F:B ratio. The SWC reduced by 21.5% under drought conditions, and by 12.6% and 28.8% between light and extreme drought, respectively. Droughts elevated soil pH, MBC:MBN ratio and F:B ratio by 1.5%, 12.5% and 25.9%, respectively. Whereas both light and extreme drought significantly increased soil pH, MBC:MBN ratio and F:B ratio by 1.8% and 3.6%, 11.0%, and 26.9%, and 25.5% and 108.2%, respectively. Soil AP, C:N ratio, BG activity, and Shannon diversity index varied differently in various drought treatments. The soil C:N ratio and AP did not change significantly in the treatment of drought, but BG activity and Shannon diversity index significantly decreased by 11.1% and 3.9%, respectively. Light drought decreased soil C:N ratio (6.8%) and significantly increased Shannon diversity index (2.7%), but had no prominent impact on BG activity and AP. The Shannon diversity index (13%), BG activity (10.2%) and AP (9.8%) experienced a clear downturn during periods of extreme drought, although C:N ratio altered tinily.
3.2 Variation of key edaphic properties in different soil layers
3.2.1 Dynamics of SWC under drought conditions
Drought events unequivocally reduced SWC across both topsoil (0-15cm) and the subsoil (15-45cm). SWC was diminished by 23.3% in the topsoil and by 20.5% in the subsoil (Fig. 3(a)). Moreover, light drought and extreme drought had various effects on SWC in two soil layers. The effect of light drought was relatively slight. Topsoil and subsoil SWC attenuated by 10.7% and 12.7%, respectively (Fig. 3(a)). However, extreme drought exerted the most substantial impact, diminishing soil water content by 29.3% and 26.9% in the topsoil and 15–45 cm soil layer, respectively (Fig. 3(a)). These findings indicated that extreme drought had a relatively greater impact on topsoil SWC compared to the subsoil, whereas light drought had a relatively greater impact on the subsoil SWC compared to topsoil.
3.2.2 Major chemical properties variations in response to droughts
Compared to the 0-15cm soil layer, the response of soil C:N ratio, pH and MBC:MBN ratio to droughts were more pronounced in the 15-45cm soil layer. Specifically, MBC:MBN ratio was more responsive to drought events. Overall, extreme drought had a stronger effect on key soil chemical properties than light drought. In the 0-15cm soil layer, no significant changes were detected in soil C:N ratio (Fig. 3(b)) and soil pH (Fig. 3(c)), since MBC:MBN ratio showed a various increase of 9.6% (Fig. 3(d)). Under light drought, soil C:N ratio surprisingly decreased by 4.6% (Fig. 3(b)), meanwhile soil pH did not show interesting changes (Fig. 3(c)), and MBC:MBN ratio significantly enhanced by 7.9% (Fig. 3(d)). Under extreme drought, soil C:N ratio and soil pH varied unnotably, whereas MBC:MBN ratio remarkably increased by 26.5% (Fig. 3(d)).
In the 15-45cm soil layer, significant alterations were observed under general drought conditions in soil C:N ratio, soil pH, MBC:MBN ratio, and BG activity. Specifically, soil C:N ratio notably diminished by 8.8% (Fig. 3(b)). Soil pH significantly augmented by 6.5% (Fig. 3(c)). MBC:MBN ratio were found to enhance significantly by 24.5% (Fig. 3(d)), and BG activity decreased by 18.6% (Fig. 3(e)). Light droughts precipitated a reduction in soil C:N ratio (-11.1%) (Fig. 3(b)). While it elevated soil pH and MBC:MBN ratio by 4.9% (Fig. 3(c)) and 17.5% (Fig. 3(d)), respectively. Under extreme drought, soil C:N ratio, soil pH and MBC:MBN ratio showed a same tendency, soil C:N ratio significantly decreased by 15.6% (Fig. 3(b)), soil pH significantly increased by 10.2% (Fig. 3(c)), and MBC:MBN ratio significantly increased by 32.4% (Fig. 3(d)). BG activity did not experience remarkably variations under light drought or extreme drought conditions.
3.3.3 Biological properties varied during drought episodes
Compared to the topsoil, drought events significantly attenuated the F:B ratio in the subsoil, with these changes being especially significant under extreme drought conditions. In the topsoil, drought intensities did not have a significant impact on the F:B ratio (Fig. 3(f)). However, in the subsoil, drought had a primary effect on the F:B ratio. Specifically, the F:B ratio experienced a downturn by 142.3% (Fig. 3(f)). Light drought and extreme drought provided valuable a detrimental effect, with a notable 119.6% and 217.4% decrese, respectively (Fig. 3(f)).
3.3 Soil properties variance in the rhizosphere vs. non-rhizosphere of drought duration
3.3.1 Changes in SWC in rhizosphere and bulk soil
SWC exhibited notable fluctuations in the rhizosphere and non-rhizosphere soil during drought periods. Notably, extreme drought conditions had a more substantial impact on SWC in the rhizosphere than in the non-rhizosphere, nevertheless light drought appeared to affect the non-rhizosphere SWC more. All categories of drought led to a reduction in SWC in both the rhizosphere and non-rhizosphere. In the rhizosphere soil, SWC decreased by 52.7% in extreme drought, by 4.5% in light drought, and by 29.8% in drought (Fig. 4 (a)). In the non-rhizosphere soil, SWC decreased by 26.5% in extreme drought, by 12.4% in light drought, and by 21.2% in drought (Fig. 4 (a)).
3.3.2 Characteristics of key chemical properties
Drought events did not significantly impact soil AP content in either the rhizosphere or bulk soil. However, other chemical properties such as soil pH, MBC:MBN ratio, and BG activity were more noticeably affected by drought. In the rhizosphere, soil pH, MBC:MBN ratio, and AP remained largely unchanged under drought and light drought. The exception was BG activity, which saw an increase of 29.7% under drought and 25.0% under light drought (Fig. 4 (e)). In the non-rhizosphere, soil pH and MBC:MBN ratio significantly increased by 1.6% (Fig. 4(c)) and 19.7% (Fig. 4(d)), respectively. BG activity decreased by 13.2% (Fig. 4(e)). Light drought led to no significant variations in soil AP (Fig. 4(g)), while to mitigate the C:N ratio by 10.4% (Fig. 4(b)) and BG activity by 13.5% (Fig. 4(e)). Also, it ameliorated soil pH level by 2.2% (Fig. 4(c)) and MBC:MBN ratio by 20.2% (Fig. 4(d)).
3.3.3 Responses of selected biological properties
The response of soil biological properties to drought events varied significantly between the rhizosphere and non-rhizosphere soil. For the rhizosphere, light drought had a greater impact on F:B ratio compared to extreme drought. However, for the non-rhizosphere, extreme drought had a greater impact on F:B ratio compared to light drought. Furthermore, changes in soil microbial diversity in response to drought were more distinct in the non-rhizosphere.
In the rhizosphere, F:B ratio significantly decreased by 31.1% under drought conditions (Fig. 4(f)), while the microbial Shannon diversity index did not show various dynamics (Fig. 4(h)). Under light drought, there was a significant decrease of 19.3% in F:B ratio (Fig. 4(f)), and no significance under extreme drought. In the bulk soil, drought conditions varied the soil F:B ratio and microbial Shannon diversity index. There was a restraint in the soil Shannon diversity index (-7.6%) (Fig. 4(h)) and a ameliorate in the soil F:B (+ 43.3%) (Fig. 4(f)). Light drought and extreme significantly enhanced F:B ratio by 86.5% and 161.4% (Fig. 4(f), respectively.
3.4 Drought-induced impacts of experimental environments on edaphic properties
In field experiments, drought had a suppressive effect on SWC, BG activity, and AP, but it had a promoting effect on soil pH and MBC:MBN ratio. In controlled potted environments, positive variations were more pronounced in SWC and MBC:MBN ratio, but BG activity changes were reversed. Whatever was potted environments or open field, SWC patterns always revealed a reduction under both light drought and extreme drought. But SWC showed more fluctuations with extreme drought (a reduction of 26.5% in open field and 52.7% in potted conditions) than light drought (a reduction of 12.4% in open field and 4.5% in potted conditions) (Fig. 5(a)).
Light drought and extreme drought had reversed effects on soil C:N ratio. Light drought caused soil C:N ratio significant decrease (-10.4%) in open field, and extreme drought had no remarkable influences (Fig. 5(b)). Extreme drought reduced it notably by 23.5% in potted conditions, and light drought did not affect it (Fig. 5(b)).
In field experiments, light drought and extreme drought enhanced soil pH by 5.0% and 9.3% (Fig. 5(c)), respectively. In pot experiments, light drought significantly mitigated soil pH by 1.7% (Fig. 5(c)), while extreme drought remarkable augmented it by 3.1% (Fig. 5(c)). Drought led to soil MBC:MBN ratio increase (+ 15.8%) in open field and decrease (-18.9%) in potted conditions (Fig. 5(d)). Under extreme drought, this variation was pronounced in open field with significant increasing by 34.7% and in potted environment with notable decreasing by 17.3% (Fig. 5(d)). For soil AP, it experienced a reduction in open field experiments by10.4% (Fig. 5(g)). And extreme drought led soil AP to reduce by 9.7% in field conditions (Fig. 5(g)). In contrasts, during lighter droughts, its variations were reversed in control potted environments with increasing by 18.2% (Fig. 5(g)). Drought had a significant suppressive on soil BG activity in open field experiments, with a decrease of 13.1% (Fig. 5(e)). It experienced a downturn (-13.5%) during periods of light drought. However, light drought led to a various increase (+ 25%) in soil BG activity within control potted experiments (Fig. 5(e)).
In open field, drought promoted the soil F:B ratio by 21.9% (Fig. 5(f)). On the contrary, it diminished by 14.7% in control potted conditions (Fig. 5(f)). Light drought had a positive effect on soil F:B in open field, evidenced by a 28.0% rise, and it had no influence in potted environment (Fig. 5(f)).
3.5 Soil properties change by land-use alteration during drought occurrences
3.5.1 Influence of grassland on edaphic properties
Droughts had a notable impact on SWC in grassland. Specifically, light drought and extreme drought attenuated SWC by 17.0% and 26.1% (Fig. 6(a)) respectively. Extreme drought ameliorated soil C:N ratio, soil pH and BG activity. Extreme drought and light drought augmented soil MBC:MBN ratio by 82.9% and 58.5% (Fig. 6(d)), respectively. Soil F:B ratio down-regulated markedly (-8.1%) (Fig. 6(f)), and soil AP enhanced 2.6% (Fig. 6(g)) during drought conditions.
3.5.2 Soil physicochemical and biological properties in forest
Among the forest, drought remarkably attenuated SWC by 19.6%. (Fig. 6(a)). Extreme was found to decreased SWC for forest by 25.6%, whereas light drought conditions had a moderately impact on SWC (-12.9%).
Under conditions of extreme drought and light drought, forest soil C:N ratios were exhibited marked reductions by 10.5% and 10.4%, respectively (Fig. 6(b)). In general, drought ameliorated soil pH levels (+ 2.7%) (Fig. 6(c)) and MBC:MBN ratio (+ 21.9%) (Fig. 6(d)). They were enhanced by 5.2% and 17.6% during light drought, and increased by 5.4% and 27.2% under extreme drought (Fig. 6(c, d)). Under light drought and extreme drought, forest soil MBC:MBN ratio was found to enhance by 17.6%, and 27.2% (Fig. 6(d)). Light drought caused a significant decrease of 17.0% in forest soil AP (Fig. 6(g)). Soil BG activity showed restraint and diminished by 12.8%. Furthermore, light drought and extreme drought precipitated a reductions in the soil BG activity by 13.5% and 24.1%, respectively (Fig. 6(e)).
Forest soil F:B ratio experienced an upturn during drought periods. In general, drought enhance F:B ratio by 74.6%, but light drought and extreme drought led to an increase by 91.5%, and 161.5%, respectively (Fig. 6(f)). In additional, Shannon diversity index increased by 2.7%, which indicated forest soil microbial diversity were ameliorated under some drought conditions (Fig. 6(h)).
3.5.3 Assessing cropland soil physicochemical-biological properties
SWC performed a reduction (-25.7%) in the cropland during drought periods. Light drought and extreme drought reduced cropland soil SWC by 3.3% and 43.1%, respectively (Fig. 6(a)). Extreme drought exacerbated SWC variations compared to light drought.
Drought conditions enhanced the soil C:N ratio by 15.3% in cropland (Fig. 6(b)). Extreme drought significantly decreased the pH by 0.7% (Fig. 6(c)). There was a downturn in the MBC:MBN ratio under drought conditions. The soil AP significantly attenuated by 8.0% during period of light drought (Fig. 6(g)). Drought was found to diminish soil BG activity in cultivated land by 25.7% (Fig. 6(e)). Yet, extreme drought attenuated it by 6.6% (Fig. 6(e)).
Generally, soil F:B ratio showed a decreasing trend in cropland and was downturn by 34.6% under drought (Fig. 6(f)). For soil microbial diversity, drought increased the Shannon diversity index by 0.3% (Fig. 6(h)), but the change was not significant.
3.6 Interactions edaphic physicochemical and biological properties affected by droughts
Correlating physicochemical and biological properties under drought conditions, we found that there was a significant correlation (p < 0.05) between soil F:B ratio and soil physicochemical properties (Fig. 7). F:B ratio was significantly negatively (p < 0.05) correlated with BG activity and soil C:N ratio. F:B ratio was significantly positively correlated with SWC, soil pH, AP and MBC:MBN ratio.
Figure 8 showed a significant negative correlation (P < 0.001) between MBC:MBN ratio and Soil physicochemical properties, including SWC and BG activity. Soil MBC:MBN ratio had the more stronger correlation coefficient with BG activity, with an R2 of 0.57, while it had the smallest correlation coefficient with SWC. Additionally, the positive correlation coefficients were showed between MBC:MBN ratio and soil pH, C:N ratio and AP. Although, these values were relatively small, but still exhibited significant (p < 0.001) relationships except for between MBC:MBN and soil AP. Therefore, we speculate that changes in soil F:B ratio and MBC:MBN ratio are not only influenced by drought episodes but also, to some extent, by BG activity.
Figure 9 illustrated the variations of eight key soil properties, the correlations between soil microorganisms and physicochemical properties under light and extreme drought. Both light drought and extreme drought had positive effects on soil F:B ratio, MBC:MBN ratio, and soil pH. And this impact became more pronounced with increasing drought intensity. That was to say increasing effects of extreme drought precipitated than light drought. Light drought and extreme drought had negative effects on SWC, C:N ratio and BG activities. These responses of SWC and BG activity to extreme drought became stronger compared to light drought. By contrast, light drought caused soil C:N ratio more fluctuations than extreme drought.
The effects of light drought and extreme drought on soil AP and microbial diversity exhibited contrasting patterns. Soil AP and microbial Shannon diversity index exhibited reduction under light drought compared to extreme drought.
4.1 Responses of edaphic properties to the drought conditions
The response of soil physicochemical and biological properties to varying degrees of drought intensity demonstrates a complex interaction. Cumulatively, extreme drought emerges as the most influential stressor, markedly impacting these characteristics. Extreme drought precipitate a substantial depletion in SWC. (Wang et al., 2021) proposed drought mitigated SWC by 29.6%. Also, numbers of researches emphasized that SWC is mostly affected by extreme drought (Hicks et al., 2018) (Canarini et al., 2018; Olatunji et al., 2019; Sun et al., 2020). Under extreme drought conditions, soil AP decreases significantly, while light drought augmented levels of the AP. Phosphorus is an essential element for plant growth (Lynch, 2011), and most phosphorus only be absorbed by plants after mineralized and transformed by soil microorganisms. Extreme drought impacted on microorganism activities.
On one hand, extreme drought enhanced F:B ratio significantly by 108.2%, indicating that fungi were more drought tolerant than bacteria. Firstly, bacteria are more sensitive to SWC compared to fungi (Gao et al., 2021; Liu et al., 2022; Zhou et al., 2017), and fungi are more adapted to low moisture environments (Yuste et al., 2011). Secondly, the chitinous cell walls of fungi enhance their tolerance to water stress, which bacteria do not possess (Sun et al., 2020). On the other hand, extreme drought diminished soil microbial diversity notably, with the Shannon diversity index reduced by 13%. In contrast, light drought increases soil microbial diversity, with a 2.7% increase in the Shannon diversity index.
In addition, drought precipitated markable reduction in soil microbial biomass and bacterial abundance (Ren et al., 2018). Drought intensities altered microbial community structure (Vasquez-Dean et al., 2020). However moderate water scarcity (such as light drought) could augment microbial activity (Frey et al., 2013). Thirdly, enzyme activity was sensitive to soil quality changes (Acosta-Martinez et al., 2014), and was inhibited by water stress in bioreactions caused by soil microorganisms (Lefi et al., 2004). This undermined the resilience and resistance of microbial communities to drought stress (Wang et al., 2015). Moreover, the BG activity is closely related to biological activity(Acosta-Martinez et al., 2014). We found that extreme drought significantly reduced BG activity by 10.2%. And there was a strongly correlation between BG activity and soil microbial activity. Extreme drought diminished the diffusion of soluble substrates in the soil and degraded the microbial migration rate, which could lead to diminish the microbial activity and enzyme activity(Bastida et al., 2006; Tietema et al., 2008). Furthermore, drought lowed acidic soil pH levels (Liu et al., 2019), and elevated alkaline soil pH levels (Deng et al., 2021), soil pH drove spatial distribution of soil bacteria(Chu et al., 2010; Fierer and Jackson, 2006; Jesus et al., 2009), whereas fungi were more adaptable to pH changes(Lauber et al., 2008). Our research proposed that droughts enhanced the soil F:B ratio and soil pH, and they had a significant positive correlation. This instructed that soil pH could have an effect on the composition and structure of soil microbial communities .
4.2 Effects on soil properties: topsoil (0-15cm) vs. subsoil (15-45cm)
Our findings delineate a differential response to drought in topsoil versus subsoil. The subsoil demonstrated heightened sensitivity to light drought compared to the topsoil, which is consistent with the findings of Sun et al. (2020). This is because vegetation growth exhibits a water threshold effect (Huang et al., 2020; Song et al., 2019), where a certain range of SWC promotes plant growth. Under light drought conditions, vegetation growth is hardly affected and may even be beneficial to some extent (Fariaszewska et al., 2017; Song et al., 2019), leading to an increased draw on water from the roots that exceeds the drought’s direct effect on soil moisture. Consequently, fluctuations in SWC are more accentuated in the subsoil under light drought due to vegetation’s water requirements. Conversely, inhibitory effect of extreme drought on topsoil SWC was more pronounced compared to subsoil. Severe water deficit leads to a restraint of vegetation coverage, transpiration and net primary productivity (Song et al., 2019), which affect vegetation absorbing and utilizing soil moisture. In addition, growth inhibition or death of vegetation enhanced soil evaporation significantly (Zheng et al., 2018), which leads to notable fluctuations in topsoil SWC Drought conditions have a considerably inhibitory effect on topsoil C:N ratio (Sun et al. (2020). However, Chen et al. (2019) though that light drought enhanced topsoil C:N ratio. Vegetation coverage types could be drive these variations. Compared to sensitive vegetation, drought-tolerant vegetation could vary the geometric shape of soil pores around the roots and the relative abundance of soil bacteria (Rabbi et al., 2021). Moreover, vegetation cover types also strongly influence microbial activity and functional diversity (Fisk et al., 2003). Compared to soils with single vegetation cover, they maintained a 10% higher microbial diversity due to mulched multiple vegetation covers(Zhang et al., 2022). To some extent, soil microbial diversity contributes to the variability in soil C:N ratio. Extreme drought had a inhibitory effect on subsoil C:N ratio with a notable 8.8% decrease in soil C:N ratio. Nevertheless, Hicks et al. (2018) highlighted that soil C:N ratio was not affected prominently by extreme drought. Soil depths and vegetation cover variations were likely responsible for this result.
The subsoil MBC:MBN ratio varied relatively unnoticeable. Different depth soils had heterogeneity in soil nutrients, hydrothermal conditions and aeration status, which led to their differences of soil properties under drought (Brockett et al., 2012). Furthermore, the abundance of aerobic bacteria in the topsoil leads to poor habitat conditions for microorganisms, and the litter in the surface layer also provides organic matter (Schindlbacher et al., 2011). As described above, they could be accountable for great fluctuations of topsoil MBC:MBN ratio during drought periods.
Soil F:B ratio had a significant positive correlation with soil pH. And there was a same performance between F:B ratio and BG activity. Therefore, we proved significant variations in the response of soil pH, F:B ratio, and BG activity to drought intensity across different soil layers. The most pronounced responses were observed in the subsoil. Firstly, the soil pH, F:B ratio and BG activity in the topsoil were highly sensitive to environmental changes, and even slight disturbances could have a conspicuous performance on them. For example, less intensive land use could enhance microbial growth efficiency (Malik et al., 2018). Biotic and abiotic disturbances changed the community composition of forest soil fungi and the proportion of functional diversity (Rodriguez-Ramos et al., 2021). Secondly, the number and diversity of microorganisms in the subsoil were greater (Yin et al., 2020), which could provide some resistance to environmental stresses. In addition, soil microbes community structure strongly correlated with soil pH (Bu et al., 2018) and BG activity (Figure.8). This could explain why there was a consistent variation in soil pH, F:B ratio, and BG activity under drought conditions.
4.3 Effects on edaphic properties: Rhizosphere vs. non-rhizosphere soil
Significant differences emerged in the physicochemical and biological properties between the rhizosphere and non-rhizosphere soil during drought periods. Beyond the inhibitory effect on SWC, parameters such as C:N ratio, BG activity, and Shannon diversity index generally enhanced in the rhizosphere, while they reduced in the non-rhizosphere. In the rhizosphere, the soil pH, MBC:MBN ratio, F:B ratio, and AP experienced a downturn, while a upturn was observed in the non-rhizosphere. On one hand, the transformation of soil nutrients (such as carbon, nitrogen and phosphorus) relied on the soil microbes, which worked in biogeochemical processes between vegetation and soil at different spatiotemporal scales (Chakraborty et al., 2012; Zhao et al., 2022). The rhizosphere is a microbial hotspot and is one of the most complex microbial communities on Earth (Mendes et al., 2013). On the other hand, plant root exudates drove patterns in rhizosphere microbial community, which revealed a close interaction of rhizosphere microbiome and root exudates (Geeta Singh and Mukerji, 2006). Alteration of microbial patterns, in turn, could affect root exudates, material cycling, energy flow, and information transfer within the soil system, forming a complex system (Paterson et al., 2007).
4.4 Effects on soil properties: controlled potted environments vs. open field conditions
We found that drought increased soil F:B ratio in open field environment and decreased it in control potted conditions, which was consistent with previous studies by Hu et al. (2019) and Monokrousos et al. (2020). Control potted experiments had smaller container volume and lower soil volume, which maybe have a restraint on nutrient absorption by plants (Vaknin et al., 2009) and an inadequate nutrient supply for root growth (Kiaer et al., 2013; Poorter et al., 2012). In conclusion, a reduction in root exudates led to a decrease in rhizosphere microbiome. On the other hand, the soil in potted experiments was artificially disturbed, whereas open field environments utilized undisturbed soil (Gao et al., 2020). Undisturbed soil contained a higher amount of organic matter, which was likely to enhance the microhabitat availability of microbes and made it easier to access food resources (Olle Zackrisson et al., 1996; Pietikainen et al., 2000).
Light drought significantly increased soil AP and BG activity in control potted experiments, while in open field experiments, it decreased soil AP and BG activity. Phosphorus is crucial for plant growth (Lynch, 2011), but the existing soil phosphorus storage cannot meet the demands for crops growth. The arbuscular mycorrhizal fungi (AMF) can facilitate the absorption of phosphorus by vegetation roots (Ryan et al., 2007). However, pot experiments hindered growth of plant roots under drought conditions, which led to ineffective phosphorus conversion by mycorrhizal fungi. As a result, Control potted experiments diminished the absorption of phosphorus by plants compared to open field conditions, where vegetation had better obtain its necessary nutrients. Soil BG activity was also dependent on soil enzymes, which were mainly produced by microorganisms (Zornoza et al., 2006), and were major regulators of soil biology (Marx et al., 2001). Corstanje et al. (2007) found that BG activity enhanced with phosphorus content increasing. Whereas, under drought conditions, AP variation in control potted soil could led to alterations of soil microbial activity, then impacted soil BG activity. Consequently, alterations in soil BG activity under drought could be attributable to shifts in the effective phosphorus content in potted soils.
4.5 Effects on soil properties: forest, grassland and cropland
When drought occurred, the physicochemical and biological properties of cultivated soils exhibited more variability than those of forest and grasslands, while the differences between woodlands and grasslands were not significant. This variation could be attributed to several factors: on one hand, forest had enriched species and created a diversified spatial pattern of biodiversity (Llado et al., 2017). Furthermore, forest-floor litter was primary source of soil organic matter (Trumbore and Czimczik, 2008). The protective layer formed by forest-floor litter prevented environmental disturbances (Sardans and Penuelas, 2005). Secondly, the vegetation cover of grasslands were relatively homogeneous (Dengler et al., 2014), and its rate was not high which led to ineffective soil protection and more susceptible to external disturbances (Ren et al., 2016). Deng et al. (2021) demonstrated that grasslands were more sensitive to drought than forest, but their resistances were reversed. On the other hand, frequent disturbances caused by human activities, especially conventional tillage and fertilization, maybe have shaped cropland soil physicochemical and biological properties. Long-term chemical fertilizer additives aggravated the decline of soil organic matter and fertility (Wu et al., 2020) and deteriorated structure and functional diversities of soil microbes. This maybe explain why cropland soil C:N ratio varied more notably during periods of drought compared to Forest and grassland. In additional, SWC worked on the decomposition of soil organic matter, but drought induced soil moisture decrease changed this process. This may also be a potential reason that some soil properties variations for cropland surpassed that of other lands.
4.6 Limitations and Potential Studies
Our research systematically examined the impact of drought events (especially light drought and extreme drought) on key soil properties. However, we did not survey soil properties under moderate and severe drought conditions for the lack of sufficient experimental data. We anticipate acquiring pertinent data on these intermediate drought intensities in future research endeavors, aiming to thoroughly elucidate the spectrum of soil property alterations corresponding to each drought level. In addition, our study and previous research studied by Deng et al. (2021) and Ren et al. (2018) did not investigate variations of soil textures, soil aggregates, soil porosity and soil animals across different conditions under drought episodes, which should be explored in the future.
This comprehensive study systematically quantified the responses of soil phtsicochemical and biological properties to varying intensities of drought across different environmental conditions. The findings underscore the complexity of soil system reactions to drought and provide valuable insights for land management and agricultural practices. The main conclusions were as follows:
- Drought general lead to reduce SWC and increased soil pH, MBC:MBN ratio, and F:B ratio. These variations were aggravated with drought stress increasing. Soil C:N ratio decreased under the light drought, whereas AP and BG activity were reduced during extreme drought occurrence. Light drought led to an augmented level in the soil microbial diversity. In contrast, it diminished significantly under extreme drought.
- Extreme drought and light drought showed contrary effects on SWC in different soil depths. Extreme drought had a greater effect on the SWC of the topsoil (0-15cm) compared to the subsoil (15-45 cm), and it was contrary to light drought. Compared to the topsoil, subsoil MBC:MBN ratio and F:B ratio varied significantly to extreme drought.
- The rhizosphere experiences a more pronounced reduction in SWC due to extreme drought than the non-rhizosphere, which is more affected by light drought. Soil BG activity enhanced in the rhizosphere and decreased in the non-rhizosphere under light drought, respectively. Soil pH and MBC:MBN ratio were significantly higher in the non-rhizosphere, while they were not significantly different in the rhizosphere. The F:B ratio and microbial diversity show contrasting responses in the rhizosphere and non-rhizosphere, with diversity significantly decreasing in the non-rhizosphere under drought.
- Under light drought, SWC varied more in open field experiment compared to controlled pot conditions. In contrasts, extreme drought reversed it in the pot environment. Field soil C:N ratio varied significantly during the light drought. But potted soil C:N ratio varied markedly when extreme drought occurred. Soil MBC:MBN ratio, AP and BG activity experienced greater changes in open field conditions than in controlled potted environments. Soil MBC:MBN ratio were ameliorated in the field during extreme drought, but it diminished in potted conditions during light drought. Light drought enhanced field soil F:B ratio, while potted soil F:B ratio showed no significant change.
- Compared to forest and grassland, SWC varied notably in cropland when the extreme drought occurred. Differently, under the light drought, the most pronounced changes in SWC were observed in grassland rather than others. Extreme drought reduced significantly forest soil C:N ratio, MBC:MBN ratio, and BG activity. Grassland soil C:N ratio and BG activity exhibited non-significance, while MBC:MBN ratio showed a significant increase. BG activity decreased significantly in the cropland. Soil pH and F:B ratio increased significantly in the forest. Soil pH decreased significantly in the cropland. Under light drought, soil AP reduced significantly in forest and cropland, while the F:B ratio increased significantly in the forest.
- A significant correlation is found between physicochemical and biological properties, particularly between F:B ratio and MBC:MBN ratios versus BG activity. This indicates that changes in soil F:B ratio and MBC:MBN ratio were influenced not only by drought intensity but also to some extent by BG activity.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this research.
Author Contribution
Y.M. and B.Q. wrote the main manuscript text.Zl.Y. and Mj.Y. manuscript revision.Yj.X collecting data.Bs.W and Dh.Y provide technical guidance.
Acknowledgments
We greatly appreciate all authors whose data and studies were included in this meta-analysis, and those who proposed the constructive comments. This research was funded by the National Natural Science Foundation of China (grant number 52109038 and 52209038) and Postdoctoral Research Grant in Henan Province (grant number 202102014).
Data availability
The datasets used and analysed during the current study available from the corresponding author on reasonable request.
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No competing interests reported.