Search

Understanding historical soil moisture (SM) variations and their relationship with temperature in high‐latitude humid regions is essential for predicting hot droughts under widespread warming. This paper presents the first‐ever annual‐resolution summer surface SM reconstruction (1736–2006 CE) in Sweden, located in northern Europe (NE). The reconstruction utilizes the paleoclimate proxy, tree‐ring δ18O, which exhibits a strong correlation with reanalysis SM data during 1948–2007 CE (r = −0.67, p < 0.001). In contrast to an unprecedented trend suggested by previous studies, our reconstruction exhibits a wetting trend in NE since the 1960s falling within the historical spectrum of trends. Both reanalysis SM data and our reconstruction show significant negative correlations with temperature at interannual timescales. However, Coupled Model Intercomparison Project Phase 6 climate models failed to capture the significant SM‐temperature relationship during the historical (1850–1947 CE) and observation periods (1948–2006 CE). This proxy‐model comparison represents a critical step toward enhancing our understanding of SM dynamics and the reliability of SM projections in high‐latitude humid regions. Plain Language Summary: Soil moisture (SM) refers to water in any form contained in soils, and it plays an important role in sustaining plant growth and influencing climate dynamics. SM deficits can result in ecological/agricultural droughts. However, our understanding of historical SM changes is limited due to the scarcity of long‐term records. Consequently, modeling historical SM changes proves challenging and prone to inaccuracies. In this study, we utilized a tree‐ring record to reconstruct centuries‐long SM changes in Sweden, located in northern Europe. Our analysis revealed a significant negative correlation between SM and temperature in both our reconstruction and SM data sets. However, state‐of‐art models failed to capture the significant relationship observed at our study site. These findings highlight the need to improve climate models to increase the accuracy of climate forecasts and better understand SM dynamics in high‐latitude humid areas. Key Points: This study presents the first 271‐year historical soil moisture (SM) reconstruction in northern Europe (NE) using the tree‐ring δ18O proxySignificant correlations between historical SM and temperature in NE were observed at interannual timescalesOur results implied that Coupled Model Intercomparison Project Phase 6 models might underestimate the SM‐temperature relationship [ABSTRACT FROM AUTHOR]

Soil microorganisms are vital components of the karst terrestrial ecosystem. However, their responses to the vegetation succession on karst mountain peaks remain unclear as to whether soil microbial diversity and community compositions change with vegetation succession. We investigated the diversity and community compositions of soil bacteria and fungi and associated environmental factors along a vegetation succession from moss crusts (MC) to moss crusts with sparse grasses (MCG) to sparse grasses (G) on karst mountain peaks. The results indicated that soil organic carbon and total nitrogen generally increased, and soil pH changed in the range of 8.19–8.44 and slightly declined with vegetation succession. Overall, there was an increase in microbial biomass along the vegetation succession, with the dominant phyla of bacteria, including Proteobacteria, Acidobacteria, Actinobacteria, Choroflexi, Gemmatiomnadates, Bacteroidetes, and Planctomycetes, and the dominated phyla of fungi, including Basidiomycota and Ascomycota. Notably, both the bacterial and fungal community compositions were different among the three successional stages. Spearman's correction analysis showed that soil organic carbon and total nitrogen had stronger and more significant influences on the soil microbial community compositions compared to soil water content, pH, and C:N ratio. Overall, our results provide evidence for the changes and influencing factors of the microbial community with the succession vegetation on karst mountain peaks. [ABSTRACT FROM AUTHOR]

Asymbiotic N2 fixation (ANF) is a major nitrogen (N) input pathway to terrestrial ecosystems. However, there is considerable uncertainty about how lithology affects ANF. Here, we measured ANF rates in soil, litter, and moss in forests underlain by limestone (limestone forest), dolomite (dolomite forest), and clasolite (clasolite forest), respectively, in southwest China. Effects of lithology on soil ANF rate varied seasonally. The rate was highest in the dolomite forest during the wet season, but was highest in the limestone forest during the dry season. The overall soil ANF rate was significantly higher in the limestone forest than in the clasolite forest. Litter ANF rate was significantly higher in the limestone forest and dolomite forest than in the clasolite forest regardless of season. There was no significant difference in moss ANF rate among the three types of forests in both seasons. The annual N2 fixation rate was highest in the limestone forest (1.72 ± 0.27 kg N ha−1 y−1) but lowest in the clasolite forest (0.70 ± 0.08 kg N ha−1 y−1). In the wet season, the variation of ANF rates was best explained by soil nitrate and available iron for soil, and by litter N and calcium content for litter. In the dry season, the variation of ANF rates was best explained by soil water content, ammonia and total phosphorus for soil, and by litter water content and calcium content for litter. No strong explanatory variables were identified for ANF in moss during the wet or dry season. Our findings suggest that lithology significantly affected ANF in soil and litter, but not in moss, and hence should be considered in Earth system models to facilitate better prediction of N inputs via biological N2 fixation under global change. [ABSTRACT FROM AUTHOR]

Purpose: Information on root-zone soil water content (SWC) is essential for vegetation restoration, irrigation scheduling, and hydrological modeling. However, measurements of SWC within a variety of land uses may be time-consuming and labor-costing. This study tested whether SWC at a depth of a land use can be used to predict profile SWC of other land uses in terms of temporal stability analysis at a karst depression area in southwest China.Materials and methods: A total of 30 datasets of root-zone SWC from 0.1- to 0.5-m depths were collected by time domain reflectometry probes for three typical land uses from March 12 to November 8, 2015.Results and discussion: Results showed that the profile mean SWC and its associated standard deviation (SDP) and coefficient of variation (CVP) differed significantly (P < 0.05) among the grassland, farmland, and forestland. The profile SWC was more temporally stable according to the apparently lower CVT in comparison with CVP. The similarities of the vertical patterns of SWC were strong for the same land uses, while were relatively weak between the different land uses. The SWC measurements of the most temporally stable depth can be used to accurately predict profile SWC for both the same land use and other land uses.Conclusions: This study further expands the application of the temporal stability analysis and can aid water resource management in areas with diverse land uses. [ABSTRACT FROM AUTHOR]

Soil nitrogen (N) accumulation enhances carbon (C) sequestration and subsequently the restoration of degraded ecosystems, but the changes in soil N properties after vegetation restoration in such ecosystems are poorly understood. We collected data from 358 fixed points in the karst catchment in China to determine the trends, magnitude, and mechanisms of soil N sequestration after 10 years of revegetation. The revegetation types were tillage fields that were abandoned or were converted to pasture, planted forest, or converted to mixed pasture and forest and pasture lands that were abandoned (P_Aban) or converted to mixed pasture and forest (P_Mix). The changes in soil N density (g m−2) over time were insignificant, and only P_Aban, P_ Mix, and the overall catchment experienced significant increases in soil N concentration (g kg−1). Structural equation modeling indicated that bedrock exposure affected the change in soil N concentration (∆NC) and the soil N sequestration value (∆ND) via affecting the change in soil C concentration, by directly affecting bulk density, soil depth, and Ca2+. The soil C:N ratio increased significantly, and the increase in soil N was slower than that of C, suggesting a decoupling of soil C and N accumulation in the initial stage of revegetation. ∆ND was significantly related to the change in soil C:N ratio, indicating that the coupling between soil C and N may be constrained by insufficient soil N sequestration during revegetation. These findings suggest that the restoration of degraded karst ecosystems requires an increase in soil N input early during vegetation restoration. [ABSTRACT FROM AUTHOR]

This study aims to evaluate the effects of soil physicochemical properties and environmental factors on the spatial patterns of surface soil water content (SWC) based on the state-space approach and linear regression analysis. For this purpose, based on a grid sampling scheme (10 m × 10 m) applied to a 90 m × 120 m plot located on a karst hillslope of Southwest China, the SWC at 0–16 cm depth was measured 3 times across 130 sampling points, and soil texture, bulk density (BD), saturated hydraulic conductivity (Ks), organic carbon (SOC), and rock fragment content as well as site elevation (SE) were also measured at these locations. Results showed that the distribution pattern of SWC could be more successfully predicted by the first-order state-space models (R2 = 67.5–99.9% and RMSE = 0.01–0.14) than the classic linear regression models (R2 = 10.8–79.3% and RMSE = 0.11–0.24). The input combination containing silt content (Silt),Ks, and SOC produced the best state-space model, explaining 99.9% of the variation in SWC. And Silt was identified as the first-order controlling factor that explained 98.7% of the variation. In contrast, the best linear regression model using all of the variables only explained 79.3% of variation. [ABSTRACT FROM PUBLISHER]

• As high as 30.04% of the monotonically increasing vegetation NPP (41.08%) can maintain the increasing trend. • Soil moisture and land-use changes interacted with other variables to affect NPP increment. • The coupling and shared effects had greater influences than the pure effects on NPP increment. • The shared and coupling effects dominated the persistent and non-persistent NPP increment, respectively. In the framework of ecological engineering, vegetation net primary productivity (NPP) has grown dramatically in the Three-north Shelter Forest Program area (TSFPA), which is influenced by both human activities and climate change, as well as their interaction. However, it remains unclear how the individual (pure effects) and interactive (shared and coupling effects) impacts affect and maintain the growing vegetation NPP trend. In this study, the monotonically increasing NPP trend from nonlinear trends was detected using Ensemble empirical mode decomposition (EEMD), and its persistence was investigated using boosted regression tree (BRT) models. We investigated the relative contributions of environmental factors and variable interactions, as well as spatially identified the multiple effects of climate change and human activities on the increasing trend and its persistence over TSFPA. The results indicated that: (1) The vegetation NPP was dominated by the monotonically increasing trend (41.08 %), and 30.04 % of all pixels can still maintain NPP increment. (2) Although soil moisture exhibited the highest interaction with the precipitation seasonality (32.21 %) on the monotonically increasing NPP trend, land-use change substantially interacted with soil moisture (26.97 %), snowmelt (23.34 %), and net solar radiation (13.43 %). (3) The monotonically increasing vegetation NPP was caused not only by the pure effects of climate change (23.57 %) and human activities (11.69 %) directly but also by their coupling effects (26.64 %) or shared effects (38.10 %) interactively. (4) The shared effects (50.94 %) dominated the persistently increasing NPP trend, whereas the coupling effects (78.78 %) dominated the non-persistently increasing NPP trend. The findings highlight the importance of the pure, shared, and coupling effects of human activities and climate change interactively for elucidating the driving of the vegetation NPP increment and its persistence over ecological engineering areas. [ABSTRACT FROM AUTHOR]

Knowledge of the temporal variability of soil water content (SWC) at the hillslope scale is essential for guiding rehabilitation strategies and for optimizing water resource management in the karst region of southwest China. This study aimed to use temporal stability analysis to upscale point-scale measurements to represent mean areal SWC on two typical karst hillslopes. Based on a grid sampling scheme (10 m × 10 m) applied to two 90 m × 120 m plots located on two hillslops, the SWC at a depth of 0-16 cm was measured 11-12 times across 259 sampling points, using time domain reflectometry (TDR) from April 2011 to October 2012. Soil texture, bulk density (BD), saturated hydraulic conductivity ( K ), organic carbon (SOC), rock fragment content (RFC), and site elevation (SE) were also measured at these locations. Results showed the hillslope with more shrub cover was wetter than the hillslope with mixed grass-shrub cover. This difference was related to the differences in soil texture, soil hydraulic permeability, and topography. Through a comparison of values obtained with the Spearman correlation coefficient ( r ), standard deviation of mean relative difference (SDRD), and mean absolute bias error (MABE), we inferred that there is a higher degree of temporal stability for SWC in wet conditions than in drier conditions on the two hillslopes. Based on the values of the index of temporal stability (ITS), which combine the mean relative difference (MRD) and SDRD, the two locations were determined to be representative of mean SWC on both hillslopes. Moreover, these locations captured changes in mean SWC (NSCE = 0.69, and 0.65, and RMSE = 1.96, and 1.96 %, respectively). This demonstrates the feasibility of using the temporal stability of SWC to acquire mean SWC on karst hillslopes of southwestern China. The indirect method, which estimates mean SWC by considering the offset between the mean and the measurement value at a time-stable location, predicted mean SWC (NSCE = 0.86, and 0.76, and RMSE = 1.29, and 1.63 %, respectively) more precisely than the direct method (mean SWC directly measured at a time-stable location), because it eliminates deviation by introducing a constant offset (MRD). We recommended the use of the indirect method to acquire mean SWC values, when an allowable bias of 5 % for both MRD and SDRD can not be achieved. In addition, we found that soil texture, RFC, and elevation affect the pattern of SWC on the shrub hillslope. These results are expected to be useful for monitoring soil water dynamics on karst hillslopes, especially for restoration purposes. [ABSTRACT FROM AUTHOR]

Soil saturated hydraulic conductivity ( K) is one of the most important parameters that determine soil water redistribution. However, it is little known in a karst area with thin, discontinuous, and rocky soils. The purpose of this paper was to investigate the spatial variability of K and its influencing factors in a small karst catchment with high heterogeneity in southwest China. Undisturbed surface (0-10 cm) soil samples were collected with metal cylinders from an 80 m × 80 m grid for measuring K, bulk density (ρ), non-capillary porosity (NCP), and capillary porosity (CP). Disturbed surface samples were taken with a shovel to measure soil organic carbon (SOC) and rock fragment content (RC). Environmental factors including topographical location (slope and depression), land-use type (forestland, shrubland, shrub-grassland, and farmland), slope gradient, slope exposure, and elevation were investigated for each sampling point. The results showed that K value was relatively high (9.10 m/d) with a moderate variation, and had a strong spatial dependence (nugget/sill value = 0.06 %). NCP showed a very significantly ( p < 0.01) positive correlation but ρ showed a very significantly negative correlation with K. RC and SOC had significantly ( p < 0.05) positive correlations but CP had no obvious effect on K. This indicated that NCP and ρ were the dominant influencing factors of K in a karst area. The effects of topographical locations and land-use types on K were not significant ( p > 0.05), but their interaction effects were significant. This suggested that the influencing factors were more complex and they should be considered together in a karst area. The mean K value was the lowest in the forestland on the hillslope and in the farmland in the depression, indicating where the surface runoff may appear easily. Such results could help to better understand the soil hydrological processes and contribute to the building of hydrological models in small karst catchments. [ABSTRACT FROM AUTHOR]

Geostatistical and statistical analyses were combined to examine the spatial distribution of soil water content under four vegetation types during the dry season, in the peak-cluster depression in the karst region in northwest Guangxi, southwest China. The soil water content significantly increased from farmland to plantation, secondary forest, and primary forest; whereas the variation coefficients, the sill ( C+ C), and total spatial variance increased, although the range decreased. The spatial distribution of soil water content in the different vegetation types had a high spatial autocorrelation. Different models produced a best fit for the semivariograms of the four vegetation types. Elevation and slope position were the primary factors influencing the spatial distribution of soil water content, with other key factors differing between the four vegetation types. Moreover, even though different specific factors influenced soil water content in the four vegetation types, the correlations and degrees of associations between the soil water content and these various factors differed. Therefore, the corresponding strategies for rational usage and management of water resources should be different for the four vegetation types in this region. [ABSTRACT FROM AUTHOR]

The cosmic-ray neutron sensing (CRNS) is an emerging method for continuously monitoring soil water content (SWC) at an intermediate scale. However, when multiple hydrologic units are present within its footprint, the potential application of CRNS in water resources management is restricted. Here we propose a new strategy to predict point-scale SWC in root zone established on CRNS-based soil moisture and improved relative difference method. A total of 768 days of soil moisture data were collected by CRNS at the intermediate scale and EC-TM sensors at the point scale in a karst catchment. The original and improved mean relative difference methods predicted point-scale SWCs within and without the effective measuring depth, respectively. The mean effective measuring depth was 13.16 cm, ranging from 10.13 to 19.23 cm. Both land use type and soil structure played essential roles in regulating point scale SWC in the soil profile. Point-scale SWC in root zone can be predicted accurately (P < 0.001) based on SWC data derived from the CRNS system. The prediction accuracy of point scale SWC can be improved by increasing the averaging time of the soil moisture values. Our results demonstrated that the proposed strategy was reliable for CRNS to predict SWC beyond the effective measurement depth. This study provides a good perspective for effectively managing of water resources in areas with complex hydrological processes. • Both land use type and soil structure have important effects on soil moisture. • CRNS can accurately predict deep soil moisture at point scale. • The prediction accuracy of soil moisture can be improved by increasing averaging time. • CRNS is an effective tool for water resources management in small watershed. [ABSTRACT FROM AUTHOR]

Soil and epikarst play an important role in the hydrological cycle in karst regions. This paper focuses on investigating the seasonal recharge and mean residence time (MRT) of soil water and epikarst water in a small karst catchment of southwest China. The deuterium contents in precipitation, creek, soil baseflow (direct recharge of the saturated soil water to the stream), epikarst spring, and soil waters were monitored weekly for two years, and MRT was calculated by an exponential model (EM) and a dispersion model (DM). The obvious seasonal variation of deuterium in rainfall was buffered in epikarst water, indicating sufficient water mixing. Soil baseflow contained less rainy-season rainwater than epikarst spring discharge, reflecting the retarded effect of soil thickness on rainwater recharge. MRTs of all water bodies were 41-71 weeks, and soils in the depression extended those of shallow groundwater. This demonstrated that the deep soil layer played an important role in karst hydrological processes in the study catchment. The creek was recharged mostly by rainfall through epikarst, indicating its crucial role in water circulation. These results showed epikarst had a strong water-holding capacity and also delayed water contact time with dolomite. [ABSTRACT FROM AUTHOR]

Knowledge of dynamics of profile soil moisture is of importance for soil moisture prediction and modeling. However, information on the relationships between soil moisture at the surface and subsurface layers is relatively inadequate. The objective of this study was to examine the similarities in the overall and scale specific temporal patterns of soil moisture across soil profile. Soil moisture was measured at five soil depths using time-domain reflectometry in two sampling plots over 242 days in a karst depression. Spearman rank correlation and wavelet coherency analyzes were used to determine overall and scale-dependent similarities of temporal patterns of soil moisture, respectively. Results showed that soil moisture in farmland was higher than grassland especially in 10–50 cm depths. The surface depth had higher variability for both farmland and grassland. Spearman rank correlation coefficients between any two soil depths were significant, indicating strong depth persistence in the overall temporal pattern of soil moisture. The similarity decreased as the distance between depths increased. Wavelet coherency analysis indicated that soil moisture temporal pattern had a higher depth persistence in the farmland than grassland. Soil depth had a larger influence than land uses on the similarity in soil moisture temporal pattern, and the relative humidity and precipitation had larger effect than air temperature controlling the change of soil moisture at larger scales. These scale specific similarity in the temporal pattern of soil moisture can be used to guide estimating deeper soil moisture and to improve sampling strategies for soil moisture prediction with a higher prediction accuracy. [ABSTRACT FROM AUTHOR]

• Surface and bedrock topography are closely coupled in shallow soil-covered plot. • Soil moisture in thick soil plot is notably affected by the surface topography. • Runoff in thinner soil plot is remarkably affected by the bedrock topography. • Soil thickness is a critical factor in distinguishing hydrological processes. Topography plays a crucial role in predicting rainfall-runoff processes under climate change, as it affects the spatial distribution of water flow pathways. However, understanding the potential impact of high micro-topography heterogeneity on runoff generation remains challenging. We conducted high-resolution monitoring of soil moisture, instantaneous water table, and subsurface runoff in two 3D plots with different soil thicknesses (66 cm vs. 35 cm) in the karst region of southwest China. The topographic features were characterized by the soil thickness, relative elevation, and topographic wetness index obtained from a 1 × 1 m grid. The topography of the shallow-soil plot (SSP) exhibited a higher level of complexity, with a greater variability (approximately 80.0 %) in soil thickness compared to that of the deep-soil plot (DSP; approximately 61.5 %). Additionally, the SSP was prone to forming a micro-topography characterized by a combination of exposed bedrock and soil cover. In DSP, the variation in soil moisture was strongly controlled by surface topography, with elevation contributing to 52 % and 81 % of the explanation for the soil moisture at 30 cm and soil–bedrock interface, respectively. In SSP, the contribution of topography to the variation in soil moisture did not exceed 31 %. The controlling effect of topography on runoff is highlighted in SSP, with the contribution rates of surface and bedrock relative elevation reaching 79% and 97%, respectively.The weaker water storage capacity of the SSP resulted in a higher volume of subsurface runoff (161 t vs. 116 t) and frequency of runoff events (2248 vs. 1994) compared to the DSP. Accordingly, this study emphasizes that the soil–bedrock spatial heterogeneity will control the relationship between soil moisture, subsurface runoff, and terrain, which will contribute to providing a basis for flood prediction and model simulation in regions with complex topography. [ABSTRACT FROM AUTHOR]

[Display omitted] • The size of the carbon sink across SWC has increased quadruply. • The contribution of CCR to the regional carbon sink was increasing. • Water and temperature controls on the karst carbon sink varied with the lithological background. South-West China (SWC) is a pivotal region for global greening, recognized as having great carbon sink potential. Multiple evidence underscores the expanding contribution of SWC's carbon sink to the global carbon sink enhancement. However, our understanding of carbon sink dynamics and the response to climate across bedrock remains limited. In this study, we investigated the divergence in trends in carbon sink across bedrocks, evaluating their respective contributions of bedrocks to the overall carbon sink in SWC. Additionally, we assessed the dominant climatic factors and examined how bedrocks shape the response of carbon sink to climate change. Our results revealed a notable increase in the regional carbon sink, with an average rate of 3.58 TgC/yr from 1981 to 2019. Continuous Carbonate Rocks (CCR) exhibited the highest increased rate of total carbon sequestration (1.57 TgC/yr), surpassing Discontinuous Carbonate Rocks (DCR) by threefold. Non-karst areas contributed the most to both the mean and interannual variations of the overall carbon sink, with CCR exerting the most contribution to the trends. Over time, the contribution of CCR to the overall carbon sink escalated, while the contributions of DCR and non-karst declined. All bedrocks displayed negative correlations between NEP and temperature, with DCR showing higher susceptibility. Non-karst areas experienced adverse impacts from vapor pressure deficit, while CCR benefited from positive soil moisture effects. Our findings implied that bedrock indeed played a critical role in the variations in NEP and the response of NEP to climate change. Bedrock regulated climatic controls on carbon sink through soil water availability as soil water availability is affected by the lithology of basement carbonate. This novel understanding emphasizes the necessity for tailored ecological restoration initiatives that consider the distinctive lithological features of the karst ecosystem, and holds significance for implementing ecologically sound projects in karst regions. [ABSTRACT FROM AUTHOR]

• SWC was scale-dependent on meteorological factors. • The higher relationships between controls and SWC was mainly found at large scales. • Soil depth had greater influence on wavelet coherency than land use. • Wavelet coherency analysis was a useful technique in solving scale problem. Temporal soil water content (SWC) patterns are affected by meteorological factors that operate at different intensities at different scales. Pearson's correlation analysis can only identify linear correlations at the measurement scale, therefore we examined the relationships between SWC and meteorological factors at different scales and occasions in a karst landscape using wavelet coherency analysis. SWC was measured at five soil depths using time domain reflectometry in two land uses over 242 d in a karst depression, southwest China. The results showed that the temporal SWC dynamics were influenced mainly by precipitation and land use. The temporal SWC patterns in farmland differed from those in grassland. The relationships between SWC and meteorological factors varied with scale, and a significant correlation was generally observed at large scales. Information on variation of scale for SWC improved the performance of soil moisture prediction. Soil depth had a greater impact on wavelet coherency at large scales than land use. Positive and negative correlations were detected between SWC and meteorological factors using wavelet coherency analysis. This study indicated that wavelet coherency analysis has potential to reveal the scale-specific dependence of SWC on controls. [ABSTRACT FROM AUTHOR]

Abstract Precipitation in the karst regions of southwest China is plentiful, however, soils are shallow and highly permeable, which results in rapid transportation of surface water through a soil profile. Therefore, the presence and amount of water stress in a soil layer can be unpredictable. This study will investigate the soil water conditions on karst hillslopes in southwest China through dynamic in situ observations of profile soil moisture (θ). Three profiles at two slope positions on two shrub-grasslands (S1 and S2) were selected, respectively. At each profile, TDR 100 probes were set at 10, 20, 30, 50, and 70 cm and for 100 cm, only on downslopes. Soil samples (undisturbed and disturbed) were collected to measure basic physical and chemical properties. Precipitation and θ were monitored manually each week between July 15, 2012 and November 25, 2014. Results show that soil properties, which have an important effect on θ , varied among slope positions and between the two hisllslopes. A higher clay content, which led to greater water holding capacity, explained a higher θ in downslopes and on S2. Shallow soil layers (0–30 cm), especially in the coarse textured soil layers of S1, were susceptible to water stress due to the limited availability of soil water. Water stress was low during water supplying (December through April) and relatively stable periods (May to June). However, during water consuming periods (July to November), profile θ decreased greatly which resulted in severe water stress for the whole profile. Although precipitation was seasonally uneven, based on weekly observations, profile θ on both hillslopes was not temporally active (coefficients of variation of θ was smaller than 20%) and the variance of each depth was similar. Generally, the effects of precipitation on θ can last two (0–50 cm) to five (70–100 cm) weeks. Nevertheless, θ of downslope deep soil, which was (or near) saturated all year round, was barely affected by precipitation. These results can provide valuable information for hydrological models and rational strategies for ecological restoration designing in karst regions. Highlights • Soil water conditions of two karst shrublands were investigated. • Shallow soil layers (0–30 cm) were susceptible to water stress. • Soil moisture (θ) was not active with small variances at all soil depths. • Precipitation affected θ within two (above 50 cm) to five (under 70 cm) weeks. • Saturated deep soil layers were not obviously affected by precipitation. [ABSTRACT FROM AUTHOR]

Abstract Plant water use is an important component in the function of Earth's critical zone and this can be examined by decomposing isotope composition of xylem water into contributions from precipitation stored in shallow soil layers and deeper groundwater. The usual procedure for estimating the proportional use of groundwater by plants is to sample the isotope composition of soil and groundwater and determine the most probable mixing coefficients from all potential sources. Here we propose and test a novel method for achieving the same goal without sampling soil water. The method is based on analyzing variability in the stem water isotope ratios of several members of a community and the known isotope ratio of groundwater to 'triangulate' the unknown isotope ratio of stored rainwater. Using a simple water balance model, parameterized to produce the best fit between actual and estimated stem water isotope ratios, we simulated seasonal variation in the volume and isotope ratio of rainwater storage, along with species-specific groundwater use ratios. The method was applied to eight woody plant species growing on two rocky outcrops in the South China karst. Estimated average proportional groundwater use over two seasons varied between 14% and 62% and was site-dependent. For the majority of species, groundwater use increased as estimated stored rainwater volume declined. The two species with highest groundwater use were taller, deciduous or semi-deciduous trees with lower wood densities. While the new method was inspired by the inability to sample water stored in the rocky outcrops, it may have broader use in any environment where the spatial variability of soil water isotope composition is a barrier to estimating average groundwater use ratios. The broader adoption of this or equivalent methods would greatly improve the study of the Earth's critical zone. [ABSTRACT FROM AUTHOR]

Soil hydrologic functions are important for karst landscapes where soil water loss is significant. The largest global ecological restoration engineering project, namely the “Grain for Green” program, is being implemented since 2000 in karst landscapes across southwestern China. However, its effects on soil hydrologic functions are still unknown. Using data acquired in field investigations and a generalized linear model (GLM), this study examines the effects of different vegetation restoration types on soil field saturated-hydraulic conductivity ( K fs ) in karst landscapes of southwest China. The results indicate that the K fs of artificial grassland (napiergrass) was higher than in other vegetation restoration types (zenia insignis, toona sinensis, orchard, and natural restoration shrubland) and cropland at both the surface and 10 cm depths. The K fs of vegetation restoration lands was lower than that of Mulun nature reserve (primary forest) at both the surface and 30 cm depths. We found significant differences of K fs in different depths of vegetation restoration lands but not significant for primary forest. The GLM model explains 64.75% of the total variation in K fs . Among the input variables, vegetation type explained the largest proportion (11.29%) of the variation, followed by bulk density (BD), soil organic carbon, and BD and vegetation type interactions. The factors above were significantly related to K fs . This study suggests that the implementation of different vegetation restoration types can alter soil hydrologic functions and provides useful knowledge for ecological restoration practices and management in karst landscapes. [ABSTRACT FROM AUTHOR]

Spatiotemporal variability in soil water storage (SWS) is controlled by different factors operating at various intensities and scales. The traditional Pearson's correlation analysis can be used to identify the linear correlations at the measurement scale only. In this study, wavelet coherency analysis was used to investigate scale-specific relationships between SWS and selected controlling factors. SWS of 135 sampling locations were calculated along a 1,340-m long sampling transect established in a typical catchment on the Loess Plateau, China. The selected controlling factors were soil saturated hydraulic conductivity (Ks); clay, silt, sand, and soil organic carbon (SOC) contents; elevation; and aboveground biomass (AGB). The spatial pattern of SWS measured in growing and nongrowing seasons and at different soil depths was similar. At all the sampling points, except for some locations in the depression area, SWS in the growing season was relatively greater than that in the nongrowing season. The influence of factors on SWS varied with scale, with soil Ks, clay and sand contents significantly correlated with SWS at large scales. Season and soil depth had no significant effect on scale-specific relationships between SWS and the controlling factors. The wavelet coherency analysis identified the type of correlation at different scales and locations. The outcomes of this study offer meaningful insights into the hydrological processes that can be observed in the Loess Plateau region and could have significant implications for future hydrological systems and water resources management. • SWS was scale-specifically dependent on the controlling factors. • Season and soil depth had no significant effect on scale-specific relationships. • The effects of Ks, clay and sand contents on SWS were sensitive to scale variation. • Wavelet coherency analysis was a useful tool dealing with scale issue. [ABSTRACT FROM AUTHOR]

• Root-zone water bypass and replenishment are regulated by antecedent soil water condition and rooting depth. • Root-zone water bypass is widespread through two different bypass processes. • The occurrence of the temporal ecohydrological separation was verified by root-zone water bypass. Root-zone water storage is a core component of many hydrological systems, and a critical variable influencing water transport in the soil-vegetation-atmosphere continuum and biogeochemical processes. However, the dynamics of water bypass and replenishment in root-zone during precipitation infiltration processes have been rarely investigated. Here we employed a piecewise isotope balance method to estimate the proportion of root-zone water replenished by precipitation, and conducted a meta-analysis to check the patterns of water bypass and replenishment in root-zone for various species across five climate zones and examined the potential influencing factors. The results show that high proportion of root-zone water replenishment is likely to appear during the transition period from dry season/soil to wet season/soil. Root-zone water bypass is widespread under various climates, which is likely to occur by downward drainage under wet soil condition or via evaporation loss under high atmospheric demand. Antecedent soil water condition and rooting depth are found to be the two key factors regulating dynamics of root-zone water bypass and replenishment. These findings provide useful insight into our understanding of ecohydrological modeling (e.g., non-well-mixed water pool in soils) and plant responses to precipitation pulses. [ABSTRACT FROM AUTHOR]

Soil moisture ( θ ) and its stable isotope values are two of the most commonly used parameters for studying hydrological processes in soils. Despite their unique ability to aid in distinguishing between soil water evaporation and plant transpiration, the spatial variability of soil water isotope values is not fully understood. In the current study, 10 m × 10 m grids were established within a 90 m × 120 m plot on a highly heterogeneous karst hillslope. Two sampling campaigns were conducted during the early growing season, on April 15, and during the mid-growing season, on August 18, 2011. Stratified soil samples were collected from the shallow soil layer (0–30 cm) to measure θ and its stable isotope values, which were represented by soil water δD values (δD θ ). Related soil properties, land cover and topography were also measured and treated as influencing factors. On both sampling dates, θ decreased with depth, while δD θ stayed constant for all soil layers and were similar to the most recent rainfall values. Additionally, the variance of δD θ was smaller than that of θ , especially on August 18 when the most recent rainfalls had similar δD values. The high contrast between θ and δD θ caused the impacts of other factors on δD θ to be masked by the impact of the recent rainfall. Soil moisture presented a moderate to strong spatial dependence, which was consistent with the spatial variability of the influencing factors that were significantly correlated with θ . Soil water δD value exhibited weak spatial dependence and random spatial patterns that were different from the other influencing factors. Moreover, significant correlations between these same influencing factors and δD θ disappeared after a partial analysis with θ as a controlled variable, which means δD θ was indirectly affected by these influencing factors through θ . This suggests that the spatial variability of δD θ was being controlled at fine scales. Our results highlight the importance of analyzing the spatial variability of δD θ , its influencing factors and θ in shallow soil layers separately. [ABSTRACT FROM AUTHOR]

Knowledge of spatiotemporal dynamics of soil water storage (SWS) is essential for hydrological modeling and vegetation restoration in semi-arid areas. However, characterizing the temporal stability of SWS at a regional-scale requires time-consuming and labor-intensive manual sampling. Moreover, the influence of soil depth on temporal stability of SWS is not systematic. This study aimed to investigate the influences of sampling frequency and soil depth on SWS and SWS temporal stability. We measured soil moisture at 20-cm intervals in the soil profiles to a depth of 3 m using a neutron probe at 135 locations along a 1340-m long transect on 14 dates from 2012 to 2013. Results showed that sampling frequency did not influence the mean SWS (P < 0.05), while sampling frequency significantly affected temporal stability characteristics including Spearman's rank correlation coefficient (r s), standard deviation of mean relative difference (SDRD), the number of locations with SDRD < 5%, and the representative locations. Temporal stability of SWS increased with the increasing soil thickness and depth, which increases the possibility of the number of representative locations in deep soil. Although the mean SWSs of all soil depths can be predicted accurately at each sampling frequency, the prediction accuracy improved when sampling frequency was reducing. The values of R 2 ranged from 0.769 to 0.978 at 15-day sampling frequency, and from 0.987 to 0.998 at 45-day sampling frequency. Soil moisture stability may be more important than the soil water regime during prediction of soil moisture. These findings can provide guidelines for optimizing soil moisture sampling strategies and benefit management of water resources in semi-arid watershed. • Sampling frequency did not influence mean SWS. • Sampling frequency significantly influenced in temporal stability indices. • The SWS prediction accuracy can be improved by reducing sampling frequency. • SWS stability was more important than its values during the prediction process. [ABSTRACT FROM AUTHOR]

• The mean footprint radius and effective depth of CRNS were 311.51 m and 11.98 cm. • CRNS can accurately estimate intermediate scale soil moisture. • CRNS can be used for field scale water management in karst area. • Temporal stability analysis is an effective tool for trans -scale soil water prediction. Cosmic-ray neutron sensing (CRNS) is a newly-developed method for continuously measuring soil water content (SWC) at the hectometer horizontal scale. However, it is unknown whether CRNS-based SWC could be used for soil moisture prediction at field scale. In this study, we try to predict field scale SWC using the CRNS combined with the temporal stability analysis in a karst watershed. The CRNS was installed in a karst watershed as well as capacitance-based SWC sensors located in five land uses. The CRNS was calibrated by eighteen manual sampling procedures. A weather station was established about 200 m away from the CRNS for monitoring basic meteorological variables. The mean footprint radius and effective depth of the CRNS were 311.51 m and 11.98 cm, respectively. The CRNS can accurately estimate intermediate scale soil moisture, with a coefficient of determination (R 2) of 0.89. Significant correlations (P < 0.01) were observed between SWC measured by the CRNS and Em-50 instruments. Though the SWC derived from the CRNS was significantly lower than that from Em-50 instruments in different land uses, the SWCs at field scale can be predicted accurately. The influences of landforms and land uses on prediction accuracy were not obvious from our data. The CRNS has great potential to improve water resources management in areas with greater heterogeneity. [ABSTRACT FROM AUTHOR]

• Soil nematode richness is relatively low on inshore islands. • Large island maintains high soil nematode abundance and diversity. • Edaphic and climatic-geographical factors drive variations in soil nematode compositions. Factors that influence the composition and diversity of soil nematode communities on oceanic islands are poorly understood. In this study, we evaluated the soil nematode communities and associated biotic and abiotic factors on five oceanic islands in the north of the South China Sea. We detected 38 nematode genera and found that bacterivores and fungivores were the dominant trophic groups, representing 54.2–70.4% and 15.1–35.3%, respectively, of total soil nematode abundance. The abundances of total soil nematodes and of all trophic groups, maturity, structure indices and nematode diversity were positively correlated with island latitude and area, and were negatively correlated with soil total K and annual temperature. In addition, the abundances of total soil nematodes, bacterivores, herbivores and nematode dominance index were positively associated with soil available K. The abundances of total soil nematodes, fungivores, herbivores, predators and omnivores, maturity, structure, Shannon diversity indices were positively related to soil moisture. Variation partitioning analysis showed that the combination of soil properties and climatic-geographical factors explained more of the variation (26.8–61.5%). The results suggest that larger island can maintain greater nematode diversity than smaller islands due to the greater diversity of habitats on larger island. The key climatic-geographical and edaphic factors affecting soil nematode communities were latitude, annual temperature, soil moisture and total K, respectively [ABSTRACT FROM AUTHOR]