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Microbes inhabiting deep soil layers are known to be different from their counterpart in topsoil yet remain under investigation in terms of their structure, function, and how their diversity is shaped. The microbiome of deep soils (>1 m) is expected to be relatively stable and highly independent from climatic conditions. Much less is known, however, on how these microbial communities vary along climate gradients. Here, we used amplicon sequencing to investigate bacteria, archaea, and fungi along fifteen 18‐m depth profiles at 20–50‐cm intervals across contrasting aridity conditions in semi‐arid forest ecosystems of China's Loess Plateau. Our results showed that bacterial and fungal α diversity and bacterial and archaeal community similarity declined dramatically in topsoil and remained relatively stable in deep soil. Nevertheless, deep soil microbiome still showed the functional potential of N cycling, plant‐derived organic matter degradation, resource exchange, and water coordination. The deep soil microbiome had closer taxa–taxa and bacteria–fungi associations and more influence of dispersal limitation than topsoil microbiome. Geographic distance was more influential in deep soil bacteria and archaea than in topsoil. We further showed that aridity was negatively correlated with deep‐soil archaeal and fungal richness, archaeal community similarity, relative abundance of plant saprotroph, and bacteria–fungi associations, but increased the relative abundance of aerobic ammonia oxidation, manganese oxidation, and arbuscular mycorrhizal in the deep soils. Root depth, complexity, soil volumetric moisture, and clay play bridging roles in the indirect effects of aridity on microbes in deep soils. Our work indicates that, even microbial communities and nutrient cycling in deep soil are susceptible to changes in water availability, with consequences for understanding the sustainability of dryland ecosystems and the whole‐soil in response to aridification. Moreover, we propose that neglecting soil depth may underestimate the role of soil moisture in dryland ecosystems under future climate scenarios. [ABSTRACT FROM AUTHOR]

Bulk density (BD) is one of the most important soil physical properties in the hydrological processes of Earth's Critical Zone (CZ). The heterogeneity and related controlling factors of surface BD have received extensive research attention. However, it is unclear whether the effects of controlling factors on BD are homogeneous or vary spatially. This study quantified topsoil BD with a high‐density sampling strategy (10 m × 10 m; n = 1134) across the ShuangChaGou watershed in the critical zone of the Chinese Loess Plateau (CLP‐CZ). Stepwise multiple linear regression (SMLR), regression kriging (RK), random forest (RF) and geographically weighted regression (GWR) were established as modelling approaches to (i) compare the performance of these four models in explaining BD variation, (ii) assess the main factors driving BD, and (iii) evaluate the spatial non‐stationary relationship between BD and its explanatory variables. GWR explained the BD variation to the same extent as the RK model (35%), outperforming both the SMLR (15%) and RF models (19%). The variation in topsoil BD depended mainly on: soil organic carbon (SOC) content, normalised difference vegetation index (NDVI), plan curvature, and slope length. The spatial distribution of the local R2, regression coefficient magnitude, significance and sign calculated by GWR revealed that the effects of variables accounting for variation in BD varied spatially. The explanatory power of these variables in terms of BD variation was strongest in northern and northeastern areas of the watershed and weakest in southern areas of the watershed. Both significant negative and positive correlations between BD and silt content, NDVI, and plan curvature were found. This study enriched traditional research linked to the complicated relationships between BD and controlling factors by incorporating spatial non‐stationarity, which is useful for improving the accuracy of hydrological modelling in the CLP‐CZ and in similar CZs worldwide. Highlights: Non‐stationary relationships between BD (N = 1134) and its explanatory variables were captured at the watershed scale.GWR model performed on par with the RK and better than the SMLR or RF model.Silt content, NDVI, and plan curvature showed both significant negative and positive effects on BD.Spatial non‐stationarity of BD should be considered for improving the performance of hydrological modeling. [ABSTRACT FROM AUTHOR]

• Deep rooted vegetation significantly leads to soil water deficit with the depth and precipitation. • Relative deep soil water deficit in artificial forest decreases with precipitation increase. • Precipitation self-regulation responses to deep soil water deficit in space. • Soil water deficit is more sensitive in the sub-shallow soil layers at a depth of 1800 cm. In semi-arid and semi-humid regions, deep soil water resources are crucial for vegetation restoration to mediate the ecosystem sustainability. Yet it is still limited on understanding soil water deficit (SWD) caused by vegetation restoration and its response to soil depth and precipitation. To obtain a more accurate measurement of regional SWD, we conducted a two-year soil water monitoring to a depth of 1800 cm under typical vegetation restoration with a precipitation gradient ranging from 300 mm/y to more than 600 mm/y across the Loess Plateau of China. With increasing precipitation gradient, soil water storages in 0–1800 cm depth under both arable cropland and artificial vegetation significantly increased (p < 0.05). Compared to arable cropland, artificial vegetations (forest and shrub) had a significantly greater SWD regardless of either precipitation gradient or soil depth. In particular, the deep-rooted vegetations (R. pseudoacacia and C. korshinskii) on the Loess Plateau demonstrated the greatest SWD in the 0–1800 cm layer due to their great water consumption. Indeed, a positive relationship between the accumulated SWD and the accumulated root biomass was detected, suggesting that the increased root biomass in soil depth may promote soil water consumption. In addition, the relative SWD in artificial vegetation at a depth of 1800 cm decreased significantly (p < 0.05). These findings reveal that there is a mechanism of precipitation self-regulation on the deep SWD, of which is more sensitive in the sub-shallow soil layers. We conclude that the SWD depends on vegetation restoration, soil depth, and precipitation self-regulation, offering a reference for how afforestation affects deep soil water in the Loess Plateau region. This finding also benefits to mediate water balance processes and develop sustainable land management in water-limited regions around the world. [ABSTRACT FROM AUTHOR]

• Spatiotemporal DSL variation and associated forcings were explored at a semi-humid catchment. • Land use type was the dominant control on DSL variation at a catchment scale. • DSL response to meteorological factors differed between the semiarid and semi-humid catchments. • The mixed "forest/shrubland + grass" plantation type is recommended for the semi-humid catchment. The formation of dried soil layers (DSLs) is a broad consequence of climate change (i.e., through temperature increases and/or precipitation decreases) and poor land management practices, where these layers exist has severe negative effects (i.e., retardation of vegetation, negative water cycle balance) on the sustainability of restored ecosystems and health of agriculture. Because of the effort and time required, only sporadic in situ measurements on spatial and temporal DSL variation have been reported, particularly at a catchment scale. However, this information is essential for understanding mechanisms associated with evolutionary DSL processes as well as developing effective land management techniques. Accordingly, this study monitored soil water content to a 5 m depth over a 5-year period in a semi-humid catchment on the Loess Plateau while also investigating temporal DSL dynamics and dominant controls. In the semi-humid catchment, DSLs occurred at 90.7 % of all sampling sites. Spatially and temporally averaged DSL formation depth (DSLFD), DSL thickness (DSLT), and DSL soil water content (DSL-SWC) within the 0–5 m profile were 170 cm, 262 cm, and 9.52 %, respectively. Forest and shrubland DSL regimes were drier than those of grassland. These three DSL indices exhibited significant correlation with temperature variables, while only DSLFD variables significantly correlated to precipitation. Land use type was the predominant control of DSL variation at a catchment scale. A spatial DSLT variation comparison under different land use treatment positions determined the feasibility of planting grass at an upslope position and mixed forest/shrubland + grass at mid-slope and downslope positions in the semi-humid catchment, which provides a good case study for afforestation practices at a catchment scale. Findings from this study are intended to aid in catchment soil and water conservation, soil drought mitigation, and sustainable vegetation management practices on the Loess Plateau as well as other water-limited regions worldwide. [ABSTRACT FROM AUTHOR]