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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]

• 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]

Soil water storage (SWS) can effectively be used as an important indicator to manage soil water resources in water-limited ecosystems, being strongly linked to land-use and land management changes. The Government of China launched the Gully Land Consolidation (GLC) project in 2011 to reclaim landforms and resolve land scarcity and food shortages in the Chinese Loess Plateau (CLP) without endangering the environment. Corollary effects of this project were alterations to original landscapes and associated hydrological processes. However, temporal dynamics in available soil water of the newly formed land for cultivation purposes remain unclear. Accordingly, this study measured SWS to a 5 m soil depth over a 5-year period at two gully sites, one newly formed under the GLC project (i.e., the experimental site) and the other left untreated (i.e., the control). SWS exhibited similarly weak temporal variation at both gully sites (coefficient of variation [CV] = 6% at the experimental site; CV = 11% at the control site). Although SWS always negatively correlated to mean temperature and wind speed (p < 0.05) with no observable hysteresis, SWS responded to precipitation at both sites with obvious hysteresis (experimental site = two month hysteresis; control site = three months hysteresis). Seasonal SWS characteristics confirmed the lag response to precipitation. Moreover, spatial SWS patterns differed distinctly at both sites. The mean proportion of available SWS to the available water holding capacity was 69% at the experimental site, indicating that stored soil water was available for crop cultivation without the threat of water shortages. Our results provide new insight into spatial and temporal SWS variation in newly formed land engineered by restoration projects. Furthermore, clarifying SWS temporal dynamics and spatial distributions based on long-term monitoring is essential to evaluate, model, and manage water resources in areas affected by anthropic activities. • Temporal SWS dynamics in the 0–5 m layer were investigated at two gully sites. • SWS response to precipitation differed between the two gully sites. • Spatial SWS patterns at the two gully sites distinctly differed. • SWS at the newly formed land site was sufficient for crop cultivation. • Spatiotemporal SWS under restoration projects is critical to land management. [ABSTRACT FROM AUTHOR]

• Surface K s variation (N = 1195) under different landscape units was explored across a watershed. • K s magnitude and variation differed significantly among Tableland, Gully-land and Slope-land. • BD, SSWC, silt, plan curvature, NDVI, and slope gradient shaped K s variations in the watershed. • Factors controlling K s variations were different under the three landscape units. • Distinct landscape units should be considered for evaluating the K s heterogeneity in a watershed. The spatial heterogeneity and related driving factors of the saturated hydraulic conductivity (K s) in the Earth's critical zones (CZs) have been documented widely in the literature. However, information regarding the effect of distinct landscape units (i.e., Slope-land, Gully-land, and Tableland) on such heterogeneity at the watershed scale in CZs characterized by complex topographic conditions remains scant. In the current study, we collected undisturbed and disturbed soil samples from the surface soil layer (0–5 cm) at 1195 sampling sites across the ShuangChaGou watershed (0.22 km2) in the Chinese Loess Plateau (CLP). According to classical and geostatistical analyses combined with the structural equation model (SEM), the mean K s followed the sequence Slope-land (0.25 cm·min−1) > Gully-land (0.187 cm·min−1) > All samples (0.186 cm·min−1) > Tableland (0.14 cm·min−1) (p < 0.05), with the K s values exhibiting moderate variations except for the Gully-land (strong variation). The LogK s spatial variation characteristics (i.e., the anisotropy ratio, effective range, and C/(C 0 + C)) considering the influence of topography varied among the landscape units and the entire watershed. The SEM identified that soil bulk density, saturated soil water content, silt content, plan curvature, NDVI, and slope gradient were the controlling factors in sequence affecting the K s variation for All samples (AS). In contrast, factors controlling K s variations were different under the Tableland, Gully-land, Slope-land, and AS. The magnitudes, spatial variations and controlling factors of the surface K s were greatly affected by the landscape units. Our study provided a robust assessment of the spatial variation of K s in CZs involving complicated topographic features, highlighting the utility of dividing distinct landscape units when evaluating the K s variation. A good understanding of this information is helpful for developing distributed hydrological models and for evaluating water and solute transportation in CZs of the CLP and similar regions around the world. [ABSTRACT FROM AUTHOR]

Soil aggregates (SA) play crucial roles in soil organic carbon (SOC) sequestration. Different SA fractions contribute differently to the sequestration of SOC. However, few studies have examined the factors controlling SA fractions and associated SOC contents across a watershed. Soil samples were collected at 0-10 cm (surface layer) and 10-20 cm (subsurface layer) from 88 sites across a semi-humid watershed (1.1 km 2 ) on the Loess Plateau, China. These samples were separated into macroaggregates (MA), microaggregates (MI), and silt + clay fractions (SC) by wet-sieving, and SOC content of each fraction was determined. The objectives were to: 1) investigate the spatial variability of SA fractions and associated SOC contents as well as their main controls across an entire watershed, and 2) explore the linkages between soil aggregation and SOC sequestration. The bulk and aggregate SOC contents of all SA fractions showed moderate variability, with coefficient of variations of 23.3-31.9%. Geostatistical analysis indicated that the spatial patterns of SA fractions and SOC content varied with aggregate size. From combined Spearman's correlation analysis and structural equation modelling, we found that soil texture was an important control on the spatial variability of all SA fractions and associated SOC contents. Vegetation dynamics and management practices associated with land use were also important controls on MA and MI and their associated SOC contents, especially in the surface layer. However, SC and its associated SOC content were more sensitive to eco-hydrological processes related to topography. Among the land uses, grassland had the greatest SOC sequestration potential. The fine roots of herbs can wrap MI in MA and increase SOC content within MA, which is the primary mechanism responsible for SOC sequestration in grasslands. These results indicate that using vegetation with fine root systems for restoration is a good strategy to increase SOC sequestration in this region., Competing Interests: 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 paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)