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Characterizing the structure of the critical zone (CZ), which is often tens of metres deep, is challenging and crucial to understand earth surface processes. Existing findings on the effect of topography on CZ thickness focused exclusively on mountainous regions with steep slopes, not considering low hilly regions with gentle slopes. Seismic refraction and borehole surveys were conducted to characterize the effects of topography on a red soil CZ structure in the low hilly region of Jiangxi Province, subtropical China. This CZ is derived from Quaternary red clay (QRC) overlying Cretaceous sandstone bedrock. Regolith with a thickness of 8.2–9.5 m consisted of an upper layer of QRC (4.8–6.2 m thick) and a lower layer of weathered sandstone (2.6–4.0 m thick). The boundary between weathered and unweathered sandstone was identified by a Ca depletion profile and calcite weathering front. The boundary between the QRC and weathered sandstone was identified by morphological and textural features and a feldspar weathering front. The QRC had seismic refraction velocities <1.3 km s−1, the weathered sandstone had velocities of 1.3–1.7 km s−1, while the sandstone bedrock had velocities >1.7 km s−1. Following the profile sequence downslope, regolith thickness decreased linearly with decreasing ground elevation. The linear fitted regression slope was <1. This was predominantly determined by QRC thickness, which in turn was mainly controlled by the difference between slopes of ground and bedrock surface and by surface erosion. Following the profile sequence across the slope, regolith thickness decreased linearly with decreasing ground elevation, with a slope of ~1. This was controlled by QRC thickness, which in turn was mainly controlled by surface erosion only, since the bedrock surface was nearly level. These results suggest that topography can control CZ structure in hilly regions with gentle slopes. [ABSTRACT FROM AUTHOR]

Depth patterns of soil microbial distribution have not been well characterized and little is known about the balance between stochastic and deterministic processes in shaping microbial community through soil profiles. We examined how the composition, diversity, and assembly processes of bacterial communities change to a depth of 4 m along three cores sampled through a typical upland red soil Critical Zone in subtropical China. The sampled soils at the center of the Critical Zone were acidic, highly weathered, and clay loam to clay in texture, and developed from Quaternary red clay. Bacterial richness and phylogenetic diversity decreased with depth in the upper soil zone (0–90 cm), but was constant in the deeper zone (90–420 cm). In both the upper and deeper zones, richness was strongly and positively correlated with total organic carbon, dissolved organic carbon and total nitrogen. Depth-dependent changes of community structure were observed above the upper but not deeper zones. Stochastic processes were more important in the upper zone, whereas deterministic processes were dominant in the deeper zone. The distinct depth-dependent patterns of bacterial communities and assembly processes exist through deep soil profiles and are influenced by both contemporary and historical pedogenetic factors and processes. [ABSTRACT FROM AUTHOR]

Abstract Nitrate accumulation has been reported in the top 1 m and subsurface soil (>1 m) across arid to semi-humid regions, but not in humid regions. Nitrate inventories through the whole regolith, referred to collectively as soil and saprolite, in humid regions have received little attention to date, likely due to previously assumed low nitrification rates and large nitrogen (N) losses by severe surface runoff and erosion. In order to understand if and how reactive N exists in the below ground (soil and saprolite) in humid environment, the amount of NO 3 −-N, NH 4 +-N and dissolved organic N (DON) present in the regolith to a depth of 9 m in a typical red soil Critical Zone was investigated under different land uses (upland, woodland and paddy field). The Red Soil Critical Zone Observatory is located in the subtropical Jiangxi Province, China, with a mean annual precipitation of 1795 mm and mean annual potential evapotranspiration of 1229 mm. The examined regoliths were acidic, highly weathered, and mainly clay loam to clay in texture. Results showed that on average 92% (827 ± 97 kg N ha−1) of NO 3 −-N and 82% (521 ± 153 kg N ha−1) of DON were stored at depth (from a depth of 1 m to the bedrock surface) in the upland regolith, while 92% (283 kg N ha−1) of NO 3 −-N and 78% (820 kg N ha−1) of DON were stored at depth in the woodland regolith. Nitrate N significantly accumulated with depth in the upland regolith from the 1- to 4-m depth interval (p < 0.01), while the inventory (632 ± 75 kg N ha−1) in the top 3-m zone accounted for on average 71% of the total. Dissolved organic N significantly accumulated with depth in the upland regolith from the 0- to 3-m depth interval (p < 0.01), while the inventory (408 ± 75 kg N ha−1) in the top 3-m zone accounted for on average 64% of the total. There was no significant accumulation for NH 4 +-N throughout the upland regolith (p = 0.35). No substantial accumulation of dissolved N was measured at depth in paddy field regoliths with different cultivation ages. The finding that large reservoirs of reactive N can exist in deep regolith rather than in the routinely investigated solum of subtropical regions shows a missing part of the terrestrial N budget and raises concerns about potential groundwater nitrate pollution. Graphical abstract Reactive nitrogen accumulates beneath 1 m depth. Unlabelled Image Highlights • Reactive nitrogen accumulates at depth in the red soil Critical Zone. • The majority of reactive nitrogen is stored below 1 m in the deep regolith. • Land uses play a key role in determining reactive nitrogen accumulation. • Pattern of nitrate accumulation negatively correlates with porosity variation. [ABSTRACT FROM AUTHOR]

• Nitrate sources from surface water through regolith to groundwater were traced. • Nitrate with light N isotopes was preferentially adsorbed by variable charge soils. • Soil organic N contributed highest to the upland and paddy subcatchment streamflow. • Chemical fertilizers contributed highest to the groundwater and regolith. • Most of the nitrate in the Critical Zone was from CF and SON. Nitrate contamination of water bodies is a serious concern in regions with intensive agriculture. Previous studies, which only considered surface water and groundwater but not the regolith, do not provide a comprehensive accounting of nitrate sources and their proportional contributions in the Critical Zone, which extends from the top of the vegetation canopy down to the bottom of the fresh groundwater. In this study, dual nitrate isotopes, hydrochemical compositions and a Bayesian isotope mixing model were used to investigate nitrate sources, processes and individual source contributions across surface water, regolith and groundwater at the Red Soil Critical Zone Observatory of China. Denitrification was found to be weak in the Critical Zone. Dilution processes were important in eliminating nitrate from streamflow and groundwater. More than 90% of the nitrate inventory was stored at depths from 1 m to the bedrock surface in the upland regolith. Nitrate with light nitrogen (14N) isotopes was preferentially adsorbed by the variable charge soils with the help of weak electrostatic attraction. The contribution of soil organic N (SON) to the nitrate in the upland catchment streamflow was found to be highest with a mean of 56%, followed by manure and sewage (MS, 26%), NH 4 + in chemical fertilizers (CF, 17%). For the paddy subcatchment streamflow, the contribution of SON (68%) was highest, followed by CF (19%) and MS (13%). In contrast, for the groundwater, the contributions of CF and SON were 60% and 36%, respectively. For the regolith, the contributions of CF and SON were 72% and 25%, respectively. Thus the most important sources of nitrate were SON for the streamflow and CF for the groundwater and regolith. These findings show that the different partitioning and isotopic fractionation within the regolith strongly affect the sources of nitrate reaching groundwater, compared to the sources reaching surface water. [ABSTRACT FROM AUTHOR]

• Controls on nitrate accumulation at depth in variable charge soils were revealed. • Regolith pH, crystalline Fe, AEC and bulk density dominated nitrate accumulation. • The greater amount of Fe oxides gave its larger contribution than Al oxides. • Nitrate accumulation was controlled by adsorption and low hydraulic conductivity. Nitrate accumulation at depth in variable charge soils in humid tropics and subtropics affects nitrogen (N) leaching to groundwater. However, the mechanisms on the nitrate accumulation in relation to regolith (soil and saprolite) properties are poorly understood. In this study, four upland regoliths were selected in a typical red soil Critical Zone of humid subtropical China. These regoliths with a thickness of 6–8 m, which developed from Quaternary red clay underlain by sandstone bedrock, were acidic, highly weathered, and clay loam to clay in texture. We analyzed the relationships between nitrate concentrations and the regolith physiochemical properties. The results showed that regolith pH was negatively correlated with nitrate concentrations, whereas crystalline iron (Fe) and aluminum (Al) and anion exchange capacity (AEC) were positively correlated with nitrate concentrations at depths below both soil surface and 1 m. Bulk density (BD) was significantly related to nitrate concentrations only at depths below soil surface, while depth, clay, amorphous Fe and Al, total organic carbon (C), and total N were significantly related to nitrate concentrations only at depths below 1 m. There was no significant correlation between nitrate concentrations and sand, silt, silicate bound Fe or Al, C/N, or moisture at depths below both soil surface and 1 m. Based on the random forest analysis, the relative contributions of pH, crystalline Fe, depth, AEC, BD, crystalline Al and amorphous Fe to explaining nitrate concentration variations were 28.3%, 9.5%, 4.3%, 3.4%, 2.9%, 2.8% and 2.6%, respectively. The concentration of crystalline Fe was ten times that of crystalline Al, which would account for its larger contribution to the nitrate variations. Nitrate accumulation at depth was most likely controlled both by electrostatic adsorption on crystalline Fe and Al oxides and low hydraulic conductivity of the regolith at depth, which appeared to be largely influenced by regolith pH and competitive anions. Our findings shed light on regolith internal property controls on nitrate legacy in the subtropical vadose zone and may help control nitrate contamination of groundwater in intensified agricultural regions. [ABSTRACT FROM AUTHOR]