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Climate models project changing patterns of precipitation and increases in temperature that modify soil moisture dynamics. Land use and changing frequency and intensity of precipitation can induce changes in soil structure and rooting abundances at timescales shorter than commonly considered. Soil structure is a critical ecosystem that governs water flow through soil profiles and across landscapes, and can influence weathering rates and thus solute release and soil development. We hypothesize that the altered soil structure and modification of rooting depth distributions linked to land use change can influence soil solute concentrations, and that those shifts in solute release are dependent on patterns of precipitation. We installed suction lysimeters to collect soil water for ~3 y in two grassland regions with distinct mean annual precipitation (800 mm y-1, 1100 mm y-1) in native prairie, agriculture, and post-agriculture land uses at depths of 10, 40, and 120 cm. We linked solute concentrations to soil moisture, aggregate-size distribution, pore geometry, and rooting depth distributions to assess how land use change and the altered rooting abundance it imposes can modify soil structure and hydrologic fluxes, and to infer how soil weathering can shift deep in the subsurface. We reveal how soil moisture residence time and the soil pore network can govern solute production, and the importance of precipitation and thus of soil moisture accumulation over growing seasons for mineral weathering and solute production. Specifically, we find that the solubility potential of multiple weathering products and organic carbon increases with precipitation, dominance of relatively small aggregates at the surface, and fewer coarse roots. Enhanced solute concentrations at depth may also reflect transport down-profile. Our findings reveal unintended consequences of land use change that influence important hydrologic dynamics and nutrient cycling in the vadose zone and how deeply and how persistently unexpected consequences of changes in land cover can propagate. 

Aspect influences critical zone (CZ) function, particularly in mountainous terrain where it is an ecosystem-defining geographical feature. Distinct insolation across aspects is linked to differences in water availability and flows, land cover and vegetation productivity, soil thickness and rooting depths, frost cracking, weathering rates, and solute concentrations. Relatively few studies have explored any changing influence of aspect on vegetation productivity, which governs soil water storage and runoff. We probe the hypothesis that the productivity benefit of growing on aspects with greater radiation inputs in mountain systems has been declining over the past few decades as warming has accelerated. We quantify how forest productivity varies with aspect from 1985 to 2021 across the world’s mountain ranges using a monthly-averaged, satellite-derived measure of greenness (NDVI). Globally, most montane forests exhibited increasing greenness over time. Mountainous forests ~15° to ~40° latitude N or S of the equator exhibited behavior consistent with our hypothesis by increasingly favoring shadier aspects, particularly during growing seasons when rainfall and soil moisture can be limiting to productivity. In contrast, closer to the poles where climates are coolest and aspect has an even greater influence on annual solar radiation, the benefit of a sun-facing aspect appears to be increasing across all seasons, consistent with poleward forest community migration hypotheses. We also demonstrate greater increases over time in montane forest greenness on east-facing slopes compared to west-facing slopes; north of ~40° latitude this pattern appears less robust. These observations reveal that it is increasingly disadvantageous for montane forests growing on sunnier, hotter aspects at relatively low latitudes during the hottest times of the year. Given known linkages between ecosystem productivity and CZ functions like water storage, provision, and flows, soil development, solute production, and regolith thickness, these analyses cast light on yet-underappreciated consequences of a rapidly warming climate on Earth’s montane forests and their capacity to shape CZ processes. 

Clarifying the mechanisms that control variability in the spatial distribution of soil organic carbon (SOC) is key to accurate estimates of soil C fluxes. Mobile organic C (MOC), here defined as the fraction of SOC that is not strongly bound to mineral surfaces but that can be transported hydrologically as dissolved or particulate organic C, represents the portion of SOC whose residence time can be modulated via movement down profiles and across landscapes. The relationship between the spatial arrangement and turnover time of SOC is especially evident in the widely observed correlation between soil depth and mean residence time; deeper SOC tends to persist for relatively long periods in the profile.  Moisture can promote microbial mineralization of SOC to CO2, but water also can transport MOC throughout profiles and landscapes. Controls on the movement of MOC have not been fully elucidated however, and the relationship between MOC and the spatial arrangement of SOC has not been thoroughly explored. Using data collected from five distinct ecosystem types across North America we evaluate the hypothesis that moisture dynamics throughout the soil profile as driven by seasonality, vegetation productivity, and topographical position influence the spatial distribution of MOC, and thus the observed heterogeneity of SOC and its persistence. We demonstrate that, in soils with surplus water availability and structural features that permit sufficient flow, transport drives the accumulation of disproportionately large concentrations of MOC deep in the profile and in downslope topographical positions. Our results further demonstrate that the vertical and lateral transport of MOC is also regulated by variation between energy- and water-limited systems in the amount of seasonally-available water moving through the profile: at times and in places where relatively more surplus water is available, MOC is more readily translocated. Excursions from these patterns of transport and accumulation result from soil textural and structural characteristics that immobilize organic C or inhibit flow. These findings reveal the nuances of how soil moisture dynamics regulate vertical and lateral distributions of MOC, thereby promoting the development of heterogeneous SOC stores as well as deep, relatively persistent SOC pools.