Root distributions, precipitation, and soil structure converge to govern soil organic carbon depth distributions

The depth distribution of soil organic carbon (SOC) is governed by the interaction of many ecosystem features, including differential C inputs in shallow and deep soils and the redistribution of C via water flow through the profile. In C-rich Mollisols in particular, we need to better understand the degree to which the conversion of native prairie to cultivated lands is changing C loss and retention. We probed multiple mechanisms driving these processes using two approaches: one leverages a regional-scale dataset derived from the Natural Resources Conservation Service (USDA-NRCS) National Cooperative Soil Survey (NCSS) Characterization Database; and a second focusses on a local-scale, more detailed dataset representative of the climatic and land-use gradients invoked in the larger database. The first approach focused on parameterizing SOC depth distributions of Mollisols across a climatic gradient in the US Midwest to investigate how land use and effective precipitation affects vertical gradients of SOC. The second approach furthered the investigation of SOC depth distribution drivers by quantifying biological, physical, and chemical properties of multiple soil profiles across Kansas, US. SOC declined more gradually with depth as water availability increased in native prairie soils, prompting the hypothesis that increased water flow through the profile carries C to deep layers, particularly where high root abundances promote soil porosity. Analyses of multiple soil profiles indicate that surficial changes driven by land conversion propagate their influence to deep soil horizons in ways significant for the coupling of C cycling across depths. Our findings support the hypothesis, and specifically suggest linkages between decreased root abundances and increased flows of soluble C downward under agriculture, and associated changes in soil structure that affect the propensity of SOC to form aggregates. The interplay between rooting depth abundances and water availability in different land uses thus appears to influence the arrangement of soils particles and voids in ways important for vertical water flow and C transport. Our work illuminates the convergence of multiple important mechanisms driving changes in the shape of SOC depth distributions across timescales shorter than typically assumed, with consequences for projecting soil C cycling and storage in the Anthropocene.