To what extent does preferential flow regulate soil organic carbon depth distributions and processing across diverse environments?

Relative to the rest of the soil matrix, soil preferential flow (PF) disproportionately influences large-scale processes like groundwater recharge and solute flux via rapid transport of water through soil pore spaces. Despite the ubiquity of PF occurrence across land uses and climates, there is lack of understanding around the feedbacks between PF generation and carbon transport and transformation, both of which result from abiotic (e.g., water content, clay content) and biotic (e.g., root and microbial activity) processes. We can assess the depth distribution of soil carbon parameters in relation to PF occurrence through soil profiles to decipher the effects of PF generation on carbon dynamics. Recent work from our group shows that the probability of PF occurrence increases with rainfall intensity, net primary production, and clay content. In addition, total rainfall amount increases PF propagation to deeper soil layers. We hypothesize that these same factors increase the transport of carbon deeper into the subsoil and influence carbon transformation at depth. Specifically, we hypothesize that high peak rainfall intensity, net primary production, and clay content will be associated with greater availability of extractable organic carbon relative to total soil organic carbon (SOC) at depth due to rapid transport via PF, high root abundance, and high retention of mineral-associated SOC, respectively. To identify the dominant drivers of PF and the resulting dynamic feedbacks with soil carbon, we synthesized metrics of soil PF, carbon, and enzyme activity from observations at eight U.S. sites spanning climatic gradients, land-cover types, and soil orders. We derived PF metrics using a soil water velocity threshold method based on observations of high frequency soil moisture and precipitation data. PF occurred when the observed soil water flux exceeded the saturated hydraulic conductivity of the soil matrix estimated from a pedotransfer function. Preliminary findings indicate that land-cover is a primary control on extractable DOC availability and propagation with soil depth. PF may also be associated with DOC transformations, given the coupling of land cover and net primary production. Understanding which, and to what extent, abiotic and biotic factors promote generation of PF through feedbacks with soil carbon dynamics advances our knowledge of the potential effects of changing climate and land use on water and carbon fluxes through soils.