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Linking biotic drivers of soil structure to solute flows through soil profiles
Recent work highlights the powerful role that biota (here, roots and the microbes that surround them) play in governing soil structure over timescales much shorter than previously assumed. If robust across ecosystems, this observation suggests that the rapid response of biota to anthropogenic forcings (e.g., land use change, climate change) may impose yet-faster changes to soil structure in the Anthropocene. Such changes have great import for flows of water, nutrients, and dissolved organic carbon through soil profiles, and thus for ecosystem productivity and soil carbon fates. As part of a suite of projects at multiple sites in the U.S. funded by the NSF and the USDA, we seek participation in EMSL’s MONet project to provide detailed descriptions of soil properties in select locations and thus aid in hypothesis testing (see below). We propose to collect cores for MONet analyses at two established research sites: Coal Creek in Colorado’s Rocky Mountains, and Reynolds Creek in Idaho. At these sites, we already have quantified rooting depth distributions every 1 cm to the base of the accessible profile. At Coal Creek, ongoing research quantifies soil [CO2] and [O2], soil moisture and temperature, and sapflux from canopy dominants. The graduate student who will benefit from MONet data has a funded project at Coal Creek exploring the role of transpiration-induced soil water movement in governing spatial heterogeneity of dissolved organic carbon; MONet data will aid in his data interpretation. At Reynolds Creek, ongoing research is probing the role of juniper encroachment into sagebrush stands in modifying soil water and carbon dynamics. There, we are currently generating the same rooting depth distributions already obtained for Coal Creek, and soil pits will be instrumented in late May 2023. The graduate student who will benefit from MONet data is quantifying the rates of aggregate formation and collapse in a lab setting; MONet data will help her expand her work to a field setting. At both places, basic soil chemistry and physical attributes have been quantified and assays to quantify aggregate size distributions are in process. We seek to augment our understanding of biotic influences on soil attributes via MONet data. Specifically, we hypothesize that because high relative abundances of roots and microbes are linked to relatively plentiful dissolved organic C and microbial necromass, in these locations we see promotion of relatively large soil aggregates and greater microporosity. We further hypothesize that these structural features are linked to greater hydrologic flow from surface horizons down-profile, resulting in greater depth distribution of surface horizon dissolved organic C. We therefore predict that MONet-provided soil hydraulic properties will exhibit a greater propensity for water flow where mean aggregate diameters (which we will quantify) are larger, that these tendencies will be exhibited more so where roots and microbes (and possibly FTICR and amino sugar estimates of necromass) are more abundant, and that organic matter characteristics will reveal a greater abundance of material similar in characteristics to surface material at depth than where flow rates are more tempered by smaller pores. Probing these hypotheses depends on controlling for soil texture and depth. At each site, we will sample soil cores representing a gradient of rooting types: at Coal Creek, from forest into meadow, and at Reynolds Creek from juniper-dominated into sagebrush-dominated. If our hypotheses appear valid, we will have demonstrated the importance of root and microbial abundances to soil structure for governing water and solute flows, features that until now have been studied in disparate fields. An alternative hypothesis is that root and associated microbial turnover results in pore clogging to a greater extent than macroporosity generation. With either outcome, the work will advance our ability to project how well-documented, human modification of ecosystem rooting depths, and thus of the depth distributions of rhizosphere microbial communities, can alter soil structural characteristics critical for ecosystem water, nutrient, and carbon flows. Please note that this abstract contains more information than the research proposal itself due to word limitations.