<p>Soil organic carbon (OC) is one of the most important terrestrial carbon pools and plays a major role in climate regulation, water quality, provisional services, and numerous other ecosystem functions. The conversion of natural vegetation and the supporting soil to intensively managed agricultural systems put soil at risk for loss due to erosion and enhanced microbial degradation with loss rates increased by orders of magnitude above the pre-managed system. The process has negatively impacted agricultural productivity on hillslopes by diminishing soil health, as well as the quality of stream water and coastal aquatic environments, and it is an important but as of yet poorly quantified factor in the region’s terrestrial C budgets. There have been substantial debates on the role of erosional and depositional processes on the landscape as a control on exchange of C between the land surface and the atmosphere. A central aspect of the debate stems from the limited data regarding the fate of soil erosion-induced transport of OC through stages of detachment and splash, transport and redistribution, deposition and burial. The overarching purpose of this thesis is to evaluate how dynamic patterns of soil OC erosion due to intensive agricultural management influences soil aggregate strength, the chemical nature of mobilized organic particles, and connectivity and sourcing between hillslope and streams. Using both simulated and natural, short-term, event-based erosive rainfall processes, with a multiproxy geochemical approach, we attempt to develop a comprehensive understanding of how upland watershed mechanistic controls soil movement and associated chemical alterations to the material exported through dissected segments from hillslope to the fluvial network. </p> <p>Our results demonstrate that erosive processes on hillslope connects between terrestrial sources to receiving potential deposition settings, actively ‘filter’ soil aggregates and particles and associated OC at each erosional stage (i.e., detachment and transport downhill/downstream), with distinct geochemistry in low relief and poorly drained agricultural systems, like the CCW. Complex interactions among tillage intensity, tillage practice-induced, oriented surface roughness, and storm-induced hydrological connectivity, that potentially impact the fate of these transported OC upon decomposition, deposition and burial, and have important implications for predicting landscape level heterogeneity in surface and buried soil chemistry upon mobilization and burial, as well as the dynamics of sourcing and transformation of material exported to inland water systems.</p>

<p dir="ltr">In the Midwest, tile drainage is used to lower water tables and remove excess water from the soil to improve crop production. This network of underground pipes (i.e., tiles) and expansive agriculture also increases nutrient export, contributing to ecological harm in local lakes and rivers and further downstream in the Gulf of Mexico. Conservation practices that avoid, control, or trap nutrients can mitigate these losses, but studies quantifying their impact at the watershed scale are challenging. This work uses water quality monitoring data collected throughout the Midwest to identify potential nutrient sources and pathways, the hydroclimatic variables influencing them, and the effects of conservation practices. In a study in northeast Indiana, nutrient travel times for total phosphorus, soluble reactive phosphorus, nitrate, and dissolved organic carbon were observed to be faster during winter storm events, likely due to a lack of vegetative processes. Tile drains were the primary contributor to in-stream nitrogen and phosphorus during spring storms but were not a primary contributor for phosphorus in the winter. Data from nitrate sensors across the Midwest were used to quantify the effect of sampling frequency on hysteresis and flushing indices, showing that sampling intervals greater than 8 hours estimates could lead to inaccurate values, and that caution should be used when interpreting outcomes when using longer sampling intervals. Wet antecedent conditions were associated with a dilution pattern of nitrate during storm events, and tile drainage exacerbates this by causing greater leaching during wet periods. A systematic review of water quality monitoring studies at the watershed scale showed the limits using current data, and suggested how providing better statistics could be used to facilitate a more robust meta-analysis to determine effect sizes and sources of heterogeneity among studies. In a monitoring study located in the central Indiana, agricultural conservation practices reduced nitrate concentrations by 27% in an artificially drained watershed. While tile drainage is a critical pathway for nutrients in the Midwest, the combined effect of various conservation practices can improve water quality at the watershed scale.</p>