ABSTR ACT: Long-term row crop agricultural production has dramatically reduced the pool of soil organic carbon. The implementation of cover crops in Midwestern agroecosystems is primarily to reduce losses of nitrogenous fertilizers, but has also been shown to restore soil carbon stocks over time. If labile carbon within agricultural soils could be increased, it could improve soil health, and if mobilized into subsurface drainage, it may positively impact watershed biogeochemistry. We tested for potential differences in water-extractable organic carbon (WEOC) at two different soil profiles (0-5 cm and 5-20 cm) between plots planted with cereal rye/daikon radish (cover crop), corn, and zero control (no vegetation) within the Illinois State University Research and Teaching Farm. We also tested for potential differences in denitrification within the upper soil profile throughout the growing year. We modeled excitation-emission matrices from soil cores through parallel factor analysis. We found no difference in WEOC concentrations between each crop treatment (P = 0.2850), but concentrations of WEOC were significantly lower in the 5-20 cm profile than that in the upper (0-5 cm) profile (P = 0.0033). There was a significant increase in WEOC after each treatment in samples after cover crop termination. The parallel factor analysis model found humic and fulvic acids to be the dominant fractions of WEOC in all soils tested. Humic and fulvic acids accounted for ~70% and 30% of model variation. Denitrification rates did not differ across treatments (P = 0.3520), which is likely attributed to soil WEOC being in limiting quantities and in primarily recalcitrant fractions. After three years, cover crops do not appear to alter soil WEOC quantity and type. Restoring the availability of carbon within agricultural soils will not be a short-term fix, and fields will likely be a net carbon sink, contributing minimal labile carbon to receiving waterways.
Nutrient stoichiometry within a wetland is affected by the surrounding land use, and may play a significant role in the removal of nitrate (NO3–N). Tile-drained, agricultural watersheds experience high seasonal inputs of NO3–N, but low phosphorus (PO4–P) and dissolved organic carbon (DOC) loads relative to surface water dominated systems. This difference may present stoichiometric conditions that limit denitrification within receiving waterways. We investigated how C:N:P ratios affected denitrification rates of sediments from tile-drained mitigation wetlands incubated for: 0, 5, 10, and 20 days. We then tested whether denitrification rates of sediments from surface-water and tile-drained wetlands responded differently to C:N ratios of 2:1 versus 4:1. Ratios of C:N:P (P < 0.05) and incubation length (P < 0.05) had a significant effect on denitrification in tile-drained wetland sediments. Carbon limitation of denitrification became evident at elevated NO3–N concentrations (20 mg L−1). Denitrification measured from tile water and surface water wetland sediments increased significantly (P < 0.05) at the 2:1 and 4:1 C:N treatments. The results from both experiments suggest wetland sediments provide a limiting pool of labile DOC to maintain prolonged NO3–N removal. Also, DOC limitation became more evident at elevated NO3–N concentrations (20 mg L−1). Irrespective of NO3–N concentrations, P did not limit denitrification rates. In addition to wetting period, residence time, and maintenance of anaerobic conditions, the availability of labile DOC is playing an important limiting role in sediment denitrification within mitigation wetlands.
Watershed biogeochemistry throughout Midwestern agroecosystems has been altered through hydrologic manipulation and over-application of nitrogenous fertilizers.As a result, nitrate (NO 3 -N) export from subsurface drainage has negative impacts on local and downstream ecosystem health. Wetland installation has proven to be a viable option for targeted management where a large proportion of NO 3 -N is removed through the bacterially mediated process denitrification. For denitrification to maintain high rates under prolonged NO 3 -N saturation, a stable supply of labile dissolved organic carbon (DOC) is required. The focus of this dissertation was to study how the stoichiometry of agricultural wetlands limits denitrification within a controlled laboratory and field-scale applications.Denitrification rates within wetlands that retain subsurface tile drainage were limited by the availability of DOC. The limitation of DOC became more evident under high NO 3 -N concentrations, suggesting that wetland sediments have an insufficient pool of labile DOC to maintain elevated rates of denitrification during seasonally intense NO 3 -N inputs. It was than hypothesized that terrestrial DOC contributions delivered by surface water would serve as an effective DOC subsidy for denitrifying bacteria. It was found that wetlands retaining drain tile or surface water exhibited similar changes in denitrification to 2:1 and 4:1 C:N ratios.Bacterial production and denitrification were measured under low (1:1) and high (4:1) C:N ratios to test how bacteria allocate DOC at differing ratios. There was no change in denitrification in sediments incubated at the 1:1 ratio, while bacterial production significantly increased throughout the incubation period. The 4:1 ratio did result in a significant increase in denitrification rates and bacterial production, but bacterial production did not differ between the 1:1 and 4:1 treatments.Changes in NO 3 -N removal and denitrification in response to DOC availability (1 mg/L and 10 mg/L) were observed throughout replicate 5 days. Denitrification rates increased significantly at both DOC concentrations throughout the study period.Reductions in NO 3 -N concentration were observed at low and high DOC availability, however significant reductions only occurred at the 10 mg/L DOC treatment wetlands.The contribution cover crops have on WEOC in agricultural soils was tested by modeling spectral data from dissolved C fractions extracted from soil cores. Within the first years of cover crop implementation, the amount and types of WEOC did not differ between plots with or without cover crops. This also translated into similar rates of denitrification observed throughout the study period, suggesting that terrestrial denitrification was limited by C availability, similar to receiving aquatic systems.
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