Role of a grass riparian zone in controlling the fate of nitrogen in a poorly drained agricultural landscape Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/7p88cj59f

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  • Riparian ecosystems, through their unique positions in the agricultural landscape, have the potential to mitigate nutrient loading to streams. This study was conducted to gain a better understanding of N cycling in a poorly drained Grass Riparian Zone (GRZ) and adjacent Perennial Ryegrass seed Field (PRF) in the Willamette Valley, Oregon, U.SA. Tracer studies showed higher recovery of ¹⁵N in the PRF than the GRZ due to higher plant uptake and soil retention by the PRF. Prolonged flooding in the GRZ most likely resulted in higher denitrification and lower plant uptake and soil retention of ¹⁵N. More PRF plant biomass N was available for mineralization and NO₃⁻ accumulation during the summer. Shallow (30-45 cm) and deeper (135-150 cm) soil water NO₃⁻ concentrations were higher in the PRF than the GRZ. Dilution, from precipitation, and biological consumption were most likely responsible for decreases in soil water NO₃⁻ seen in the PRF and as water moved into the GRZ. When soil water NO₃⁻ that was not processed in the PRF entered the GRZ, it was consumed within 6 m in shallow soil water and 17 m for deeper soil water. High carbon and low oxygen in the shallow soils of the GRZ most likely contributed to highest denitrification potential (DNP) rates just inside the GRZ. Dissimilatory nitrate reduction to NH₄⁺ (DNRA) rates in the GRZ surface soil indicated this process together with plant uptake and denitrification, could easily account for the consumption of NO₃⁻ in incoming shallow soil water. Differences in the management of the GRZ and PRF translated into differences in subsoil carbon and NO₃⁻ but did not influence DNP or DNRA rates in the subsoil. Based on measured DNP rates, it was unlikely that denitrification alone could have accounted for decreases in deep soil water NO₃⁻. The discrepancy between DNP rates and loss of soil water NO₃⁻ may have been due to DNRA or unmeasured hotspots of denitrification activity. This study showed that the low maintenance, poorly drained grass riparian zone supported conditions that promoted the consumption of soil water NO₃⁻.
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