N processing in grass seed crops differing in soil drainage and disturbance in western Oregon, U.S.A. Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9s161883d

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  • A better understanding of grass seed crop and soil fertility is necessary to improve fertilizer practices and preserving water quality in Willamette Valley, Oregon, where 55% of land-use is in grass seed production that directly impacts adjacent waterways containing native protected fish species. I determined tillage effects on soil N mineralization processes at two sites contrasting in soil drainage (moderately well drained, MWD) and well drained, WD)) and studied the relationship between temporal changes in soil N, N mineralization, crop N uptake and biomass accumulation, and microbial biomass carbon MBC). Net mineralization was determined using the in- situ buried bag method and MBC by fumigation extraction. Nitrate was the dominant inorganic-N form at both sites. Annual net nitrification was significantly (a=O.05 level) different between tillage treatments for fallow and seed years 1 and 2 at the WD site (fallow no till (NT) 75 kg N/ha/yr; conventional till (CT) 153 kg N/halyr; first seed year NT 202 kg N/halyr; CT 243 kg N/halyr; second seed year NT 51 kg N/halyr; CT 79 kg N/halyr). Annual net nitrification was significantly (a=0.05 level) different between tillage treatments for seed year 2 at the MWD site (year NT 46 kg N/halyr; CT 63 kg N/ha/yr). MWD had higher rates of nitrification in the fall, compared to the WD site where nitrification rates were greatest in the spring. Tillage had no effect on fine fescue and tall fescue seed yield during the three years of production. MBC under NT was significantly (ct=0.05 level) higher by 20-30% irrespective of soil drainage or time of year. WD had twice the amount of MBC compared to MWD. Crop N was directly related to the amount of crop biomass produced. Across all seed years tall fescue accumulated 300 kg N ha⁻¹ yr⁻¹ and fine fescue approximately 200 kg N ha¹ yr⁻¹. Each grass seed crop was fertilized in the spring with 134 kg N ha⁻¹. Crop N uptake was lowest during the fall crop re-growth period and highest when soil N was elevated in the spring. Spring mineralization., supplemented with fertilizer N, seemed to efficiently meet crop N demand for biomass and seed production. Overall, findings indicate that soil NO₃-N loss occurs during the high precipitation winter months of late-fall and winter.
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