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Soil & Water Management & Conservation

Tillage Effects on Nitrogen Dynamics and Grass Seed Crop Production in Western Oregon, USA

USDA-ARS, National Forage Seed Production Research Center, Corvallis, OR 97331

^* Corresponding author (griffits{at}onid.orst.edu)

Understanding N soil fertility in grass seed crops will lead to improved fertilizer practices and preserve water quality in Willamette Valley, Oregon. This study determined the effects of conventional tillage (CT) and no tillage (NT) on N dynamics and grass seed crop growth and seed yield on moderately well-drained (MWD) and a well-drained (WD) soils either in tall fescue (Festuca arundinacea Schreb.) or fine fescue (F. rubra L.) production. Temporal changes in soil N, N mineralization and immobilization, crop N uptake and biomass accumulation, and microbial biomass C (MBC) were determined. Net N mineralization was determined using the in situ buried bag method and MBC by fumigation extraction. Tillage treatment had no effect on fine fescue and tall fescue seed yield during the 3 yr of production. Soil MBC, under NT, was 20 to 30% higher (P = 0.05), regardless of soil drainage class or time of year, compared to the CT soil. Soils at the WD site had twice the amount of MBC compared to MWD. Crop N uptake was lowest in the fall and highest when soil N was elevated in the spring. Tillage enhanced annual total net N mineralization at the better-drained site (WD) resulting in more potential soil NO₃ to be leached the following winter high precipitation months when the crop's demand for N is low. This was especially true for fallow years when an actively growing crop was lacking. Net N mineralization was little affected by tillage in the more poorly drained soil.

Abbreviations: CT, conventional tillage • GDD, growing degree days • MBC, microbial biomass C • MWD, moderately well-drained • NT, no tillage • SOM, soil organic matter • WD, well-drained

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