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

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 welldrained (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 5 0.05), regardless of soil drainage class or time of year, compared to the CT soil. Soils at theWD site had twice the amount of MBC compared to MWD. Crop N uptake was lowest in the fall and highest when soil Nwas elevated in the spring. Tillage enhanced annual total net N mineralization at the betterdrained 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. IN WESTERN OREGON, grass seed crops grow and produce seed from September to June at a time when 97% of the 1109 mm of annual precipitation is received. This precipitation regime facilitates soil NO3 flushes to shallow and surface waters because adequate plant and soil microbe N sinks are weak (Griffith et al., 1997c). During the winter months NO3 flushing is greatest (Griffith et al., 1997c). Research data indicates that the source ofmuch of theNO3 flushed to shallow ground and surface water was from soil mineralization processes (Griffith et al., 1997c), whereas, directly applied fertilizer N inputs to this flushing phenomenon appear minimal (S.M. Griffith, unpublished data, 2002). Thus, with regard to reducing N loss from grass seed fields and minimizing potential effects of off-site NO3 movement on water quality, a better understanding of mineralization processes in westernOregon is imperative because information is currently lacking. It is especially important to understand how tillage management, which can greatly influence soil NO3 levels, affects soil Nmineralization (N source) and N use by the crop (N sink). Soil ammonium, directly from applied fertilizer or soil N mineralization, readily binds to soil making these ions relatively immobile. Dispersion of NH4 to waterways, through processes of soil erosion or after being nitrified to NO3, can create water quality problems for fish and aquatic wildlife at certain concentrations, temperature, and dissolved oxygen conditions. Nitrification processes transformNH4 to NO2 andNO3. These soil processes can occur rapidly by soil microbes with adequate soil moisture and temperature under oxidizing conditions. Nitrite and NO3 are relatively mobile, and when not absorbed by plants or microorganisms, can easily move to ground and surface water and adversely affect aquatic wildlife at certain concentrations (Mueller and Helsel, 1996). Soil drainage, tillage, and crop residue have key influences on mineralization processes, thus it is important to understand howmineralized N are affected by grass seed crops under both conventional and conservation tillage and high residue management conditions on different classes of soil drainage. Currently, there is a trend in western Oregon grass seed production systems to flailchop postharvest residue and not incorporate it into the soil, and to adopt NT practices. The large amount of soil disturbance of CT accelerates C loss and promotes N mineralization (Stewart and Bettany, 1982). Previous work has demonstrated that NT management increases microbial biomass in agricultural soils (Doran, 1987; Gravatstein et al., 1987; Carter, 1991), as well as the closely related content of active N (Belvins et al., 1977; Lamb et al., 1985; Havlin et al., 1990; McCarty et al., 1995). Compared with NT, CT systems decrease potentially mineralized C and N (Woods and Schuman, 1988) and the soil’s ability to immobilize and thus conserve mineral N (Follett and Schimel, 1989). Greater potentially mineralizable N under NT compared to CT, in the upper 7.5 cm of soil, has been associated with a larger microbial biomass (Doran, 1980). A common observation is that the N supplying potential of the soil increases after NT practices have been utilized for a few growing seasons (Franzluebbers et al., 1994), as does the closely related content of active N (McCarty et al., 1995). Information is lacking in western Oregon about factors that regulate soil N mineralization, including the effects of crop production practices such as tillage. In situ soil N mineralization is rarely measured in agricultural field studies and never in western Oregon. Estimates of N mineralization are commonly based on laboratory incubations conducted under controlled environments and have little relevance to temporal changes in field N mineralization and how it relates to fluctuating crop N sinks. In contrast, methods for measuring soil N mineralization in situ have been routinely used in forest and M.A. Nelson, S.M. Griffith, and J.J. Steiner, USDA-ARS, National Forage Seed Production Research Center, Corvallis, OR 97331. Received 26 July 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:825–831 (2006). Soil & Water Management & Conservation doi:10.2136/sssaj2005.0248 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA 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. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 825 Published online March 29, 2006