Although legumes have been widely studied for their nitrogen-fixing ability, it is uncertain to what extent legume cover crops achieve their nitrogen-fixing potential under the climatic conditions encountered in western Oregon. Furthermore, it is unknown what factors control the proportions of legume cover crop N that are either sequestered into soil organic matter, or that contribute to the N requirements of the following summer crop. Soil was sampled in mid-September 1997, after harvest of a summer broccoli crop, from plots located at the North Willamette Research and Extension Center, Aurora, Oregon. Soil was sampled from main plots that had been either winter cover cropped with red clover (LN��� and LN���) or fallowed during the winter period (FN��� and FN���), and specifically from sub-plots in which the following summer crop had received either zero (N���) or an intermediate (N���) rate of N fertilizer as urea. Levels of total organic carbon (TOC), total Kjeldahl nitrogen (TKN), and readily mineralizable C and N were measured in both whole soil samples and in different aggregate-size classes (<0.25, 0.25-0.5 0.5-1.0, 1.0-2.0, and 2-5mm) prepared by dry sieving the soil. Aggregate size-class distribution was not affected by the cover crop treatment. Although there was no significant effect of cover crop treatment on either TKN or TOC levels in whole soil samples, TOC levels were consistently higher in the small aggregate size-classes <1 mm of the fallow than the legume treatment. There was a significantly higher level of mineralizable C in the <0.25 mm size class of the legume than the fallow treatment. There was a trend for the level of mineralizable N to be greater in soil from the legume than the fallow treatment. However, N fertilizer had a significant positive effect on the level of readily mineralizable N in both fallow and legume cover-cropped treatments, it had a negative effect on TKN levels among all aggregate-size classes. There were differences in the levels of mineralizable N measured among the aggregate-size classes, and immobilization of N between 20 and 40 days of incubation also differed among the aggregate-size classes. / Graduation date: 1999

Thesis (M.S.)--California Polytechnic State University, 2009. / Title from PDF title page; viewed on December 2, 2009. Major professor: Brent G. Hallock, Ph.D. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Agriculture with specialization in Soil Science." "November 2009." Includes bibliographical references (p. 67-71). Also available on microfiche.

The effect of cover crops on N and weed dynamics was assessed within a pea (Pisum sativum L.) – cover crop – sweet corn (Zea mays L.) rotation. Cover crops of oat (Avena sativa L.), perennial rye (rye) (Secale cereale L.), oilseed radish (OSR) (Raphanus sativus L. var. oleoferus Metzg Stokes), and OSR plus perennial rye (OSR+rye) increased plant available N (PAN) over the cover crop growing season compared to the no cover control at the Bothwell site only. However, at neither site did cover crops result in increased PAN for the sweet corn, indicating that these cover crops will not reduce required N fertilizer applications. Also, cover crops posed neither an increased or decreased need for weed management during sweet corn production. However, OSR may be useful in pesticide reduced programs due to its potential ability to reduce fall herbicide applications, provided it does not set viable seed.

The use of high N fertilizer in sweet corn (Zea mays L.) and wheat (Triticum aestivium L.) production often results in leaching losses and contamination of ground water. Cover crops planted after harvest of sweet corn and wheat may reduce residual soil NO$ sp- sb3$-N levels by crop N uptake and subsequently minimize NO$ sp- sb3$-N content in gravitational water. Field experiments were conducted on a Ste. Rosalie heavy clay (Humic Gleysol) and a St. Bernard sandy clay loam (Melanic Brunisol) to determine the contribution of the six cover crops to nutrient uptake, subsequent N release, leaching losses, denitrification rates and soil properties such as aggregate stability, organic matter. The cover crops were red clover (Trifolium pratense L.), crimson clover (Trifolium incarnatum L.), forage radish (Raphanus sativus L.), canola (Brassica rapa L.), barley (Hordeum vulgare L.), annual rye grass (Lolium multiflorum L.). Three replicates were used in a split plot arrangement of a randomized complete block experiment. Sweet corn and wheat were grown at three fertilizer N rates, 0-75-150 kg N ha$ sp{-1}$ for sweet corn and 0-45-90 kg N ha$ sp{-1}$ for wheat. Cover crop of forage radish, canola and barley were more effective at absorption or soil N than rye grass and clover species at all sample times. Levels of soluble N in the soil were reduced with cover crop in the off-season. Cover crop plots had higher NO$ sp- sb3$-N levels than control plots in the spring, indicating net mineralization and nitrification. Gravitational water NO$ sp- sb3$-N contents were higher in the control plots relative to cover crop plots at both sites. Therefore growing cover crops after harvest of sweet corn and wheat can reduce residual NO$ sb3$-N level in the soil and thus restrict ground water contamination with fertilizer N.

Thesis(M.S.)--Auburn University, 2005. / Abstract. Vita. Includes bibliographic references.

Thesis (M.S.) in Plant, Soil and Environmental Sciences--University of Maine, 2008. / Includes vita. Includes bibliographical references (leaves 88-93).

Thesis--Auburn University, 2008. / Abstract. Vita. Includes bibliographical references (p. 54-59).

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 89-96). Also available on the World Wide Web.

Thesis (Ph. D.)--Auburn University. / Abstract. Vita. Includes bibliographical references.

Thesis (M.S.)--Oregon State University, 2008. / Printout. Includes bibliographical references (leaves 159-167). Also available on the World Wide Web.