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
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/33785 |
| Date | 24 November 1998 |
| Creators | Ndiaye, Aissatou |
| Contributors | Bottomley, Peter J. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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