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Dep. of Soil Science
Dep. of Crop Science and Plant Ecology, Univ. of Saskatchewan, Saskatoon, SK, Canada S7N OWO
*Corresponding author (vankessel{at}usask.sask.ca)
ABSTRACT
Topography influences the distribution of water and N, thereby exerting an indirect control on N2-fixing activity of legumes. A study was conducted to assess the (i) variability of N2 fixation in pea (Pisum sativum L.) in a rolling field, and (ii) degree of landscape-scale control on N2 fixation. A 90 by 100 m sampling grid, with 10-m spacings, was established in the field. Each sampling point was classified as either footslope or shoulder. The percentage of N derived from the atmosphere (% Ndfa) was estimated using natural 15N abundance and the A-value approaches. Spring soil water content and inorganic N were most concentrated in the footslopes, whereas mean estimates of % Ndfa showed inverted spatial distribution patterns. At flowering, natural 15N abundance estimates of % Ndfa did not differ between shoulders and footslopes. In contrast, A-value mean estimates of % Ndfa were 84% for shoulders and 92% for footslopes. The two approaches gave similar mean estimates of % Ndfa at maturity, with values of approximately 72 and 84% for footslopes and shoulders. A random spatial pattern existed for total aboveground N accumulation, indicating that pea was able to adjust its N2-fixing activity according to the available soil N. Despite similar spatial patterns for % Ndfa estimated using the two approaches, the correlation between the A value and natural 15N abundance approaches was poor (r = 0.213 at flowering and r = 0.377 at maturity). The poor correlation suggests that N2 fixation by pea was partially controlled at the landscape scale, whereas strong micro-scale controls may have existed that ultimately regulated N2 fixation.
Contribution no. R763 of the Saskatchewan Center for Soil Research.
Received for publication September 8, 1994.
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