Dinitrogen Fixation in Pea: Controls at the Landscape‐ and Micro‐Scale

Topography influences the distribution of water and N, thereby exerting an indirect control on N 2 ‐fixing activity of legumes. A study was conducted to assess the (i) variability of N 2 fixation in pea ( Pisum sativum L.) in a rolling field, and (ii) degree of landscape‐scale control on N 2 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 15 N 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 15 N 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 N 2 ‐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 15 N abundance approaches was poor ( r = 0.213 at flowering and r = 0.377 at maturity). The poor correlation suggests that N 2 fixation by pea was partially controlled at the landscape scale, whereas strong micro‐scale controls may have existed that ultimately regulated N 2 fixation.