Landscape-scale estimates of dinitrogen fixation by Pisum sativum by nitrogen-15 natural abundance and enriched isotope dilution

Abstract

Topography and slope position influence the soil and environmental factors that affect N2 fixation by legumes. The present study was conducted to (1) estimate N2 fixation by field peas in a gently rolling farm field using the natural 15N abundance and the 15N-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N2 fixation at the landscape scale. Whereas soil available water capacity, available NHinf4sup+, total crop yield, and percent N derived from N2 fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil NOinf3sup-levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NHinf4sup+but not with available soil water, pH, electrical conductivity, NOinf3sup-, or particle size. Estimates of the % Ndfa using enriched 15N ranged from 0 to 92.8%. The highest median value (68.6%) for % Ndfa using enriched N occurred on the divergent footslopes, with the lowest value (28.1%) on the convergent shoulders. Estimates of % Ndfa using natural abundance ranged from 13.2% to 96.9%. Smaller fluctuations during the growing season in the δ 15N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched 15N. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched 15N (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on N2 fixation, large variations in N2 fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N2 fixation.