LANDSCAPE-SCALE ESTIMATES OF DINITROGEN FIXATION BY PISUM-SATIVUM BY N-15 NATURAL-ABUNDANCE AND ENRICHED ISOTOPE-DILUTION

Topography and slope position influence the soil and environmental factors that affect N-2 fixation by legumes. The present study was conducted to (1) estimate N-2 fixation by field peas in a gently rolling farm field using the natural N-15 abundance and the N-15-enriched isotope dilution techniques and (2) identify soil and environmental factors that influence N-2 fixation at the landscape scale. Whereas soil. available water capacity, available NH4+, total crop yield, and percent N derived from N-2 fixation (% Ndfa) estimated using enriched N were significantly affected by landform patterns, soil No-3(-) levels, seed yield, and the % Ndfa estimated using natural abundance did not follow landform patterns. The % Ndfa using natural abundance was correlated with NH4+ but not with available soil water, pH, electrical conductivity, NO3-, or article size. Estimates of the % Ndfa using enriched N-15 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 delta(15)N of the available N pool may have resulted in less variability for % Ndfa using natural abundance compared to enriched N-15. Despite similar mean values for % Ndfa using natural abundance (44.5) and enriched N-15 (49.6), no significant correlation between the two estimates was found. These results suggest that although topography may exert gross controls on Nz fixation, large variations in Nz fixation at the microsite level may preclude correlations between individual estimates and limit detection of landscape scale patterns of N-2 fixation.