Biological nitrogen (N) fixation is the largest N input to most terrestrial ecosystems worldwide. Multiple nutrient cycles regulate biological N fixation, though feedbacks between N-fixation and coupled element cycles are rarely studied. We examined nutrient limitation and suppression of soil heterotrophic N-fixation across 24 temperate conifer forests in Oregon. These forests span a wide range in soil N and other nutrients, reflecting legacies of prior symbiotic N-fixation by trees. We sought to discern how these soil N legacies influenced free-living heterotrophic N-fixation in surface organic soil. We also sought to understand how couplings of soil N to other nutrients may control potential molybdenum (Mo) and phosphorus (P) limitation of soil heterotrophic N-fixation.
Our preliminary results show that soil heterotrophic N-fixation was lowest in high N sites, consistent with N inhibition of nitrogenase enzymes and observed declines in soil C quality. Direct experimental N fertilization also suppressed heterotrophic N-fixation. Fertilization experiments revealed widespread Mo limitation of heterotrophic N-fixation, with the strongest limitation at sites where soil Mo was scarce relative to C. Widespread Mo limitation and absence of P limitation were consistent with high P:Mo ratios in soil. Changes in N-fixation following additions of Mo (positive) and N (negative) were negatively correlated across sites, evidence that multiple nutrient controls of heterotrophic N-fixation were more common than single-nutrient effects. Soil N concentrations correlated positively with C, organic Mo and organic P in both organic and mineral horizons, suggesting that legacies of obligate symbiotic N-fixation increased the abundance of multiple elements important to N-fixation. We present a conceptual model to explain element interactions and feedbacks to soil heterotrophic N-fixation across N fertility gradients. We propose that symbiotic N-fixation promotes coupled N, C, P, and Mo accumulation in soil, creating positive feedback that relaxes nutrient limitation in obligate symbiotic N-fixing trees, while fostering high N accumulation and negative feedback to soil heterotrophic N-fixation.