Nutrient feedbacks to soil heterotrophic nitrogen fixation in forests

Biogeochemistry
By: , and 

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Abstract

Multiple nutrient cycles regulate biological nitrogen (N) fixation in forests, yet long-term feedbacks between N-fixation and coupled element cycles remain largely unexplored. We examined soil nutrients and heterotrophic N-fixation across a gradient of 24 temperate conifer forests shaped by legacies of symbiotic N-fixing trees. We observed positive relationships among mineral soil pools of N, carbon (C), organic molybdenum (Mo), and organic phosphorus (P) across sites, evidence that legacies of symbiotic N-fixing trees can increase the abundance of multiple elements important to heterotrophic N-fixation. Soil N accumulation lowered rates of heterotrophic N-fixation in organic horizons due to both N inhibition of nitrogenase enzymes and declines in soil organic matter quality. Experimental fertilization of organic horizon soil revealed widespread Mo limitation of heterotrophic N-fixation, especially at sites where soil Mo was scarce relative to C. Fertilization also revealed widespread absence of P limitation, consistent with high soil P:Mo ratios. Responses of heterotrophic N-fixation to added Mo (positive) and N (negative) were correlated across sites, evidence that multiple nutrient controls of heterotrophic N-fixation were more common than single-nutrient effects. We propose a conceptual model where symbiotic N-fixation promotes coupled N, C, P, and Mo accumulation in soil, leading to positive feedback that relaxes nutrient limitation of overall N-fixation, though heterotrophic N-fixation is primarily suppressed by strong negative feedback from long-term soil N accumulation.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Nutrient feedbacks to soil heterotrophic nitrogen fixation in forests
Series title Biogeochemistry
DOI 10.1007/s10533-017-0341-x
Volume 134
Issue 1-2
Year Published 2017
Language English
Publisher Springer
Contributing office(s) Forest and Rangeland Ecosystem Science Center
Description 15 p.
First page 41
Last page 55
Country United States
Other Geospatial Oregon Coast Range