Soil nitrogen availability increases the abundance of nitrogen-fixing plants in subtropical forests

Abstract

The energy consumption theory suggests that the abundance of nitrogen-fixing plants (N-fixers) should decrease with increasing soil N availability. However, this theory fails to explain why there are more abundant N-fixers in N-rich tropical regions compared to N-limited temperate regions, which is known as the “N paradox.” We investigated changes in N-fixer abundance as soil N availability increased along two mountainous elevational gradients with different lithologies (granite vs. slate soil parent material) in subtropical forests located in Guangdong Province, China. Bayesian regression analysis revealed that soil N availability was the main factor influencing changes in N-fixer abundance across sites with different elevations in each study location. Despite comprising less than 2 % of total tree abundance across sites, N-fixers were more abundant in high-N slate forests than in low-N granite forests, and their abundance increased at higher elevation sites that had greater soil N availability in both locations. We attribute this pattern to N-fixers enhancing their competitiveness in high soil N forest locations by down-regulating symbiotic N fixation rates, accumulating N in leaves, and improving soil phosphorus acquisition. Our findings provide a case study explanation for the “N paradox,” and can help to improve predictions of changes to N-fixer abundance under increasing atmospheric N deposition.