Contrasting controls on symbiotic and asymbiotic nitrogen fixation rates along altitudinal gradients in subtropical forests

Symbiotic and asymbiotic nitrogen fixation (SNF and ANF), two forms of biological nitrogen (N) fixation, are the main pathways for external N inputs into natural terrestrial ecosystems. However, the regulatory mechanisms of SNF and ANF, particularly in response to changing environmental conditions, remain poorly understood. Here, we investigated changes in SNF and ANF rates along two altitudinal gradients in two subtropical forests on soils with granite and slate parent materials. Our results revealed distinct patterns for SNF and ANF rates. SNF rates consistently declined with increasing altitude, whereas ANF rates initially increased at lower altitudes but declined at higher altitudes. These contrasting trends were attributed to divergent regulatory mechanisms of SNF and ANF rates. Specifically, the decrease in SNF rates was primarily driven by increased soil N availability and decreased air temperature. However, the drivers of ANF rates shifted from soil properties (e.g., phosphorus, iron, and moisture) at lower altitudes to climatic factors (e.g., air temperature) at higher altitudes. We also observed opposite trends of SNF and ANF between forests on granite and slate, demonstrating that lithology is an important driver of both SNF and ANF. Collectively, our findings highlight the divergent mechanisms regulating SNF and ANF in subtropical forests, which contribute to improving the mechanistic representation of biological N fixation in Earth system models.