Nitrogen fixation is a critical ecological process that converts atmospheric nitrogen into bioavailable forms, essential for plant growth and carbon sequestration. This study, published in Forest Ecosystems, focused on two primary forms of biological nitrogen fixation (BNF): symbiotic nitrogen fixation (SNF), which occurs within the root nodules of nitrogen-fixing plants, and asymbiotic nitrogen fixation (ANF), which is carried out by free-living microorganisms in soil and litter. Understanding the environmental controls on these processes is crucial for predicting how ecosystems will respond to changing conditions.
The research was conducted in the Chebaling National Nature Reserve in Guangdong Province, China. The team examined two altitudinal gradients in forests with different parent materials—granite and slate. They collected soil, litter, moss, and leaf samples from 19 sites across 57 plots, measuring SNF and ANF rates using established methods.
The study revealed that SNF rates consistently declined with increasing altitude, driven by increased soil nitrogen availability and decreased air temperature. In contrast, ANF rates exhibited a hump-shaped pattern, initially increasing at lower altitudes but decreasing at higher altitudes. This divergence suggests that different factors regulate SNF and ANF, with soil properties such as phosphorus, iron, and moisture influencing ANF at lower altitudes, while climatic factors like air temperature become more significant at higher altitudes.
Regarding different parent materials, SNF rates were higher on granite than on slate, whereas ANF rates exhibited the opposite trend. It demonstrated that lithology is also an important driver for both SNF and ANF, shaping their divergent patterns through differences in soil nutrient availability.
These findings underscore the importance of distinguishing between SNF and ANF in ecological studies and Earth system models. "By identifying the shifts in controlling factors along altitudinal gradients—from soil nutrients to climatic drivers—our study provides valuable insights for assessing N fixation dynamics in diverse ecosystems.", the study concludes. The results also highlight the role of lithology and climate in shaping nitrogen fixation processes, emphasizing the need for spatially specific approaches in future research.
