Climatic controls on the isotopic composition and availability of soil nitrogen across mountainous tropical forest

While tropical forests play a critical role in global carbon (C) and nitrogen (N) cycles, how their biogeochemical dynamics will respond to changes in climate, especially warming, is uncertain. To shed light on links between climate and N cycling in tropical forests, we measured bulk surface soil C and N concentrations and isotopic content at 40 forested sites spanning an 1800 m elevation transect in Central America, possessing wide variation in mean annual temperature (MAT; range = 10°C) and precipitation (MAP; range = 1.2 m). Climate and terrain attributes were extracted from gridded data sets and regressed against soil variables, and then, empirical relationships were combined with a mass balance model to scale up to the larger landscape. Across the remote study region, elevation and soil δ15N values displayed a strong negative relationship, while elevation was positively related to percent of soil C, N, and C:N ratios. As elevation was tightly correlated with MAT and MAP, soil chemical and isotopic content varied strongly with climate. For example, for every degree increase in MAT, soil δ15N values—an indicator of relative gaseous N losses—increased by a factor of 0.4, and soil C:N ratios, which affect net N mineralization and N availability, declined by a factor of 1.1. With the 40 sites binned into bioclimatic life zones, montane, premontane, and wet-premontane transition forests showed distinct clustering of soil chemical and isotopic properties, yet forest type alone explained less variation compared to continuous elevation–climate parameters. Results of the spatially applied 15N mass balance model implied shifts in the contribution of gaseous-to-total N loss, from 10% or less in cool, wet high-elevation forests to upwards of 60% at warmer, drier, low-elevation sites. Climate variation was thus associated with significant shifts in N dynamics across this montane tropical region, yet more work is needed to decouple direct vs. indirect climatic controls. While the mechanisms deserve further study, observed shifts in indicators of N availability and gaseous loss may be useful in managing and modeling tropical forests under climate change.