Drivers of Carbon Stocks in Forest Soils at Varying Elevations in the Northwest Andes of Colombia

Abstract Soil represents the most important terrestrial carbon sink on Earth. Using data from 9 forest sites located across an elevational gradient (range: 167–2928 m a.s.l.) in northwestern Colombia, which spans a wide range of climate, forest, and soil types, we aim to answer the following research questions: (i) How do SOC stocks change along the elevational gradient or between lowlands and highlands? (ii) What are the main drivers of SOC stocks along elevational gradients? Using structural equation modeling (SEM), we found that climate and soil fertility explained 67% of the variation in SOC stocks across the entire dataset, with SOC stocks declining under warmer and wetter conditions, but increasing with higher soil N:P ratios. Because soil mineralogy was closely correlated with elevation (all lowland were kaolinites while all highland sites were allophanes), SEM models for lowland and highland forests were also run separately. Lowland forests were dominated by trees associated with arbuscular mycorrhizas and nitrogen-fixing symbiont root associations, which could also increase decomposition rates, and thus, reduce the SOC stocks. This finding suggests that greater soil P availability stimulated microbial activity and decreased SOC. In highland forests, which had a wider range of climatic conditions and a greater proportion of trees with ectomycorrhizal associations, decreased temperatures as well as ectomycorrhizal modification of soil N:P ratios slowed soil C cycling, resulting in a greater accumulation of SOC. In conclusion, the increase in SOC stocks across either lowlands or highlands in the Northern Andes was driven by different combinations of abiotic and biotic factors. Since increases in temperature are expected to modify forest functioning and composition along elevational gradients, which in turn depends on soil conditions, improving our understanding on the likely fate of the large amount of C stored in soils should be seen as a priority in tropical montane ecosystems.