Data from: Drivers of carbon stocks in forest soils at varying elevations in the northwest Andes of Colombia

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.