Soil carbon and microbes in the warming tropics

Climate warming could destabilise the Earth's largest terrestrial store of reactive carbon (C), by accelerating the decomposition of soil organic matter. A third of that C store resides in the tropics. The potential for tropical soils to sequester C, or to act as an additional source of CO2, will depend on the balance of C inputs and outputs, mediated by the response of soil microbial communities and their activity to perturbation.

We review the impact of warming on microbial communities and C storage in humid tropical forest soils over multiple time-scales.

Recent in situ experiments indicate high sensitivity of tropical forest soil C mineralisation to warming in the short term. However, whether this will translate into long-term soil C decline remains unclear. At decadal time-scales, high sensitivity of soil C mineralisation to warming is consistent with the correlation between the inter-annual variation in the tropical land surface temperature and atmospheric CO2 growth rate, and with simulations using the Carnegie-Ames-Stanford Approach biosphere model. This observed sensitivity may further contribute to climatic change over millennial time-scales, suggested by radiocarbon dating of organic matter in river basins showing a twofold acceleration in tropical soil C release during the late-glacial warming period. However, counter to this evidence, long-term stability of tropical soil C is suggested by observed steady-state soil C turnover across temperature gradients with elevation, and by the presence of C in tropical soils that pre-dates the Holocene Thermal Maximum and late-glacial warming periods.

To help reconcile these recent experimental findings and long-term observations, we propose mechanisms to explain tropical soil C and microbial responses to warming across multiple time-scales. Combined in situ experimental and monitoring approaches—large-scale and cross-site—are urgently needed to resolve the interplay of these mechanisms across spatial and temporal scales, to shape a better understanding of the relationship between soil microbes and C storage in tropical soils.