Global warming increases soil temperatures and promotes faster growth and turnover of soil microbial communities. As microbial cell walls contain a high proportion of organic nitrogen, a higher turnover rate of microbes should also be reflected in an accelerated organic nitrogen cycling in soil. We used a metatranscriptomics and metagenomics approach to demonstrate that the relative transcription level of genes encoding enzymes involved in the extracellular depolymerization of high-molecular-weight organic nitrogen was higher in medium-term (8 years) and long-term (>50 years) warmed soils than in ambient soils. This was mainly driven by increased levels of transcripts coding for enzymes involved in the degradation of microbial cell walls and proteins. Additionally, higher transcription levels for chitin, nucleic acid, and peptidoglycan degrading enzymes were found in long-term warmed soils. We conclude that an acceleration in microbial turnover under warming is coupled to higher investments in N acquisition enzymes, particularly those involved in the breakdown and recycling of microbial residues, in comparison with ambient conditions.

Tropical forests are biodiversity hotspots, but it is not well understood how this diversity is structured and maintained. One hypothesis rests on the generation of a range of metabolic niches, with varied composition, supporting a high species diversity. Characterizing soil metabolomes can reveal fine-scale differences in composition and potentially help explain variation across these habitats. In particular, little is known about canopy soils, which are unique habitats that are likely to be sources of additional biodiversity and biogeochemical cycling in tropical forests. We studied the effects of diverse tree species and epiphytes on soil metabolomic profiles of forest floor and canopy suspended soils in a French Guianese rainforest. We found that the metabolomic profiles of canopy suspended soils were distinct from those of forest floor soils, differing between epiphyte-associated and non-epiphyte suspended soils, and the metabolomic profiles of suspended soils varied with host tree species, regardless of association with epiphyte. Thus, tree species is a key driver of rainforest suspended soil metabolomics. We found greater abundance of metabolites in suspended soils, particularly in groups associated with plants, such as phenolic compounds, and with metabolic pathways related to amino acids, nucleotides, and energy metabolism, due to the greater relative proportion of tree and epiphyte organic material derived from litter and root exudates, indicating a strong legacy of parent biological material. Our study provides evidence for the role of tree and epiphyte species in canopy soil metabolomic composition and in maintaining the high levels of soil metabolome diversity in this tropical rainforest. It is likely that a wide array of canopy microsite-level environmental conditions, which reflect interactions between trees and epiphytes, increase the microscale diversity in suspended soil metabolomes.

As the climate warms, drought events are expected to increase in intensity and frequency, with consequences for the carbon cycle. Soil respiration (Rs) accounts for the largest flux of CO2 from terrestrial ecosystems to the atmosphere. While the drought responses of Rs have been well studied, it is uncertain how they will be modified in a future world, when higher temperatures will occur in combination with elevated atmospheric CO2 concentrations. In a global change experiment in a managed temperate grassland, we studied drought and post-drought responses of Rs dynamics under current versus likely future conditions (+3°, +300 ppm CO2). Furthermore, to understand the soil CO2 production (Ps) and transport dynamics underlying Rs fluxes we continuously monitored in-situ soil CO2 concentrations across the soil profile. Our results show that Rs was higher and that drought-induced reductions in Rs were delayed under future compared to current conditions. Peak drought reductions and post-drought pulses of Rs were more pronounced in the future scenario. Annual Rs was reduced by drought only under current but not under future conditions. An in-depth analysis of soil CO2 gradients and fluxes across the soil profile showed that elevated CO2 stimulated Ps primarily in the main rooting horizon and that warming affected Ps also in deeper soil layers. We found that both in the current and the future scenario drought led to the strongest reductions of Ps in the most productive soil layers, which also exhibited the largest depletion of soil moisture. We conclude that a future warmer climate under elevated CO2 amplifies soil CO2 production and efflux and their peak drought and post-drought responses, but delays the onset of the drought responses and thereby eliminates the overall drought effect on annual soil CO2 emissions.