Soil carbon and nitrogen cycling at the atmosphere-soil interface: quantifying the response of biocrust-soil interactions to climate change

Almost half of Earth’s terrestrial surface is covered by drylands, where limitation of water restricts vascular plant growth. In these ecosystems, a substantial part of primary production instead takes place directly at the soil surface, within complex microbial communities forming biological soil crusts (biocrusts) that include intricately bound soil particles. These communities, composed mainly of cyanobacteria, algae, fungi, and bryophytes, are fundamental actors for dryland biogeochemical cycles, as they fix atmospheric CO₂ and N₂ and constitute one of the primary, if not the only, sources of soil C and N. Due to the vast spatial extent of biocrusts, accounting for up to 70% of the living land cover in drylands, their importance for C and N cycling extend to the global scale, i.e. accounting for 7% of global net primary production of total terrestrial vegetation. However, warming temperatures and increasing soil dryness following climate change are estimated to have critical implications on these systems; recent studies show i.e. warming-induced reduction of biocrust cover and, thus, reduced CO₂ uptake.

Our aim is to contribute to this relatively new research field by providing insights into biocrust-soil-microorganism interactions under elevated temperatures and drought at the process-scale. This was realized in a phytotron incubation experiment of soil-biocrust mesocosms with experimental warming and drought, during which CO₂ uptake and heterotrophic respiration was monitored. Dual labelling pulses (¹³CO₂ and ¹⁵N₂) were applied to follow the fate of recently fixed ¹³C and ¹⁵N into both particulate and mineral-associated SOM pools via physical fractionation and into microbial biomass via PLFA. Further, hyperspectral VIS-NIR images of the surface were recorded to quantify and determine crust cover and composition.

The results revealed clear drought effects—not only in a distinct reduction in CO₂ fixation by the biocrusts, but also in the elemental distribution of soil C underneath; the effect extended down into underlying soil layers, where biocrust-derived C contents were reduced by half due to drought. The change in the translocation of biocrust-derived C into the underlying soil was reflected in the ¹³C-PLFA profiles, showing how mainly fungi transform recently fixed C from the biocrust into their biomass in the biocrust layer, extending down into the underlying soil via fungal hyphae expansion. While drought clearly restricted the microbial abundance, warming further induced a microbial community shift, where a greater relative fungal dominance was determined under experimental warming—a shift, however, that was not reflected under dry conditions. A further combined effect was determined in N fixation, where we confirm a decrease in biocrust-derived N under drought under warming.

Our results showcase the implications of elevated temperature and drought on C and N fixation and cycling—the two most fundamental ecosystem functions in biocrust-soil systems. The results support the growing body of evidence of major implications for biogeochemical cycles in drylands in a warming world.