Nutrient additions have direct and indirect effects on biocrust biomass in a long-term Chihuahuan Desert grassland experiment

Anthropogenic activities have greatly affected some of the global biogeochemical cycles, including the nitrogen (N) and phosphorus (P) cycles. This alteration of nutrients has significantly impacted ecosystems around the globe. Arid ecosystems may be particularly vulnerable to changes in nutrient availability, in addition to their frequent limitation by water availability. We conducted a nutrient-fertilization experiment with an emphasis on the cover of biological soil crusts (biocrusts) in the Chihuahuan Desert. We manipulated N, P, and Potassium (K) in a full-factorial design. We visually estimated biocrust cover and fluorometrically measured chlorophyll a as an index of biocrust abundance. We found that there were significant interaction effects of N*P and N*K as well as significant main effects of N and P on chlorophyll a. Results from a path analysis suggest that both N and P had direct positive effects on aboveground vascular plant production and direct negative effects on chlorophyll a. There were also indirect negative effects on biocrust cover and chlorophyll a by vascular plant cover. Overall, nutrient additions had a direct positive effect on vascular plant cover, which in turn had a negative impact on biocrust cover. This indicates a possible mechanism for inter-taxon competition under conditions of increased nutrient availability.