Anthropogenic activities have greatly affected some of the global biogeochemical cycles, including the nitrogen (N) and phosphorus (P) cycles. This alteration of nutrients has resulted in a surplus that has significantly impacted ecosystems around the globe. In addition to a lack of precipitation in arid ecosystems, there is a sensitivity to increased nutrients. We conducted a nutrient-fertilization experiment with an emphasis on the cover of biological soil crusts (biocrusts) in the Chihuahuan Desert. Biocrusts are a community of soil-surface organisms that commonly occur in arid environments. They are significant primary producers in desert landscapes around the world, playing several important ecological roles (such as fixing nitrogen and decreasing soil erosion via stabilization of the soil). We manipulated N, P, K (and Control) in a full-factorial design and visually estimated biocrust cover. Biocrust cover was related to vascular plant- and litter- cover in the same experimental plots. We also measured chlorophyll a as an index of biocrust abundance.
Results/Conclusions
We found no significant effect of nutrient additions on our visual estimate of biocrust cover. However, 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. Unexpectedly, N and P had negative effects on chlorophyll a. Results from a path analysis suggest that both N and P had direct positive effects on aboveground vascular plant production. Nitrogen and P also had direct negative effects on chlorophyll a. In addition, there were also indirect negative effects on biocrust cover and chlorophyll a by vascular plant- and litter 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. Understanding how arid ecosystems, particularly those containing biocrusts, will respond to increased nutrients will allow us to better predict ecosystem dynamics.