How encroaching shrubs and nutrients affect N2-fixation in the Chihuahuan desert

Background and aims Organisms that are capable of nitrogen (N) fixation, such as some species of cyanobacteria in biological soil crusts (biocrusts), can be ecologically important especially in low-nutrient dryland ecosystems. In addition, woody plant encroachment is occurring in drylands globally, including many species of N-2-fixing shrubs and trees. We determined the effects of nutrient deposition and shrub encroachment by Larrea tridentata on N-2-fixation rates of biocrusts and compared these rates to another encroaching shrub, the N-2-fixer Prosopis glandulosa in the northern Chihuahuan Desert. Methods We used both the acetylene reduction assay (ARA) and the N-15 natural abundance techniques to determine N-2-fixation rates of biocrusts. We also assessed the effects of nutrient additions on biocrust N-2-fixation rates using a long-term N addition experiment started in 2008 that is part of the Nutrient Network (NutNet). Results There was no correlation between the ARA and delta N-15 methods. We found that P. glandulosa likely fixes more N (at least during the spring) than biocrusts based on delta N-15 content in roots. There was no significant difference in delta N-15 between P. glandulosa and our reference plant, L. tridentata. delta N-15 declined in N-fertilized plots presumably because biological nitrogen fixation (BNF) is costly or because added N has negative effects on microbial communities. Conclusions Biocrusts fixed less nitrogen than N-2-fixing legume shrubs during this sampling period. Nonetheless, biocrusts are still a significant contributor to N-2-fixation in these nutrient-poor drylands based on the large proportion of soil surface area occupied by these organisms.