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 N2-fixing shrubs and trees. We determined the effects of nutrient deposition and shrub encroachment by Larrea tridentata on N2-fixation rates of biocrusts and compared these rates to another encroaching shrub, the N2-fixer Prosopis glandulosa in the northern Chihuahuan Desert. Methods: We used both the acetylene reduction assay (ARA) and the 15N natural abundance techniques to determine N2-fixation rates of biocrusts. We also assessed the effects of nutrient additions on biocrust N2-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 δ15N methods. We found that P. glandulosa likely fixes more N (at least during the spring) than biocrusts based on δ15N content in roots. There was no significant difference in δ15N between P. glandulosa and our reference plant, L. tridentata. δ15N 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 N2-fixing legume shrubs during this sampling period. Nonetheless, biocrusts are still a significant contributor to N2-fixation in these nutrient-poor drylands based on the large proportion of soil surface area occupied by these organisms. [ABSTRACT FROM AUTHOR]