Cyanolichen crusts are a key predictor of nitrogen cycling and fixation potential in soils from semiarid Mediterranean ecosystems

Abstract

Biocrusts play a key role in supporting nitrogen (N) cycling and bioavailability in drylands. However, the taxon-specific contribution of biocrust constituents to soil N fixation and bioavailability is far less understood. We measured the composition of soil-associated biocrusts and soil pigment content in a semiarid shrubland from central Spain, and evaluated the potential regulatory capacity of biocrusts, and their forming constituents (mosses, lichens, and cyanobacteria) over the N cycle (storage, bioavailability, and processing). We found that key biocrust species contributed to explain N fixation potential and availability. In particular, the cyanolichen Collema was positively correlated with N fixation potential, as was total cyanobacterial abundance, and, in particular, the morphotypic genera Microcoleus , and Leptolyngbya . Also, the cyanobacterial morphotype Microcoleus was associated with ammonium accumulation and negatively with ammonification potential. Total biocrust cover and the cover of the moss Tortella squarrosa were negatively correlated with soil N content in this relatively low-fertility site, and positively with N fixation potential. Our results underline the taxon-specific contributions of biocrust components, such as Collema , which emerged as a key predictor of N bioavailability. We provide a novel empirical demonstration of how different biocrust components play distinct roles in N cycling processes at the local scale, by promoting or inhibiting N fixation and/or storing organic and inorganic N, thus contributing to the efficient cycling of this critical element in the typically N-poor Mediterranean ecosystems of central Spain.