Nitrogen inputs may improve soil biocrusts multifunctionality in dryland ecosystems

Soil biocrusts (communities of cyanobacteria, algae, mosses, lichens, and heterotrophs living at the soil surface) are fundamental components of dryland ecosystems worldwide. There is increasing concern over the potential for increasing nitrogen (N) inputs to affect biocrusts. This is of special concern in Mediterranean Basin drylands that face the threat of increased N inputs however, the effect on biocrusts remains poorly studied. We evaluated the potential effects of increased N inputs on biocrust structure and functioning in surrounding Mediterranean shrublands in the seventh year of a N-manipulation field experiment. We tracked the N-driven changes in biotope (changes in bare soil and in the non-legume and the legume occupation areas, and the percentage of radiation intercepted by plant canopies), evaluated biocrust functional traits (based on pigments) and measured biocrust functioning in terms of C and N cycling, soil fertility (macro and micronutrients) and biodiversity, and integrated these multiple soil functions simultaneously (i.e. soil multifunctionality) Biocrust pigment concentration was significantly influenced by both plant legacy and N input. Biocrust pigments revealed a clear functional shift from: i) biocrusts dominated by photosynthetically inactive cyanobacteria that fix N-2 and are mostly committed to photopmtection at the expense of N-containing pigments under low N inputs; into ii) biocrusts more evenly composed of prokaryotes and eukaryotes, which are more photosynthetically active, but less committed to photoprotection and N-2 fixation under exposure to increased N inputs. The N-driven functional and structural changes in biocrusts resulted in trade-offs in biocrust functioning and processes (only N-2 fixation was affected) and an overall improvement in biocrust multifunctionality. By itself, biocrust pigment evenness accounted for similar to 50% of the observed variation in biocrust multifunctionality. The biocrust pigment functional approach we adopted to study the effects of increased N inputs from patchy developed anthropogenic landscapes provides novel and critical knowledge of biocrusts community and functioning, which may be used as a tool in biodiversity conservation strategies, ecosystem functions and ecological modelling.