Beyond Phosphorus: Nutrient Medium Composition Shapes Early Growth in Cyanobacteria-Dominated Biocrusts from a Peruvian Fog Oasis

Biological soil crusts are key components of dryland ecosystems, contributing to soil stabilization, nutrient cycling, and primary productivity. However, little is known about how nutrient availability influences the early development of biocrusts from South American desert fog oases, where fog is the dominant water source. Here, we evaluated functional responses of cyanobacteria-dominated biocrust microcosms from the Lachay National Reserve, Peru, to varying nutrient additions over a 57-day laboratory experiment. Prior to addition, the initial inoculum was characterized using light microscopy and DNA metabarcoding to establish ecological context. Responses were assessed through standardized microcosm mass gain, chlorophyll a content, and net CO2 exchange. Nutrient addition affected standardized mass gain (nonparametric repeated-measures analysis, p = 0.045), with phosphorus-containing treatments gaining more than phosphorus-free treatments during the first 30 days but not thereafter. However, the response cannot be attributed to phosphorus alone: the media differed simultaneously in nitrogen, phosphorus, potassium, micronutrients and ionic strength, and the substrate was already high in available phosphorus. Chlorophyll a and net CO2 exchange showed high variability and did not differ significantly among treatments; chlorophyll a nonetheless covaried with mass gain (ρ = 0.48) while accounting for only 0.2–0.6% of it, indicating that early mass accumulation reflects hydro-structural change rather than phototrophic biomass. These findings provide an experimental basis for exploring nutrient-informed approaches in desert fog oases, but they derive from a single crust type over 57 days under laboratory conditions and require field validation.