Responses of microbial nutrient limitation and its drivers to nitrogen addition and warming in two biocrust types of the Mu Us Sandland

Biological soil crusts (Biocrusts) regulate dryland nutrient cycling, yet how the enzyme activities and microbial nutrient limitation status of different biocrust types respond to Nitrogen (N) deposition and climate warming (W), along with their underlying underlying driving mechanisms, remains a critical uncertainty.

An in situ full-factorial experiment assessed the effects of N addition (10 kg N ha⁻¹ yr⁻¹) and warming (~1.13 °C increase via open-top chambers) on cyanobacterial and moss crusts in the Mu Us Sandland.

In cyanobacterial crusts, N addition drove a 294% biomass surge and phosphorus depletion. Path analysis revealed this expansion indirectly alleviated microbial C limitation (C-acquiring enzymes decreased 20.8%) but intensified N limitation via the cascading depletion of abiotic nutrients and microbial biomass carbon. Conversely, warming directly alleviated N limitation. In moss crusts, N addition alleviated N limitation by suppressing biocrust growth, yet simultaneously intensified both C and N limitations by heavily suppressing the internal microbial biomass N reservoir. Furthermore, warming increased moss biomass (+42%), which indirectly alleviated C limitation but intensified N limitation. Crucially, robust N × W interactions (η_p² = 0.712 and 0.855) drove diametrically opposed non-additive patterns in phosphorus-acquiring (AP) enzymes: single-factor treatments stimulated but the combined treatment suppressed activities in cyanobacterial crusts, with the exact reverse occurring in moss crusts.

Pervasive N × W antagonistic interactions effectively neutralized single-factor-driven shifts in microbial nutrient demand. Early-successional biocrusts rely on direct environmental coupling, whereas late-successional crusts regulate nutrient acquisition dual filtering through host biophysical traits and internal microbial reservoirs.