Biological soil crusts (biocrusts) are major contributors to nitrogen (N) inputs in drylands, yet their N-fixing capacity is increasingly threatened by atmospheric nutrient deposition. Although N enrichment typically downregulates N fixation, how these effects interact with phosphorus (P) availability, a key modulator of microbial metabolism, to shape biocrust function remains unresolved. We conducted a six-month field experiment on the northern Chinese Loess Plateau, testing the N fixation capacity responses of moss- and cyanobacteria-dominated biocrusts to N, P, and combined N + P additions (no nutrient addition as the control). We quantified biocrust traits (chlorophyll a), soil chemical properties, N fixation (via delta N-15), and microbial community composition (16S rRNA and nifH sequencing). Our results indicated that N addition caused the most pronounced reduction in chlorophyll a, decreasing its content in moss biocrusts by approximately 50 % compared to the control (P < 0.05). Biocrust NO3--N increased 7.5-fold due to N addition, while delta N-15 values shifted to >0 parts per thousand, confirming a strong suppression of biological N fixation in both biocrust types. In contrast, P addition enhanced N fixation in moss biocrusts, as reflected by lower delta N-15 values compared to the control, but slightly inhibited N fixation in cyano biocrusts. The combined addition of N and P treatment partially alleviated N-induced inhibition, through improving energy metabolism and increasing diazotrophic diversity. Although nutrient additions significantly altered diazotrophic community composition, the overall bacterial richness remained unchanged. Overall, we conclude that high N inputs impair biocrust N fixation by reducing photosynthetic capacity and shifting microbial strategies toward inorganic N use. However, P addition can buffer these negative effects, particularly in cyano biocrusts, by sustaining photosynthesis and nutrient balance. These findings highlight the importance of N-P interactions in regulating dryland N cycling and suggest that targeted P management could enhance the resilience of biocrust-driven nutrient processes under global change.