Biocrusts contribute to shrub nitrogen nutrition via soil transfer in resource-limited drylands

Biocrusts and xeric shrubs commonly coexist in drylands, forming shrub-biocrust patches that shape soil processes and ecosystem functioning. While biocrusts are important sources of newly fixed nitrogen (N) and carbon (C), it remains unclear how C and N move between biocrusts and shrubs, whether transfer is bidirectional or asymmetric, and which pathways mediate exchange. Using whole-atmosphere isotope pulse labeling combined with experimental manipulation of hyphal access in a semiarid shrubland, we traced biocrust-fixed N (¹⁵N₂), biocrust-fixed C (¹³CO₂), and shrub-fixed C (¹³CO₂). Biocrust-fixed N was detected in shallow soils and subsequently in shrub roots and leaves. Restricting hyphal access did not reduce ¹⁵N enrichment in shrub aboveground tissues, indicating that N transfer over the one-month observation period was consistent with soil-mediated redistribution rather than being dominated by direct hyphal transport. By contrast, biocrust-fixed C remained largely confined to biocrust and soil compartments, with no detectable incorporation into shrub tissues. Shrub-fixed C was retained within plant tissues and did not measurably enter biocrust or soil pools. Our results therefore reveal asymmetric, element-specific redistribution, with biocrust-derived N moving through soil into shrub tissues but no detectable reciprocal transfer of shrub-derived C, highlighting biocrusts as important sources of plant-available N in resource-limited drylands.