Fungal loop transfer of nitrogen depends on biocrust constituents and nitrogen form

Besides performing multiple ecosystem services individually and collectively, biocrust constituents may also create biological networks connecting spatially and temporally distinct processes. In the fungal loop hypothesis rainfall variability allows fungi to act as conduits and reservoirs, translocating resources between soils and host plants. To evaluate the extent to which biocrust species composition and nitrogen (N) form influence loops, we created a minor, localized rainfall event containing ¹⁵NH₄⁺ and ¹⁵NO₃⁻. We then measured the resulting δ¹⁵N in the surrounding dry cyanobacteria- and lichen-dominated crusts and grass, Achnatherum hymenoides, after 24 h. We also estimated the biomass of fungal constituents using quantitative PCR and characterized fungal communities by sequencing the 18S rRNA gene. We found evidence for the initiation of fungal loops in cyanobacteria-dominated crusts where ¹⁵N, from ¹⁵NH₄⁺, moved 40 mm h⁻¹ in biocrust soils with the δ¹⁵N of crusts decreasing as the radial distance from the water addition increased (linear mixed effects model (LMEM)): R² = 0.67, F_2,12 = 11, P = 0.002). In cyanobacteria crusts, δ¹⁵N, from ¹⁵NH₄⁺, was diluted as Ascomycota biomass increased (LMEM: R² = 0.63, F_2,8 = 6.8, P = 0.02), Ascomycota accounted for 82 % (±2.8) of all fungal sequences, and one order, Pleosporales, comprised 66 % (±6.9) of Ascomycota. The seeming lack of loops in moss-dominated crusts may stem from the relatively large moss biomass effectively absorbing and holding N from our minor wet deposition event. The substantial movement of ¹⁵NH₄⁺ may indicate a fungal preference for the reduced N form during amino acid transformation and translocation. We found a marginally significant enrichment of δ¹⁵N in A. hymenoides leaves but only in cyanobacteria biocrusts translocating ¹⁵N, offering evidence of links between biocrust constituents and higher plants. Our results suggest that minor rainfall events may initiate fungal loops potentially allowing constituents, like dark septate Pleosporales, to rapidly translocate N from NH₄⁺ over NO₃⁻ through biocrust networks.