Trevor B. Smart Nan Wu Alexander S. Taylor Yuanming Zhang Jayne Belnap Zachary T. Aanderud 2018 <p><span>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&nbsp;</span>¹⁵<span>NH</span>₄⁺<span>&nbsp;and&nbsp;</span>¹⁵<span>NO</span>₃⁻<span>. We then measured the resulting&nbsp;</span><i>δ</i>¹⁵<span>N in the surrounding dry cyanobacteria- and lichen-dominated crusts and grass,&nbsp;</span><i>Achnatherum hymenoides</i><span>, after 24 h. We also estimated the biomass of fungal constituents using quantitative PCR and characterized fungal communities by sequencing the 18S&nbsp;rRNA gene. We found evidence for the initiation of fungal loops in cyanobacteria-dominated crusts where&nbsp;</span>¹⁵<span>N, from&nbsp;</span>¹⁵<span>NH</span>₄⁺<span>, moved 40 mm h</span>⁻¹<span>&nbsp;in biocrust soils with the&nbsp;</span><i>δ</i>¹⁵<span>N of crusts decreasing as the radial distance from the water addition increased (linear mixed effects model (LMEM)):&nbsp;</span><i>R</i>²<span> = 0.67,&nbsp;</span><i>F</i>_2,12<span> = 11,&nbsp;</span><i>P</i><span> = 0.002). In cyanobacteria crusts,&nbsp;</span><i>δ</i>¹⁵<span>N, from&nbsp;</span>¹⁵<span>NH</span>₄⁺<span>, was diluted as Ascomycota biomass increased (LMEM:&nbsp;</span><i>R</i>²<span> = 0.63,&nbsp;</span><i>F</i>_2,8<span> = 6.8,&nbsp;</span><i>P</i><span> = 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&nbsp;</span>¹⁵<span>NH</span>₄⁺<span>&nbsp;may indicate a fungal preference for the reduced N form during amino acid transformation and translocation. We found a marginally significant enrichment of&nbsp;</span><i>δ</i>¹⁵<span>N in&nbsp;</span><i>A. hymenoides</i><span>&nbsp;leaves but only in cyanobacteria biocrusts translocating&nbsp;</span>¹⁵<span>N, 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 NH</span>₄⁺<span>&nbsp;over NO</span>₃⁻<span>&nbsp;through biocrust networks.</span></p> application/pdf 10.5194/bg-15-3831-2018 en Copernicus Publications Fungal loop transfer of nitrogen depends on biocrust constituents and nitrogen form article