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

Zachary T. Aanderud, Trevor B. Smart, Nan Wu, Alexander S. Taylor, Yuanming Zhang, Jayne Belnap
2017 Biogeosciences Discussions  
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 biocrust species composition and N form influence loops, we created a minor, localized rainfall event containing
more » ... ng <sup>15</sup>NH<sub>4</sub><sup>+</sup> and <sup>15</sup>NO<sub>3</sub><sup>&amp;minus;</sup> and measured the resulting <i>&amp;delta;</i><sup>15</sup>N in surrounding cyanobacteria- and lichen-dominated crusts and grass, <i>Achnatherum hymenoides</i>, after twenty-four hours. We also estimated the biomass of fungal constituents using quantitative PCR and characterized fungal communities by sequencing the 18S rRNA gene. We only found evidence of fungal loops in cyanobacteria-dominated crusts where <sup>15</sup>N, from <sup>15</sup>NH<sub>4</sub><sup>+</sup>, moved 40&amp;thinsp;mm&amp;thinsp;h<sup>&amp;minus;1</sup> and the <i>&amp;delta;</i><sup>15</sup>N in crusts decreased as the radial distance from the water addition increased (linear regression analysis: <i>R</i><sup>2</sup>&amp;thinsp;=&amp;thinsp;0.58, <i>F</i>&amp;thinsp;=&amp;thinsp;16, <i>P</i>&amp;thinsp;=&amp;thinsp;0.002, <i>n</i>&amp;thinsp;=&amp;thinsp;14). In cyanobacteria crusts, <i>&amp;delta;</i><sup>15</sup>N, from <sup>15</sup>NH<sub>4</sub><sup>+</sup>, was diluted as Ascomycota biomass increased (linear regression analysis: <i>R</i><sup>2</sup>&amp;thinsp;=&amp;thinsp;0.50, <i>F</i>&amp;thinsp;=&amp;thinsp;8.8, <i>P</i>&amp;thinsp;=&amp;thinsp;0.02, <i>n</i>&amp;thinsp;=&amp;thinsp;14), Ascomycota accounted for 82&amp;thinsp;% (&amp;plusmn;2.8) of all fungal sequences, and one order, Pleosporales, comprised 66&amp;thinsp;% (&amp;plusmn;6.9) of Ascomycota. The lack of loops in moss-dominated crusts and substantial movement of <sup>15</sup>NO<sub>3</sub><sup>&amp;minus;</sup> may stem from mosses effectively sequestering newly fixed N and fungi preferring <sup>15</sup>NH<sub>4</sub><sup>+</sup> for amino acid transformation and translocation. No label entered <i>A. hymenoides</i>. Our findings suggest that minor rainfall events allow dark septate Pleosporales to rapidly translocate N in the absence of a plant sink.
doi:10.5194/bg-2017-433 fatcat:u7dfup7wizdr3pvla67nu4qsuu