The fungal loop hypothesis states that soil fungi facilitate the exchange of water, carbon, and nutrients between plants and biological soil crusts (biocrusts) in drylands, supporting productivity and retaining resources in the biotic pool. Previous research has shown that carbon and nitrogen can move between plants and biological soil crusts at rates that exceed physical processes alone. We examined plant-biocrust systems at the Sevilleta and Jornada LTER sites in New Mexico as well as at a site on the Colorado Plateau near Moab, Utah. We used stable isotopic tracers of nitrogen (15N) added to biocrusts in a variety of sites, seasons, moisture conditions, label types and concentrations, and fungal exclusion mesh treatments. We primarily focused on Bouteloua eriopoda but also examined individuals of Gutierrezia sarothrae and Achnatherum hymenoides. Across our three dryland sites, we examined 219 target plants in 2017 and 232 in 2018.
Results/Conclusions
After three days, we saw little 15N movement from biocrusts to plants, with only 11/451 plants exhibiting clearly elevated 15N in leaves. Five of these were G. sarothrae plants at our Colorado Plateau site. We did however observe the label moving laterally through the surrounding surface soil in 32/58 soil samples taken from various positions within the raceways. By 7-10 days the 15N had reached leaves in 50% of plants, with some site differences, for example more movement at Jornada than Sevilleta in 2017. The Jornada plants showed uptake of the 15N tracer regardless of fungal exclusion mesh going down to 10 cm, indicating that near-surface fungal transfer was not the movement mechanism. While our data do suggest that it is possible, under particular circumstances, for nitrogen to move from biocrusts to plants, taken together, these data do not support widespread movement of nitrogen from biocrusts to plants over the short time scales (24-72 hours) previously reported.