Evidence for a fungal loop in shrublands

1. Dryland communities may mitigate the loss of limited resources by exchanging nutrients through subterranean fungal connections, termed fungal loops. In arid grasslands, fungal loops can influence community composition and primary productivity, yet their ecological significance across dryland systems remains unexplored. We investigated the functional role of fungal loops in nutrient translocation in a North American shrubland ecosystem.

2. We traced the movement of 15N from moss-dominated biocrusts to the dominant xeric shrub Larrea tridentata, and the movement of 13C from L. tridentata to biocrusts in plots established in situ in the Sonoran Desert. Measurements occurred at three time points spanning one week following a simulated 2.5 mm rainfall event, and at distances up to 1 m from tracer application. We also used ITS sequencing to investigate changes in fungal community composition in soils over the one-week period.

3. We discovered movement of 15N from biocrusts into L. tridentata foliage as well as 15N movement to other spatially isolated moss-dominated biocrust patches, yet this movement did not occur until 4-6 days post-rainfall, when significantly higher δ15N was observed in L. tridentata and biocrusts compared to previous days. We did not observe consistent patterns of 13C movement from L. tridentata into neighboring shrubs or biocrusts, suggesting differential environmental drivers for carbon movement in this system. Fungal communities exhibited a decrease in alpha diversity on the last day of the study, indicative of a delayed community response to rainfall concomitant with nutrient translocation. Fungal endophyte orders Pleosporales and Pezizales dominated all plot soils, and order Pleosporales was significantly more abundant in 15N enriched plots, suggesting that dark-septate endophytic fungi were involved in nitrogen translocation. The delay in nutrient translocation may reflect a rainfall-triggered rebuilding of mycelial networks between community members following drought.

4. Synthesis: Our results point to fungal-mediated nutrient exchange pathways in a previously uninvestigated vegetation type, shrublands, where nutrients are translocated between moss-dominated biocrusts and nearby shrubs. We provide the first evidence that nutrient transfer may be delayed up to six days following rainfall, consistent with pulse-dynamic responses in drylands, and that moss-dominated biocrusts play a role in fungal loops.

Methods
These data were collected and processed following the protocols outlined in Carvajal Janke & Coe, 2021.