Effect of foot disturbance to cyanobacteria-dominated biocrusts on microbes and nitrogen in Chihuhuahan grassland and shrubland

Interactions between plants and soil microbes influence plant
nutrient transformations, including nitrogen (N) fixation, nutrient
mineralization, and resource exchanges through fungal networks.
Physical disturbances to soils can disrupt soil microbes and
associated processes that support plant and microbial productivity.
In low resource drylands, biological soil crusts
("biocrusts") occupy surface soils and house key
autotrophic and diazotrophic bacteria, non-vascular plants, or
lichens. Interactions among biocrusts, plants, and fungal networks
between them are hypothesized to drive carbon and nutrient dynamics;
however, comparisons across ecosystems are needed to generalize how
soil disturbances alter microbial communities and their
contributions to N pools and transformations. To evaluate linkages
among plants, fungi, and biocrusts, we disturbed all unvegetated
surfaces with human foot trampling twice yearly in dry conditions
from 2013-2018 in cyanobacteria-dominated biocrusts in Chihuahuan
Desert grassland and shrubland ecosystems. Our study included
microbial communities and N pools sampled at different time points
in the disturbance treatments at one or both sites. We began our
sampling after observations in April 2018 that the chlorophyll a
content was at least double in control than disturbed plots in both
ecosystems (Chung et al. 2019). Stomping occurred in May, and we
collected soil and plant samples in June 2018 for N pools and soil
and root fungal abundance. We collected additional soil samples in
September 2018 and conducted the 15N tracer experiment to observe
rates of N transfer from biocrust to plants before the fall stomp
treatment in October. We collected chlorophyll a samples and soils
for sequencing bacteria in September of 2019, also before the fall
stomp treatment.