Soil organic matter determines the distribution and abundance of nematodes on alluvial fans in Death Valley, California

Death Valley, located within the Mojave Desert of the United States, is one of the most extreme terrestrial environments on Earth. We studied free-living soil nematode abundance and community structure in soil samples collected from alluvial fans in Death Valley between 2013 and 2018. Our objective was to establish the distribution and abundance of soil nematodes on alluvial fans with respect to soil properties, vegetation, and channels that reflect past and future water flows. The study period spanned an extreme drought for the region but also included a 2016 superbloom of spring annual plants that followed a strong El Niño event. We found that nematode communities were dominated by bacterial-feeding species and had higher densities under shrub canopies than in adjacent interplant soils. Nematode abundance was similar between areas on alluvial fans that were channelized by past water flows and adjacent desert pavement and does not appear to be linked to how water distributes on the fans following rainfall events. Nematode abundance increased during the superbloom in soils around annual plants. Across the study period, soil organic matter content was the only soil variable measured that was significantly and positively correlated to nematode abundance. Overall, nematodes in this extreme environment are adapted to drought, but they are highly dependent on the growth and distribution of both perennial and annual plants to supply the soil organic matter that their microbial food sources rely upon.