Nematode communities indicate diverse soil functioning across a fog gradient in the Namib Desert gravel plains.

Soil nematodes are fundamentally aquatic animals, requiring water to move, feed, and reproduce. Nonetheless, they are ubiquitous in desert soils because they can enter an anhydrobiotic state that allows them to persist when water is biologically unavailable. In the hyper-arid Namib Desert of Namibia, rain is rare, but fog routinely moves inland from the coast and supports plant and animal life. Very little is understood about how this fog may affect soil organisms. We investigated the role of fog moisture in the ecology of free-living, soil nematodes across an 87-km fog gradient in the gravel plains of the Namib Desert. We found that nematodes emerged from anhydrobiosis and became active during a fog event, suggesting that they can utilize fog moisture to survive. Nematode abundance did not differ significantly across the fog gradient and was similar under shrubs and in interplant spaces. Interplant soils harbor biological soil crusts that may sustain nematode communities. As fog declined along the gradient, nematode diversity increased in interplant soils. In areas where fog is rare, sporadic rainfall events can stimulate the germination and growth of desert ephemerals that may have a lasting effect on nematode diversity. In a 30-day incubation experiment, nematode abundance increased when soils were amended with water and organic matter. However, these responses were not evident in field samples, which show no correlations among nematode abundance, location in the fog gradient, and soil organic matter content. Soil nematodes are found throughout the Namib Desert gravel plains under a variety of conditions. Although shown to be moisture- and organic matter-limited and able to use moisture from the fog for activity, variation in fog frequency and soil organic matter across this unique ecosystem may be biologically irrelevant to soil nematodes in situ.

Characterization of soil nematode communities in three cropping systems through morphological and DNA metabarcoding approaches.

We used complementary morphological and DNA metabarcoding approaches to characterize soil nematode communities in three cropping systems, conventional till (CT), no-till (NT) and organic (ORG), from a long-term field experiment. We hypothesized that organic inputs to the ORG system would promote a more abundant nematode community, and that the NT system would show a more structured trophic system (higher Bongers MI) than CT due to decreased soil disturbance. The abundance of Tylenchidae and Cephalobidae both showed positive correlations to soil organic carbon and nitrogen, which were highest in the ORG system. The density of omnivore-predator and bacterial-feeding nematodes was reduced in NT soils compared to CT, while some plant-parasitic taxa increased. NT soils had similar Bongers MI values to CT, suggesting they contained nematode communities associated with soils experiencing comparable levels of disturbance. Metabarcoding revealed within-family differences in nematode diversity. Shannon and Simpson's index values for the Tylenchidae and Rhabditidae were higher in the ORG system than CT. Compared to morphological analysis, metabarcoding over- or underestimated the prevalence of several nematode families and detected some families not observed based on morphology. Discrepancies between the techniques require further investigation to establish the accuracy of metabarcoding for characterization of soil nematode communities.

Soil nematodes and desiccation survival in the extreme arid environment of the antarctic dry valleys.

Soil nematodes are capable of employing an anhydrobiotic survival strategy in response to adverse environmental conditions. The McMurdo Dry Valleys of Antarctica represent a unique environment for the study of anhydrobiosis because extremes of cold, salinity, and aridity combine to limit biological water availability. We studied nematode anhydrobiosis in Taylor Valley, Antarctica, using natural variation in soil properties. The coiled morphology of nematodes extracted from dry valley soils suggests that they employ anhydrobiosis, and these coiled nematodes showed enhanced revival when re-hydrated in water as compared to vermiform nematodes. Nematode coiling was correlated with soil moisture content, salinity, and water potential. In the driest soils studied (gravimetric water content <2%), 20-80% of nematodes were coiled. Soil water potential measurements also showed a high degree of variability. These measurements reflect microsite variation in soil properties that occurs at the scale of the nematode. We studied nematode anhydrobiosis during the austral summer, and found that the proportion of nematodes coiled can vary diurnally, with more nematodes vermiform and presumably active at the warmest time of day. However, dry valley nematodes uncoiled rapidly in response to soil wetting from snowmelt, and most nematode activity in the Dry Valleys may be confined to periods following rare snowfall and melting events. Anhydrobiosis represents an important temporal component of a dry valley nematode's life span. The ability to utilize anhydrobiosis plays a significant role in the widespread distribution and success of these organisms in the Antarctic Dry Valleys and beyond.