Desert soil microbial communities across a xeric stress gradient

Abstract

Deserts are the largest terrestrial ecosystem (Laity, 2009). It is estimated that 69% of desert lands that are used for agriculture around the world are either degraded or undergoing desertification as a consequence of climatic variation or intensive human activity (UNEP, 1992; Souvignet et al., 2012). Additionally, climate change models predict that the variability of rainfall events will intensify in desert regions (Faramarzi et al., 2013). Because desert environments contain a limited range of higher plants and animals, soil microbial communities are likely the most productive component of these systems as well as the dominant drivers of biogeochemical cycling (Makhalanyane et al., 2015). As a result, understanding how microbial communities respond to varying degrees of moisture input and xeric stress is important for developing sustainable resource management and agriculture practices, as well as predicting the impacts of global climate change on terrestrial systems (Paul, 2014). This study focuses on the Namib Desert in western Namibia, the oldest and one of the driest deserts on the planet (Prestel et al., 2008). In the Namib Desert, sporadic rainfall events provide 25 mm of mean annual rainfall a year; however, near the coast, consistent fog formation provides a form of available water to an area that would otherwise be hyperarid (Eckardt et al., 2013b). We examined the effect of xeric stress on microbial community structure and function in these desert soils, taking advantage of the naturally occurring gradient of water availability. Soil samples were collected every 10 km on a 190 km transect from the fog-dominated coastal region, through an inland area of high aridity, into a region of increased rainfall. Soil physicochemical properties and microbial community structure and function were assessed across the transect. Both microbial community structures and functions differentiated based on three a priori defined zones of differing xeric stress (i.e., the Fog , Dry , and Rain zones). Water availability was indicated as significantly shaping the microbial community structure and function in the central Namib Desert. In the fog dominated regions, stochastic processes dominated community assembly, while the deterministic effects of environmental filtering shaped community structure in the rest of the transect. In addition, a significant relationship between community structure and function was found (Mantel r = 0.2; p < 0.01), indicating changes in community structure coincided with changes in function.