Linkages Between Microbial and Hydrologic Processes in Arid and Semiarid Watersheds

Abstract

Microbial activity in semiarid and arid environments is closely related to the timing, intensity, and amount of precipitation. The characteristics of the soil surface, especially the influence of biological soil crusts, can determine the amount, location, and timing of water infiltration into desert soils, which, in turn, determines the type and size of microbial response. Nutrients resulting from this pulse then create a positive feedback as increases in microbial and plant biomass enhance future resource capture or, alternatively, may be lost to the atmosphere, deeper soils, or downslope patches. When rainfall intensity overwhelms the water infiltration capacity of the plant interspace or the plant patch, overland water flow links otherwise separated patches at many different scales via the transport of nutrients in water, soil, and organic matter. For example, material carried from the plant interspace is often deposited under an adjacent plant. Alternatively, material from both of these patches may be carried to rills that feed ephemeral channels, thence to seasonally intermittent and, finally, perennial streams. These inputs can either be retained by the stream–riparian ecosystem or be exported in surface flow. However, in larger perennial streams, the fate of these material inputs is confounded by the impact of storm‐driven flows on the extant aquatic biota, as flash floods can also represent succession‐initiating disturbances to the stream–riparian ecosystem on a wide range of time scales. In contrast to uplands where precipitation initiates the microbial response, nutrient transfers can support a flush of plant uptake and microbial processing, triggered by high nutrient concentrations and changed nutrient form (e.g., nitrate or ammonium). The nature and strength of the linkages between the different ecosystem components define the structure and function of arid ecosystems. Losses of materials are natural processes, but it is problematic when “conserving” systems become “leaky” via anthropogenic disturbance and losses exceed gains.

Vegetation Responses to Climate Variability in the Northern Arid to Sub-Humid Zones of Sub-Saharan Africa

“…Rainfall is usually assumed to be the only significant environmental factor that determines primary production in drylands [55,94,95]. However, the statistical and process modeling reported here indicated a wider range of environmental variables related to primary production and regional differences in the importance of these factors.…”

Soil water content drives spatiotemporal patterns of CO<sub>2</sub> and N<sub>2</sub>O emissions from a Mediterranean riparian forest soil

“…However, lower CO 2 emissions than expected for temperature dynamics were reported in summer at the intermediate and hillslope zones, likely because extreme soil dryness (soil water content < 20 %) limited respiration rates during such period (Chang et al, 2014;Goulden et al, 2004;Wickland et al, 2010). Although the mechanisms by which soil dryness may affect microbial C demand are still poorly understood, suppressed microbial respiration in summer can be attributed to a disconnection between microbes and resources (Belnap et al, 2005;Davidson et al, 2006), decreases in photosynthetic and exo-enzymatic activities (Stark and Firestone, 1995;Williams et al, 2000), or a relocation of the invested energy on growth (Allison et al, 2010). Altogether, these results suggest that soil water content may be as important as soil temperature to understand soil CO 2 effluxes, and therefore future warmer conditions may not fuel higher CO 2 emissions, at least in those regions experiencing severe water limitation.…”

Dynamics of organic carbon losses by water erosion after biocrust removal

“…Moreover, destruction of biocrusts strongly accelerates erosion and nutrient exhaustion in the disturbed patches resulting in sediments impoverished in OC, reducing the possible initial benefit. The consequent increase in runoff after disturbance once the soil seals and the biocrust is replaced by a physical soil crust, may cause part of the runoff water, sediments and nutrients, which in undisturbed conditions would have been trapped and stored in shrub patches, to run out of the system, increasing system connectivity and impairing its functioning, leading to a degraded system (Belnap et al, 2005).…”

“…Ecosystems with a high biocrust cover are within this group of ecosystems. Considering that these crusts may often represent up to 70% of ground cover (Belnap et al, 2005;Chamizo et al, 2012a;Miralles-Mellado et al, 2011) and over 60% of rainfall falling on a biologically crusted surface becomes runoff (Cantón el al., 2001;Chamizo et al, 2012a;Rodríguez-Caballero et al, 2013), which is able to export OC from biocrust constituents, suspended and dissolved by runoff and mobilized sediments, the mobilization of OC from biocrusts may be quite significant. Exported OC is often trapped by vegetated patches located downstream of crusted areas, providing an important nutrient supply essential to maintain primary productivity and biological activity in systems that are strongly limited by water and soil nutrient availability (Kidron, 2014;Whitford, 2002).…”