Grazing and edaphic properties mediate soil biotic response to altered precipitation patterns in a semiarid prairie

Global circulation models predict shifts in precipitation patterns in arid and semiarid ecosystems worldwide. Independently of changes in total precipitation, rainfall is expected to concentrate in less frequent but larger events, and this redistribution will increase drought severity in biomes naturally controlled by precipitation pulses and water availability. This study focuses on effects of altering rainfall patterns on the soil food web of a semiarid prairie, and on impacts of grazing on soil food web resilience to climate change. We worked in a greenhouse with undisturbed soil monoliths extracted from grazed and ungrazed field plots. We planted all monoliths with Bouteloua gracilis that was the dominant grass at the experimental site, and let the grass establish before applying two different rainfall treatments over a five-month period, equivalent to the duration of the growing season in the area. We simulated the current rainfall pattern in the experimental area and also an altered rainfall pattern consisting of concentrating the total growing season precipitation in half the number of rain events. Rainfall concentration increased the amplitude of the water pulse and lengthened the dry periods between rain events thus worsening soil water stress. We analyzed the experimental monoliths for effects of rainfall pattern and grazing management C and N content, soil organic matter mineralization and soil biota abundance and functional diversity. Based on the experimental data, we simulated soil food web stability and mineralization capacity. We concluded that (a) rainfall concentration causes loss of key soil functional groups and reinforces soil food web stability, (b) organic matter mineralization by the soil food web decreases with rainfall concentration, (c) coarse-textured soils are more responsive than fine-textured soils to modifications of the rainfall regime, (d) soil food web responsiveness to changing rainfall regime is higher in grazed prairies than in prairies excluded from herbivores.