Grass type drives small-scale spatial patterns in soil properties and processes

Plants are known to influence the spatial scale over which soil resources vary. While soil resource heterogeneity has been found to differ under plants of different stature (i.e. grasses vs. trees), few studies have focused on differences under comparably-sized species. Here we demonstrate that grass type controls patterns of soil resource heterogeneity over small spatial scales in a California grassland, where invasive annual grasses and native perennial grasses have similar biomass per unit area, but differ in growth form. To understand the differing effects that these grasses have on soils, we quantified the degree and scale of spatial heterogeneity in soil properties and processes in a variety of grasslands using geostatistical methods. With cross-semivariogram analysis and mapping techniques, we related aboveground patterns in annual and perennial cover to the spatial structure of soil resources and belowground processes. Perennial bunchgrass individuals found in natural stands in the absence of annual grass invasion are associated with “islands of fertility” - areas of high N and organic matter. In contrast, in experimental stands of mixed annual and perennial composition, perennial bunchgrass individuals draw down soil NO₃^- more than the surrounding annual grasses, producing zones of N depletion. Changes in perennial density are associated with changes in the scale of spatial heterogeneity in soil properties and microbial activity. While soil respiration, organic matter content, and NO₃^- were fairly uniform when only annuals grasses were present, the degree of heterogeneity increased with increases in perennial cover. The unique patterns in soil resource heterogeneity associated with cover of different grass species suggest that the shift from perennial to annual dominance in California grasslands led to changes in the small-scale spatial structure of soil properties and processes in these systems. These changes may in turn feed back to perpetuate annual grass dominance.