Modeling soil nutrients, microbial biomass, and soil activity in areas undergoing woody shrub encroachment

Arid and semi-arid grassland savannahs are undergoing prolific changes in vegetation due to woody shrub encroachment. Although the spread and proliferation of these shrubs is well documented, uncertainties related to landscape-scale soil biogeochemical consequences and spatial patterns associated with shrub invasion remain. To help constrain these uncertainties, we measured vegetation and soil properties across 52,000 ha in southeastern Arizona to determine whether emergent patterns of woody shrub influence over soils are detectable across a large range of heterogeneity in soil types, parent materials, soil age, and topographic factors. We hypothesized that shrub impacts on soil dynamics (such as nutrients, microbial biomass, and microbial activity) would vary according to soil type, water regime, and shrub biomass. Results show that plant inputs are always important predictors of soil dynamics. Considering soil nutrients, microbial biomass and soil bulk density are also important predictors. Litter depth and soil nutrients are the strongest predictors of microbial biomass. Soil pH, soil water content, and soil nutrients are the strongest predictors of microbial activity. Furthermore, soil indices are not needed to improve the accuracy of soil nutrient predictions but they are in modeling microbial biomass and activity. These results suggest the ability to accurately model belowground nutrients across large areas undergoing woody shrub encroachment utilizing solely aboveground plant data and soil texture information.