Complementary effects of encroachment and grazing intensity for soil quality in a mountain grassland

Abstract

Grasslands are globally significant ecosystems held in an ecological balance by herbivory, with a natural tendency towards shrub encroachment, creating a tension between two alternative successional trajectories. The influence of grazing pressure on soil properties is not yet accurately predicted across ecosystems, nor its interaction with encroachment, with quite variable effects found. We studied the combined effect of grazing and encroachment on soil ecophysiological processes by collecting soil samples from paired locations with and without shrub presence across a managed high-altitude pastureland of 14,000 ha. By tracking 26 sheep herds with GPS collars over multiple years, we created a spatially explicit map of grazing intensity over the landscape, permitting quantification of grazing impacts on soil properties on a continuous scale. Grazing changed soil nutrient status, increasing total soil nitrogen (TN) by up to 0.77 % over the gradient, and available phosphorus (P_avail) by up to 12 times. Grazing increased soil organic carbon (SOC) content by up to 4.5 % across the gradient, and encroachment increased SOC by 1.2 %. SOC increases under these two conditions likely represent two co-occurring paths soil carbon accumulation since particulate organic carbon (POC) was increased by 44 % under shrubs, while the effect of grazing was unclear. Grazing and encroachment impacts on the soil microbial community diverged, since microbial biomass carbon (C_mic) increased by up to 86 % with grazing intensity with a simultaneous decrease in microbial basal respiration and metabolic quotient (qCO₂), however neither were affected by encroachment. Overall, encroachment and grazing were seen to be complementary for soil protection and provision of ecosystem services, though their effects on certain parameters were contrasting. This knowledge may be useful for adaptive management in high nature value agroecological landscapes, and can improve large-scale projections of SOC stocks and other soil properties incorporating varying degrees of grazing intensity and the influence of encroachment.