In natural grassland ecosystems deficient in available P, the temporary immobilization of P in microbial biomass and its subsequent mineralization can be considered a potential mechanism of P supply to plants. The purpose of this study was to evaluate the influence of burning and grazing of grassland vegetation on the dynamics of soil P, with emphasis on the content of P immobilized in microbial biomass. The treatments consisted of the combination of grazing (presence or absence) and burning (presence or absence) in a natural grassland on slope and lowlands after 13 years under a management involving burning and grazing. Burning and grazing were arranged in a completely randomized design with four replications. Soil samples were collected from the 0-10 cm layer twice during the growing period of native pasture, always immediately after grazing. The stocking rate was calculated based on a utilization rate of 20-35 % of existing forage mass. The P content stored in the soil microbial biomass, the total P content and total organic P in soil were determined. The statistical analysis was based on analysis of variance by randomization tests. The microbial P content of natural pasture ranged from 11.4 to 57.3 mg kg-1, representing on average 38 and 32 % of total organic P in soil in the first and second evaluation, respectively. The P immobilized in microbial biomass is a potential reserve of P that can meet the demand for native species in natural pastures, and is a more sensitive indicator than the total organic P content of the soil to detect changes induced by grazing. Management of natural pastures with fire decreases the amplitude of the increase in microbial P due to grazing.
native species; microbial biomass phosphorus; organic phosphorus
