Heterotrophic Microbial Activity and Phosphorus Availability in Everglades Histosols.

Land use changes in the Everglades Agricultural Area (EAA) may alter microbial activity and organic matter decomposition and ultimately influence the fate of P in soil. The objectives of this study were to determine effects of long-term cultivation and fertilization on heterotrophic microbial activity and soil organic matter decomposition for contrasting land uses in the EAA. Soil (0-15 cm) under pasture for 100 yr and sugarcane (Saccharum sp.) for 50 yr were amended with P (0, 10, 50, 150 kg P ha-1) and the response of the microbial community measured. Long-term fertilization increased total P for cultivated soil (1227 mg P kg-1) relative to pasture (959 mg P kg-1), and changed its distribution, with greater allocation to inorganic pools for sugarcane (53%) than pasture (19%). Farming practices decreased the proportion of applied P recovered in plant-available forms due to greater P retention in inorganic pools. Microbial biomass C was not an adequate indicator of land use change, but biomass N was higher for pasture (71 mg N kg-1) than sugarcane (14 mg N kg-1), while biomass P was higher for sugarcane (3.7 mg P kg-1) than pasture (1.4 mg P kg-1). Carbon mineralization rates were higher for pasture, but organic N and P mineralization were higher for sugarcane. Phosphorus did not appear to be limiting microbial activity for either land use. The microbial community was more efficient at organic N and P mineralization for cultivated than pasture soil. Long-term fertilization and cultivation increased N and P turnover in soil, but mineralized nutrients did not accumulate in cultivated soil due to uptake by crops and retention by soil minerals. Conversion of cultivated soil to alternate land uses and subsequent elimination of crop nutrient uptake will increase nutrient accumulation in soil and may pose eutrophication hazards to proximal aquatic systems.