NUTRIENT LIMITATIONS ON MICROBIAL RESPIRATION IN PEAT SOILS WITH DIFFERENT TOTAL PHOSPHORUS-CONTENT

The effects of phosphate and ammonium additions on microbial respiration were determined using three peat soils with similar total N content (25-38 g kg-1) and different total P (TP) content: low (231 mg P kg-1), intermediate (385 mg P kg-1), and high (1.473 g P kg-1). These soils, obtained from Everglades National Park, Florida, had been formed predominantly from the partial decomposition of sawgrass (Cladium jamaicense). Soil respiration was measured by gas chromatography. Amendment of the soils with 0-100 mm PO4 or NH4 resulted in changes in the C-to-nutrient ratio that ranged from 4 to 2042 for C:P and 11.5 to 14.1 for C:N. Kinetic parameters for soil respiration were obtained by fitting the data to a kinetic model that describes soil C mineralization as the sum of an exponential decay function for readily mineralizable C and a zero-order decay function for stable C. Estimates of the zero-order rate constant for endogenous respiration in unamended soil samples increased with TP content of the soil (in mmol CO2 kg-1 d-1): low TP (36-40) < intermediate TP (49-52) < high TP (107-114). Addition of phosphate stimulated the respiration rate of the low and intermediate TP soil, but had no effect on the rate of soil respiration of the high TP soil. Ammonium additions inhibited soil respiration in the low and intermediate TP soils at most concentrations tested; the rate of respiration was inversely proportional to the amount of NH4 added. Addition of ammonium to the high TP soil stimulated respiration. Microbial respiration in the low and intermediate TP soils is limited by P availability, whereas N appears to limit respiration in the high TP soil. These results suggest that P pollution may have a marked, long-term effect on microbial respiration in organic soils with low TP content.