FACTORS LIMITING MICROBIAL BIOMASS IN THE MINERAL SOIL AND FOREST FLOOR OF A WARM-TEMPERATE FOREST

Soil respiration and microbial biomass-N, in the forest floor, and in the top 10 cm of the mineral soil, was measured in a warm-temperate forest soil in North Carolina (U.S.A.). We amended soils with nitrogen, phosphorus and sucrose at five different samplings from May 1991 to July 1992. Average microbial biomass-N was 463 mu g g(-1) in the litter layer and 52.3 mu g g(-1) in the mineral soil. N fertilization increased microbial biomass in the forest floor but in the mineral soil microbial biomass only responded to P fertilization. Phosphatase activity g(-1) of organic matter was four times higher in the mineral soil than the forest floor. Soil respiration with N, P and sucrose additions. We found seasonal differences in soil respiration and microbial biomass by P in the mineral soil is explained by the decrease of P availability due to P sorption by Al and Fe sesquioxides in the upper mineral soil, and by P uptake by roots from the forest floor. Our results suggested that in this forest microbial biomass would not act as a major N sink with decreased N deposition.