Factors limiting microbial biomass in the mineral soil and forest floor of a warm-temperate forest

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Abstract

Soil respiration and microbial biomass-N, in the forest floor, and in the top 10cm 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 μg g−1 in the litter layer and 52.3 μg 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 in the forest floor. Soil respiration increased with N, P and sucrose additions. We found seasonal differences in soil respiration and microbial biomass through the year with a winter minimum and a summer maximum. The limitation of 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 increased N deposition.

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