Response of soil microorganisms to the addition of carbon, nitrogen and phosphorus in a forest Rendzina

Soil microorganisms are believed to be controlled by energy and nutrient availability. To evaluate this hypothesis, an experiment was carried out to assess the effects of adding C, N and P on the relationship of active and total microbial biomass by comparing biomass C estimates using substrate-induced respiration (SIR) and fumigation extraction (FE). Effects on the biomass C-to-N and C-to-P ratios were also studied. In a beech forest soil, 110 g C m−2 (glucose), 4 g N m−2 (NH4NO3) and 0.3 g P m−2 (NaH2PO4) were added separately or in combinations for 15 months at biweekly (14 d) intervals. After this period, measurements were taken of soil C and N, basal respiration, biomass C by SIR and biomass C, N and P by FE in the L horizon (litter layer) and in two layers of the A horizon (mineral soil). Microbial properties in litter differed markedly from those in soil, the ratios of biomass CSIR-to-CFE, biomass CFE-to-N and CFE-to-P in litter exceeding those in soil. The differences within the A horizon were small. Total N was significantly increased in the L horizon by the addition of C, N and P. Water content and basal respiration rate were significantly increased by the addition of C, organic C was decreased by the addition of P in the two layers of the A horizon. The addition of C and P caused a significant increase in biomass N, biomass CFE and biomass CSIR, and a significant decrease in the biomass CSIR-to-CFE ratio in all three layers. In contrast, the addition of N caused a significant increase in biomass CSIR only. The addition of P generally caused a significant decrease in the biomass CFE-to-P ratio, but an increase in biomass P only in the L horizon. Although many effects of our treatments were significant, they were relatively small compared to the large amounts of C, N and P added which were many times greater than the annual input. We discussed extensively the observation that the soil and its microflora preserved its original characteristics. The differences between the biomass estimates of SIR and FE are discussed and attributed to changes in the fungal community structure.