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 av ailability. To evaluate this hypothesis, an experiment was carried out to a ssess the effects of adding C, N and P on the relationship of active and to tal microbial biomass by comparing biomass C estimates using substrate-indu ced respiration (SIR) and fumigation extraction (FE). Effects on the biomas s 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 adde d separately or in combinations for 15 months at biweekly (14 d) intervals. After this period, measurements were taken of soil C and N, basal respirat ion, 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 proper ties in litter differed markedly from those in soil, the ratios of biomass C-SIR-to-C-FE, biomass C-FE-to-N and C-FE-to-P in litter exceeding those in soil. The differences within the A horizon were small. Total N was signifi cantly increased in the L horizon by the addition of C, N and P. Water cont ent 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 th e A horizon. The addition of C and P caused a significant increase in bioma ss N, biomass C-FE and biomass C-SIR, and a significant decrease in the bio mass 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 gener ally caused a significant decrease in the biomass CFE-to-P ratio, but an in crease in biomass P only in the L horizon. Although many effects of our tre atments 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 i nput. We discussed extensively the observation that the soil and its microf lora preserved its original characteristics. The differences between the bi omass estimates of SIR and FE are discussed and attributed to changes in th e fungal community structure. (C) 1999 Elsevier Science Ltd. All rights res erved.