MICROBIAL BIOMASS AND ITS CONTRIBUTION TO NUTRIENT CONCENTRATIONS IN FOREST SOILS

Soil microbial biomass and its contribution to the available Ca, Mg, Na, K and P concentrations were studied in a wide range of forest soils. Biomass C was measured using the chloroform fumigation-incubation technique; N, P, K and Ca in biomass were estimated from biomass C values. Soils were incubated at 25-degrees-C for 10 days and the differences between the available nutrient contents in the fumigated and the unfumigated soils (the flush of nutrients) were used to estimate the contribution of the microorganisms to the available nutrient concentrations. Microbial biomass C and N ranged from 282 to 1614 mug C g-1 dry soil (d.s.) and from 42 to 242 mug N g-1 d.s. The average quantities of P, K and Ca in microbial biomass were about 86, 73 and 10 mug g-1 d.s., respectively. The death of microorganisms by CHCl3 fumigation caused an immediate flush (flush during day 0) of N, K, Na, P, and Mg with mean values of 10, 21, 8, 7 and 4 mug g-1 d.s., respectively. After 10 days of incubation the flush of nutrients (flush during day 10) showed mean values of 36 mug g-1 d.s. for N, and 20, 9 and 6 mug g-1 d.s. for the K, P. and Mg respectively, whereas the Na flush values were not significant in most soils. As for Ca the variability between the soil replicates was higher than the variability caused by the fumigation or incubation. Biomass C showed a significant relationship with the N flush measured in day 10 and with the Mg, K and P flush measured in both days 0 and 10. The results showed that soil microorganisms contained substantial amounts of both C and inorganic nutrients and that their contribution to the pool of available nutrients was large for N, important in the cases of P, K and Mg and not significant for Na.