COMPARISON OF METHODS TO ESTIMATE MICROBIAL-C AND MICROBIAL-N IN LITTER AND SOIL UNDER PINUS-RADIATA ON A COASTAL SAND

A comparison of fumigation-incubation and fumigation-extraction methods to determine microbial biomass C and N, and the substrate-induced respiration method to estimate microbial C, was made in litter and soil from Pinus radiata plantations on a nitrogen-deficient coastal sand. The pH of the samples ranged from 5.0 to 5.6, and C:N ratios from 37 to 56 for litter and 22 to 29 for soil (0-10 cm depth). In the fumigation-incubation method, a large inoculum of unfumigated sample generally appeared most suitable for litter and either a large or small inoculum for the mineral soil. Satisfactory estimates of microbial C were thereby obtained, provided a fumigated control was used. In the fumigation-extraction procedure, a mean k(EC)-factor of 0.29 (range 0.28-0.31) resulted when extractable-C flush was calibrated against these microbial C values. With litter, substrate-induced respiration generally gave similar estimates of microbial C to those of the other two methods. With soil, substrate-induced respiration probably over-estimated microbial C in samples with a low microbial biomass content. Fumigation-incubation appeared to under-estimate mineral-N flush, and hence microbial N, in these N-immobilizing samples. Specific k(EN)-factors, for converting extractable-N flush to microbial N in the fumigation-extraction method, could consequently not be determined. When microbial C was estimated by fumigation-extraction, microbial C:N ratios ranged from 8.5 to 15 in the litter and from 6.4 to 7.6 in the soil. The percentages of total C and N present as microbial C and N were broadly similar to those found elsewhere. Metabolic quotients (ratios of CO2-C production to microbial C) were high, but consistent with the young ages of these ecosystems.