SEASONAL-CHANGES IN MICROBIAL BIOMASS AND NUTRIENT FLUSH IN FOREST SOILS

Microbial biomass and N, P, K, and Mg flushes were estimated in spring, summer, autumn, and winter samples of different forest soils. The microbial biomass showed significant seasonal fluctuations with an average distribution of 880+/-270 mu g C g(-1) soil in spring, 787+/-356 mu g C g(-1) soil in winter, 589+/-295 mu g C g(-1) soil in summer, and 560+/-318 mu g C g(-1) soil in autumn. The average annual concentrations of C, N, P, K, and Ca in the microbial biomass were 704, 106, 82, 69 and 10 mu g g(-1) soil, respectively. Microbial C represented between 0.5 and 20% of the organic soil C whereas the percentage of microbial N with respect to the total soil N was two- to threefold higher than that of C; the annual fluctuations in these percentages followed a similar trend to that of the microbial biomass. Microbial biomass was positively correlated with soil pH, moisture, organic C, and total N. The mean nutrient flush was 31, 15, 7, and 4 mu g g(-1) soil for N, K, P, and Mg, respectively, and except for K, the seasonal distribution was autumn greater than or equal to spring much greater than winter greater than or equal to summer. The average increase in available nutrients due to the mineralization of dead microbial cells was 240% for N, and 30, 26, and 14% for P, K, and Mg, respectively. There was a positive relationship between microbial biomass and the N, P, K, and Mg flushes. All the variables studied were significantly affected by the season, the type of soil, and the interaction between type of soil and season, but soil type often explained most of the variance.