SEASONAL FLUCTUATIONS IN MICROBIAL-POPULATIONS AND AVAILABLE NUTRIENTS IN FOREST SOILS

Viable microorganisms, soil respiration, and available N, Ca, Mg, Na, K, and P contents were determined in samples of five different forest soils collected in spring, summer, autumn, and winter. Viable microorganisms and soil respiration were positively correlated and showed a clear seasonal trend. The soils exhibited high microbial population values in spring and autumn and low values in summer and winter; total respiration values were largely higher in autumn than in the other seasons. Seasonal variations in available Ca, Na, and K contents were much more marked than those found for available N, Mg, and P. Available N and K and the microbial population showed similar seasonal trends whereas available Ca, Mg, Na, and P did not exhibit a distinguishable and uniform seasonal pattern. The quantities of available nutrients in soils followed the order Ca > K = Na > Mg > P > N. Soils developed over basic rocks showed higher values of both microbial density and microbial activity than those in soils developed over acid rocks. All the variables analysed were clearly related to the type of soil but varied with the date of sampling; a significant seasonal effect on the microbial population, microbial activity and available nutrients was detected in all the soils studied.