A considerable part of eucalypt plantations in Brazil grows on low-fertility soils for which fertilization techniques and nutrient cycling processes are important to elevate and maintain the forest production. The response of these plantations to nitrogen fertilization has been relatively low, indicating that the soil has been capable of supplying part of the plant demand. Microbial biomass N is considered a readily available fraction, but little is known about the biotic and abiotic factors that affect its dynamics in eucalypt plantations in Brazil. Therefore, the objective of this work was to evaluate the variation of nitrogen in microbial biomass (NBM) of the soil and other related characteristics in seven-year-old eucalypt plantations in several soil and climatic conditions of the Brazilian Southeast. Soil samples (0-10 cm deep) were collected from March to November 1995. The NBM content ranged from 9.17 to 103.71 µg g-1, being significantly influenced by sampling time. This variation was explained by the combination of rainfall with soil C or N content. Clay content also had a strong influence on the variation. N-NH4+was the predominant form of mineral N. Carbon content and CBM (carbon of the microbial biomass) ranged from 2.78 to 12.32 g kg-1 and from 43.39 to 401.06 µg g-1, respectively. The RA (accumulated soil respiration) ranged from 14.57 to 79.42 µg g-1 and the qCO2 (metabolic quotient) ranged from 862 to 8026 µg g-1 h-1 . Unlike the C and N content, CBM was significantly influenced by sampling time. Simple linear regressions of N-NO3-, C, and N content, CBM and NBM with clay content allowed a predictive model to explain the variation of these attributes. Soil humidity explained 87% of the N-NO3- and N content variation; around 71% of the C content and 48 and 55% of the CBM and NBM variation, respectively. CBM or NBM, when considered together with C and N content, were good predictors of N mineralization.
organic matter; nutrient cycling; microbial activity
