Changes in microbial biomass and activity of tropical soil submitted to successive Eucalyptus rotations in the semi-arid region of Brazil

Abstract

Soil microbial biomass (SMB) is considered an important indicator of changes in the soil organic carbon dynamics. Although there are several studies on SMB in Eucalyptus plantations in Brazil, there are still no records of investigations which have evaluated the effect of successive crop rotations on soil microbial biomass. Therefore, the aim of this study was to evaluate changes in the biomass and microbial activity of tropical soil submitted to successive Eucalyptus cultivation rotations. The study was carried out in commercial plantations of Eucalyptus urophylla in the first, second and third rotations (area cultivated 4, 9 and 15 years ago, respectively) using a fragment of native forest as a reference. Soil collections were performed at a depth of 0–10 cm. The carbon and nitrogen contents of the soil microbial biomass were determined by the fumigation-extraction method and the soil microbial activity was based on the amount of CO2 evolved. The increase in Eucalyptus cultivation time caused increases in litter accumulation and soil organic carbon, which represented an increase of 50% in rotation 1 compared to rotations 2 and 3. This was reflected in increases in C immobilization in microbial biomass (MB-C) and in the efficiency of converting organic carbon to MB-C and reducing soil carbon losses. Following a similar pattern, the microbial biomass carbon (MB-C) levels showed increases of 52 and 47% in rotation 3 in relation to rotation 1 and 2, respectively. Only the metabolic quotient among the microbiological indices was sensitive to the Eucalyptus cultivation time, with a significant reduction in rotation 3 (131.0 mg g−1 day) compared to rotations 1 and 2 (average of 269.1 mg g−1 day). The change imposed by the progressive increase in the Eucalyptus cultivation time positively influenced the activity and C transformation of SMB.