Soil microbial indicators sensitive to land use conversion from pastures to commercial Eucalyptus grandis (Hill ex Maiden) plantations in Uruguay

Commercial forest plantations have increased during the last decade in Uruguay in soils of low cropping capability. Eucalyptus grandis (Hill ex Maiden) has been the main species planted due to its fast growth and adaptability to climate fluctuations. Assuming that the conversion from natural grazed pastures to commercial Eucalyptus plantations generates significant changes in the soil biological properties, we compared microbial enumeration and variables directly related to microbial activity to characterize these changes, as well as to determine the extent to which these soil biological properties change seasonally and with soil depth. The soil use conversion from pasture to forest land did not have a significant effect on the number of cellulolytic aerobes, P-solubilizers and Azotobacter spp. communities. Soil respiration, the C-mineralization coefficient, dehydrogenase, fluorescein diacetate hydrolysis and acid and alkaline phosphatase activities were affected significantly. Microbial enumeration of cellulolytics, P-solubilizers and Azotobacter spp., and parameters related to microbial biomass, soil respiration and the C-mineralization coefficient showed marked effects of sampling season. This, however, was not the case for the relative levels of the enzyme activities evaluated. These results indicated that the enzyme activities evaluated were sensitive and reliable indicators of the biochenucal changes generated by the soil use change. Spring appeared to be a better time for sampling than summer or winter because enzyme activities tended to be higher. Soil sampling depth was shown to be an important factor for obtaining consistent results, especially for the measurement of enzyme activities. For the last indicators, as well as the others, better results were obtained sampling and analyzing the upper 10 cm of the mineral soil profile. Our work suggested that microbial biomass, soil respiration, and enzyme activities are useful tools to assess biological soil quality changes due to the conversion of pasture land to planted E. grandis forest. (C) 2004 Elsevier B.V All rights reserved.