Evaluation of land use change on an andosol through physicochemical and biological indicators

The conversion of forests to agricultural land can dramatically alter soil properties, but soil resistance, which is theability of soil properties or processes to remain unchanged in the face of a specific disturbance or stress, remainsunclear. We evaluated the impact of land use change and agricultural management on changes on an andosol in theCauca department, Colombia, through the analysis of physicochemical variables and biological indicators (dimensionlessresistance index, where +1 is the highest resistance and -1 is the lowest resistance) that allowed the assessment of soilresistance. The land uses analyzed included (1st) forest, which was approximately 100 years of age, plus areas of the same forest (70% of the area), which had been replaced by (2nd) natural pastures and (3rd) forage crops in the year 1985, i.e. 30 years before the observations. All physicochemical variables except soil clay content were significantly affected by the change from forest to natural pasture. Similarly, the change from forest to forage cropping affected all physicochemical variables as well as resulting in a decrease in soil microbial biomass but an increase in microbial activity. We found that the metabolic quotient (-0.32) had the lowest resistance, followed by the microbial coefficient (0.19), microbial biomass (0.32) and microbial activity (0.39), suggesting that soil stress caused by disturbance has a marked impact on the number and activity of the soil microflora. By contrast the change from forest to natural pastures was not associated with any effect on microbial biomass and its activity, suggesting that the continuous input of organic matter to the soil through the supply of organic residues from diversified root systems and nutrients from livestock urine and manure favored the preservation and resistance of microbial processes in the soil. These findings suggest that deforestation to establish natural pasture has less impact on soil stability and health than cultivating the soil following clearing.