Kinetics of soil enzyme activities under different ecosystems: An index of soil quality

Soil microbial activity plays an important role in regulating biotransformation, nutrient cycling and hence the microbiological processes are at the center of many ecological functions. The kinetic parameters (V_max and K_m Michaelis constant) of different enzymes (amylase, invertase, protease, urease, and dehydrogenase) were determined in order to assess the metabolic response of soil. The maximum reaction velocity (V_max) represents a maximum rate of activity when all enzymes are saturated, which markedly increased in forest soil as compared to fresh mine spoil due to the gradual accumulation of soil organic matter. Smaller K_m value was estimated in forest soil (FS) as compared to fresh mine spoil (FMS), suggesting the greater affinity of soil enzymes for substrate in FS. The catalytic efficiency (V_max/K_m) reflects an impression on microbial community composition with a change in soil enzymes. These enzyme characters (activities and kinetic parameters) have greater significance as early and sensitive indicators of the changes in soil properties induced by different management systems. These parameters (V_max and K_m) can be useful markers to assess changes in microbial activity of soil, since they represent quantity and affinity of enzymes respectively. The metabolic index (dehydrogenase activity/organic carbon (OC)) was found to be correlated with V_max of dehydrogenase (r = 0.953; p < 0.01) and OC (r = 0.880; p < 0.01). Principal component analysis was able to discriminate seven different soil samples into seven independent clusters based on their enzyme activities and kinetic parameters. Indeed, the study revealed the importance of kinetics study of soil enzymes, which can be considered valid parameters to monitor the evolution of microbiological activity in soil, and hence an index of soil quality.