THERMAL-STABILITY AND ACTIVITIES OF SOIL ENZYMES AS INFLUENCED BY CROP ROTATIONS

Soil samples were collected to a depth of IO cm in 1991 and 1993 from a vegetable crop rotation experiment initiated in 1989. The two cropping treatments, with either 0 or 280 kg N ha(-1), represented the traditional vegetable rotation (TVR) and an alternative legume vegetable rotation (LVR) when a vegetable crop alternated with a red clover (Trifolium pratense L.) seed-crop that was incorporated as green manure in the following spring. The enzyme activities of L-asparaginase, amidase and beta-glucosidase were determined on whole soil and five soil aggregate size fractions: 1.00-2.00, 0.50-1.00, 0.25-0.50, 0.10-0.25 and <0.1 mm. Thermal stability of the enzymes was determined by conditioning soil samples at 85 degrees C for 2 h or by exposing soil samples to five successive freeze-thaw cycles prior to enzyme assays. Enzyme activities for LVR were significantly greater (P < 0.05) than TVR for beta-glucosidase and amidase at both N-rates in 1991. This difference in activity for these enzymes was sustained only at the high N-rate in 1993. The activity of L-asparaginase was significantly higher (P < 0.01) in the LVR in 1993. Sixty to seventy percent of the soil enzyme activity (on a mass basis) was associated with macroaggregates with specific distribution of activity across aggregate fractions varying with enzyme. Average beta-glucosidase activity decreased by 50% from 1991 to 1993. In contrast, amidase activity increased 1.43-fold over the same period. beta-glucosidase activity was sensitive to temporal trends, showing proportional decreases in activities in each system that were consistent with decreases in organic C. Furthermore, beta-glucosidase activity showed significantly higher (P < 0.05) resistance to heat-induced thermal stress in the LVR in both sampling years. Amidase and L-asparaginase activities showed no such treatment effects. The results showed that soil enzyme activity is a sensitive biological indicator of the effects of soil management practices.