Trends of lettuce and carrot yields and soil enzyme activities during transition from conventional to organic farming in an Andosol(Soil fertility)

It has been reported that crop yields drop and then increase during the first few years of organic farming, and these yield recoveries have been attributed to gradual improvements in soil properties, such as soil microbial activities to mineralize nitrogen (N) or to suppress plant disease. To clarify whether yield increase during organic transition is caused by improvement of soil microbial activities, we compared identically managed organic and conventional plots of 1-year lettuce (Lactuca sativa L.)-carrot (Daucus carota L.) rotation for 3 years (organic plots: first 3 years after switching from conventional to organic management; conventional plots: managed in the same way as organic plots for 3 years but receiving chemical fertilizer, fungicide, insecticide and herbicide) in an Andosol field. During organic transition, yields of organic lettuce and carrots were lower than those of conventional lettuce and carrots for only the first year. Yield drop and recovery of lettuce were thought to be caused by changes in the amount of N uptake, though yield fluctuation of carrots was mainly caused by damage from insects. Although soil enzyme activities may be responsible for N mineralization, various soil enzyme activities promptly responded to organic amendment to become higher under organic management than under conventional management even after the first lettuce cropping (6 months after switching to organic management; much shorter than the period of organic transition). However, discriminant analysis using activities of six soil enzymes (dehydrogenase, β-glucosidase, β-galactosidase, α-glucosidase, cellulase and protease) indicated that 18-24 months (a period close to that of the organic transition) were needed for the pattern of various soil enzyme activities to be in a steady state after switching to organic management. The pattern of soil enzyme activities fluctuating to a plateau during the second lettuce cropping seemed to show a tendency similar to that of N uptake and yield of lettuce during organic transition. Soil available N in organic plots also became higher than that in conventional plots in the third year. These results suggested that improved N uptake and yield of lettuce during organic transition in an Andosol might be caused by either improvement in various soil enzyme activities or accumulation of soil available N. Yield response of carrots demanding less N was attributed not to N mineralization but to damage from insects.