Cover crops and compost influence soil enzymes during 6 years of tillage-intensive, organic vegetable production

Soil enzymes are considered sensitive indicators of soil health but are not well understood in tillage-intensive vegetable systems. The activities of soil enzymes involved in nutrient cycling (β-glucosidase, β-glucosaminidase, alkaline phosphatase, dehydrogenase, aspartase, and L-asparaginase) were evaluated during 6 yr of commercial-scale production in five organic vegetable systems in Salinas, CA. The systems differed in yard-waste compost inputs (none or 15.2 Mg ha–1 yr–1), winter cover crop frequency (annually or every fourth year), and cover crop type (legume-rye, mustard, or rye). Large differences in cumulative organic matter input (7.4 to 136.8 Mg ha–1) from compost and cover crop shoots affected soil enzyme activities. With exception of aspartase, all enzyme activities were on average lowest without compost, intermediate with compost and infrequent cover cropping, and highest with compost and annual cover cropping. After 6 yr of vegetable production there was a positive relationship between microbial biomass and activities of all enzymes except aspartase. Despite lower inputs of cover crop shoot biomass from mustard compared with rye and the legume-rye, and differences in shoots residue quality, cover crop type had relatively little influence on enzyme activities. We conclude that soil enzyme activities were influenced primarily by annual cover cropping. These results and other attributes of soil health in this long-term study illustrate the importance of frequent cover cropping in tillage-intensive vegetable production. This raises questions about the sustainability of organic and conventional vegetable systems if cover crops are seldom used, and highlights the need for innovative strategies to increase cover cropping.