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

Authors: Brennan, Eric; Acosta-Martinez, Veronica

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2018
Publication Date: 6/27/2019
Publication URL: http://handle.nal.usda.gov/10113/5617504
Citation: Brennan, E.B., Acosta Martinez, V. 2019. Cover crops and compost influence soil enzymes during 6 years of tillage-intensive, organic vegetable production. Soil Science Society of America Journal. 83:624-637.

Interpretive Summary: Enzymes are large molecules that help organisms speed up chemical reactions. In the soil, enzymes play critical roles transforming organic matter and nutrient cycling. They can act as sensitive indicators of soil health. Soil enzymes were evaluated during six years of commercial-scale production in five organic vegetable systems in a long-term experiment in the Salinas Valley. The five systems produced lettuce, and spinach or broccoli annually, and differed in yard-waste compost inputs (none or 15.2 Mg ha-1 year-1), winter cover crop frequency (annually or every 4th year), and cover crop type (legume-rye, mustard, or rye). Cumulative organic matter input from compost and cover crop shoots over the six years ranged from 7.4 to 136.8 Mg ha-1 and caused large differences soil enzyme activities. This study provides some of the most in-depth information on soil enzymes in tillage intensive vegetable systems and indicates the importance of frequent inputs of fresh organic matter from cover crops for maintaining soil health in organic and conventional systems.

Technical Abstract: Soil enzymes may be sensitive indicators of soil health but are not well understood in tillage-intensive, high-value vegetable systems. The activities of several soil enzyme involved in nutrient cycling (ß-glucosidase, ß-glucosaminidase, alkaline phosphatase, dehydrogenase, aspartase and L-asparaginase) were, therefore, evaluated during six years of commercial-scale production in five organic vegetable systems in a long-term experiment in the Salinas Valley of California. The systems differed in yard-waste compost inputs (none or 15.2 Mg ha-1 year-1), winter cover crop frequency (annually or every 4th year), and cover crop type (legume-rye, mustard, or rye), but all produced two crops annually (lettuce, broccoli, or spinach) with the same irrigation and fertilizer inputs. This caused large differences in cumulative organic matter input (7.4 to 136.8 Mg ha-1) from compost and cover crop shoots over the six years that had clear impacts on soil enzyme activities in the loamy sand soil. With the exception of aspartase, all enzyme activities were lowest without compost, intermediate with compost and infrequent cover cropping, and highest with compost and annually cover cropping. Furthermore, after six years of vegetable production there were quadratic relationships between microbial biomass and the activities of all enzymes, except for aspartase. Despite lower inputs of cover crop shoot biomass from mustard compared with rye and the legume-rye, and differences in shoots, residue quality and cover crop type did not influence enzyme activities. We conclude that soil enzyme activities were primarily influenced by inputs of fresh organic matter from annual cover cropping, not compost. These results combined with other attributes of soil health in this long-term study illustrate the importance of frequent cover cropping in tillage-intensive vegetable production. This raises critical 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 crop adoption.