Winter cover crops increase readily decomposable soil carbon, but compost drives total soil carbon during eight years of intensive, organic vegetable production in California

Authors: White, Kathryn; Brennan, Eric; Cavigelli, Michel; SMITH, RICHARD - University Of California - Cooperative Extension Service

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/20/2020
Publication Date: 2/6/2020
Citation: White, K.E., Brennan, E.B., Cavigelli, M.A., Smith, R.F. 2020. Winter cover crops increase readily decomposable soil carbon, but compost drives total soil carbon during eight years of intensive, organic vegetable production in California. PLoS One. 15(2). https://doi.org/10.1371/journal.pone.0228677.
DOI: https://doi.org/10.1371/journal.pone.0228677

Interpretive Summary: Organic growers rely on soil organic matter to maintain soil fertility and crop productivity. The frequent tillage required to produce two cool season vegetable crops in a growing season makes maintaining soil organic matter a challenge. Compost applications are frequently used by growers to maintain soil organic matter. While less frequently grown, cover crops further increase soil organic matter, especially the fraction most beneficial to crop growth. Despite their importance to organic vegetable production little is known about the combined long-term effects of compost and cover crops on soil organic matter. Using long-term data from the Salinas Organic Cropping Systems study in Salinas, CA, ARS scientists found that annual compost application was of primary importance for maintaining soil organic carbon (a measure of soil organic matter), but the addition of annual winter cover crops further increased total soil organic carbon, in part by increasing the readily decomposable fraction responsible for increased nutrient availability and crop productivity. These results will benefit organic vegetable growers in both the Salinas Valley and elsewhere by helping to improve organic matter management for crop productivity.

Technical Abstract: Maintaining soil organic carbon (SOC) in frequently tilled, intensive organic vegetable production systems is a challenge that is not well understood. Compost and cover crops are often used to add organic matter to the soil. Compost contributes relatively stabilized carbon (C) while cover crops provide readily degradable (labile) organic matter. Our objective was to assess the effects of urban yard-waste compost, winter cover crop frequency and cover crop type on SOC and labile C stocks during eight years of intensive, organic production that usually included two vegetable crops per year in a long-term systems study in Salinas, California. Total C inputs from pelleted fertilizer, compost, vegetable residue and cover crops ranged from 30 to 90 Mg ha-1 in the five systems evaluated. Following a rapid decline in SOC stocks in year 1, compost had the largest effect on SOC stocks that increased by an average of 9.4 Mg ha-1, while increased cover crop frequency (annual vs. quadrennial) led to an additional 3.4 Mg ha-1 increase. In contrast, cover cropping frequency had the largest effect on permanganate oxidizable labile C (POX-C), increasing POX-C by 26% after 8 years. Labile POX-C was well correlated with microbial biomass C and nitrogen. Compost had the greatest effect on total SOC stocks, while increasing cover crop frequency altered the composition of SOC by increasing the proportion of labile C. These results suggest that frequent winter cover cropping has a greater potential than compost to increase nutrient availability and vegetable yields in high-input, tillage intensive vegetable systems.