Cover cropping frequency is the main driver of soil microbial changes during six years of organic vegetable production

Authors: Brennan, Eric; Acosta-Martinez, Veronica

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2017
Publication Date: 6/1/2017
Citation: Brennan, E.B., Acosta Martinez, V. 2017. Cover cropping frequency is the main driver of soil microbial changes during six years of organic vegetable production. Soil Biology and Biochemistry. 109:188-204.

Interpretive Summary: The soil food web includes a variety of important organisms (bacteria, fungi, oomycetes, protozoa, nematodes, arthropods, and earthworms) that make up the living part of the soil. These organisms play a critical role in efficient soil function and help farmers to maintain soil health and productivity. However, we have a relatively poor understanding of how the soil microbial community works in vegetable production systems that are frequently disrupted by numerous tillage operations, i.e., discing, plowing, deep plowing, and various mechanical cultivation procedures. To address this, we used soil from a long-term experiment that is focused on high-value vegetable production was in the Salinas Valley of California. This research is improving our understanding this complex community of organisms and providing practical information to develop more sustainable farming practices here and elsewhere. This super cool, unique long-term study is known as the Salinas Organic Cropping System (SOCS) and is based in the most important production area for high-value vegetables in the Unites States. The SOCS study includes several different organic systems that all produce the same vegetable crops each year (i.e., lettuce & broccoli). The systems differ in (1) whether they receive yard-waste compost, (2) whether they are fallow during the winter or are cover cropped, and (3) what type of cover crop they receive (i.e., a legume-rye mixture, mustard, or rye). Using a several molecular methods we found that frequent cover cropping is far more important that compost additions to improve and maintain soil health in intensive vegetable systems. This information suggests that organic and conventional farmers should cover crop as often as possible in these tillage-intensive systems.

Technical Abstract: Soil microbes play a key role in soil health, and understanding the functional role of this living component of soil organic matter is critical to developing sustainable systems in major vegetable production regions like Salinas, California. Soil microbial community size and composition was evaluated after six years of commercial-scale production in five organic vegetable systems in a long-term systems experiment. All 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). The same levels of irrigation, and supplemental fertilizer were applied to all systems. 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 differences in microbial biomass C (MBC) and N (MBN), and soil organic C (SOC). MBC increased by 40 mg C kg-1 soil with compost and infrequent cover cropping, and to levels that were relatively high (200-250 mg C kg-1 soil) for a loamy sand soil in systems with annual cover cropping. Changes in SOC between systems were caused primarily by compost while changes in MBC and MBN were more related to cover crop frequency. Fatty acid methyl ester (FAME) analysis revealed differences in microbial community structure that were consistent with differences between systems in MBC and MBN. Across systems and over time the ratio of fungal: bacterial FAME indicators decreased while indicators of invertebrates, and gram positive bacteria increased. High throughput sequencing revealed relatively few differences in bacterial phyla between systems, but the increase in cropping intensity across all systems changed the relative abundance of some bacterial phyla (Bacteroidetes, Deinococcus-Thermus) and genera (Flavobacterium, Nocardioidetes). Cover crop type and frequency also influenced the abundance of two bacterial genera (Pseudomonas, Agromyces). These results provide evidence of that inputs of fresh, labile forms of C from frequent cover cropping is the primary driver of changes in the soil food web and soil health in high-input, tillage intensive organic vegetable production systems.