Balancing trade-offs: an investigation into the agroecological impacts of winter legume cover crops employed in organically managed high tunnels

Abstract

High tunnels (HTs) are semi-permanent structures used in temperate climates to increase vegetable production by leveraging shoulder seasons when open fields (OFs) may be inaccessible due to cold weather or spring flooding. They have become increasingly popular in the U.S. with the advent of cost-share programs through the NRCS and are often promoted as a conservation practice. However, as an intensive practice, HTs are associated with soil structure degradation, increased pest and disease incidence, nutrient buildup, and loss of organic matter. Planting winter legume cover crops in rotation with summer vegetables is an emerging strategy to mitigate some of these adverse effects. There is limited knowledge of how winter cover crops' planting and termination time affect soil health and cash crop yield. Productivity trade-offs are expected, as early fall cover crop planting and late spring termination may interfere with lucrative cash crop production windows. However, extending the growth period of the cover crop may increase its potential to provide soil-enhancing biomass. This research evaluates the degree to which fall planting and spring termination time of two winter cover crops, hairy vetch (Vicia villosa Roth) and Austrian winter pea (Pisum sativum L.), affects 1) cover crop biomass and biomass nitrogen, 2) pepper yield, 3) permanganate oxidizable carbon (PoxC) 4) potentially mineralizable nitrogen (PMN) and 5) microbial biomass carbon (MBC) and nitrogen (MBN) in organically managed HTs. Results indicated a 176% difference in biomass production between the longest and shortest cover crop growth periods, with the longest period accumulating the most biomass. However, there was an associated decline in mean annual cash crop yield when the cover crop season was extended. Soil health indicators had mixed results but showed potential to build nutrient reserves over time, with the longest cover crop growth period increasing PMN as much as 5 times from Y1 to Y2. Microbial activity appeared to be limited in the HT, suggesting research should focus on soil biological processes in HTs.