Ava McCune

Cover crops are non-harvested plants grown to improve soil health in agricultural systems. Legume cover crops associate with beneficial soil bacteria that form root nodules and fix atmospheric nitrogen into a plant-available form via a process known as biological nitrogen fixation (BNF). Plastic covered greenhouse structures, called high tunnels, are a valuable season extension tool in cool regions of the United States, however, the resulting intensified vegetable production can negatively impact soils. Planting legume cover crops in rotation with summer vegetable crops is an emerging strategy to mitigate these negative effects, yet knowledge about specific planting times and their effects on BNF is limited. This research evaluates the degree to which planting time of two winter cover crops, hairy vetch (Vicia villosa Roth) and Austrian winter pea (Pisum sativum L.), affects: 1) plant biomass and productivity, 2) legume cover crop nodulation, and 3) BNF. I hypothesize that cover crops planted earlier in the fall will have greater biomass contributions and increased BNF, as there is a longer growth period and more time for beneficial associations with soil rhizobia to develop. Biomass was collected in December 2021 and 2022 using 0.1M quadrat frames, dried, and weighed. Nodule mass was collected in December 2022 by collecting, drying and weighing root nodules from two plants per treatment plot. Biological nitrogen fixation was measured using 15N natural abundance. In 2021, planting time and species type affected biomass production, with biomass highest in early planted vetch, and lowest in late planted vetch. Across species, both early planted pea and early planted vetch produced more biomass than their late planted counterparts.