Summer cover crops are a management tool that vegetable farmers can use to counteract the negative effects of soil degradation by physically protecting soil, contributing biomass to soil organic matter (SOM), suppressing weed growth, and enhancing nutrient cycling, but their use may be limited in the short growing season of northern climates. Evaluating the effects of cover crops on soil nutrient cycling and SOM in northern climates is an opportunity to collaborate with farmers, which is important because such collaboration improves the quality of knowledge gained from research by recognizing the incompleteness of any single perspective. Collaborative and participatory research is also a means to address the unequal power dynamics in agriculture that have systemically disadvantaged immigrant and minority farmers through interlocking challenges accessing capital, land, and information. In this dissertation, summer cover crops were evaluated in collaborative on-farm trials in northern climates for their ability to accumulate biomass, suppress weeds, affect soil C and N pools, and contribute to fall cash crop yield. Additionally, this dissertation includes a qualitative analysis of existing collaborative relationships between members of a local immigrant farmer cooperative and representatives from Extension, the Department of Agriculture, and a local agricultural non-profit. The summer cover crop trial consisted of four cover crop species treatments, grown for 30 (SD) or 50 days (LD) alongside bare fertilized and unfertilized control treatments: buckwheat (Fagopyrum esculentum) and sunn hemp (Crotalaria juncea) monocultures, and biculture of chickling vetch (Lathyrus sativus) or cowpea (Vigna unguiculata) with sorghum-sudangrass (sudax) (Sorghum bicolor x S. bicolor var. Sudanese). To quantify cover crop quantity, quality, and weed growth and seed set suppression capability, we measured cover crop and weed biomass and biomass C:N. To quantify effects on cash crops, we measured fall broccoli yield and biomass. Soil N and C cycling were quantified at cover crop peak growth (directly before termination) and one week after cover crop termination for mineral N, PMN, organic N, POX-C, extractable organic C, as well as fluorescein diacetate hydrolysis (FDA) as a proxy for microbial activity to contextualize the other measurements. Cover crops produced biomass consistent with that of more southern climates but legumes did not grow well and did not overcome weed pressure. All cover crops contribute to nutrient retention but not fertility benefits and negatively impacted fall cash crop yield. Collaborative relationships with farmers were dependent on external institutional support, and food systems professionals differed in whether they adopted an equity or equality lens.
University of Minnesota Ph.D. dissertation. July 2020. Major: Applied Plant Sciences. Advisors: Julie Grossman, Nicholas Jordan. 1 computer file (PDF); viii, 123 pages.
Agroecological approaches to warm-season cover cropping in northern climate vegetable systems and building collaborations with farmers.
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