A Social-Ecological Analysis of Ecological Nutrient Management using Cover Crops in the U.S. Midwest

Abstract

Nitrogen (N) fertilizer inputs to agricultural soils are a leading cause of nitrous oxide (N2O) emissions, contributing significantly to global climate change. As an alternative to conventional fertilizers, N2 fixing legumes can provide an organic source of N that recouples N and carbon (C) cycles. Legumes can be added to crop rotations as cover crops, which are non-harvested crops that provide critical ecosystem functions including N supply and retention. Further, mixtures of cover crop species with complementary functional traits can increase multiple ecosystem functions at once. Despite the potential benefits of widespread use of cover crops, adoption is low on farms in the U.S. Midwest. Top-down constraints including policies (e.g., the Farm Bill), dominant knowledge systems and infrastructure, and concentrated grain markets create large barriers to cover crop adoption. This dissertation applies and extends a social-ecological systems framework to link cover cropping as an ecological nutrient management (ENM) practice with the social variables that influence farmer perceptions and adoption of cover crops. Chapter 1 introduces key principles of ENM and the social-ecological systems framework that guided this interdisciplinary research. To advance ecological knowledge of cover crops, in Chapter 2, I conducted a field experiment at two sites with contrasting soil fertility properties. I tested the hypothesis that a legume-grass cover crop can decrease N2O emissions compared to a sole legume during the two-week window following tillage because grass litter can decrease N mineralization rates. I found partial support for my hypothesis: the functionally-diverse legume-grass mixture led to a small reduction in N2O emissions at one site but led to a slight increase at the other. The different treatment patterns between sites suggest that interactions between cover crop functional types and background soil fertility impact N2O emissions as cover crops decompose. In Chapter 3, I tested the hypothesis that a legume-grass mixture would provide similar N inputs as a sole legume to support corn yield, while reducing N2O losses over the whole corn growing season. The mixture supplied similar N inputs; however, I did not find significant differences in cumulative N2O emissions between treatments. A six-year N mass balance indicated N inputs and exports are approximately balanced in this agroecosystem. Historical data show that the long-term history of ENM has continued to build soil organic matter stocks, which is likely a more important driver of N cycling processes than the short-term addition of the mixture, explaining similarities between treatments. Chapter 4 is a case study of the 2017 Cover Crop Champions peer education program. Using qualitative interviews with 24 participants, I demonstrate that bottom-up actions helped farmers overcome structural constraints to cover cropping by training farmers to use language that normalized cover crop adoption, and by leveraging farmer networks to facilitate peer education and mentorship. Despite this progress, decades of research and financial incentives have largely supported large-scale commodity production, reducing access to resources needed for farmers to transition to ENM practices. Chapter 5 integrates findings across chapters, discusses the broader social-ecological systems impacts of ecological management with cover crops, and proposes further research needs. Overall, this interdisciplinary dissertation addresses gaps in our understanding of how functionally diverse cover crops influence N cycling under different soil conditions, and how social factors including farmer networks influence attitudes towards and adoption of cover crops.