The intensification revolution in dryland cropping systems: implications from field to landscape scale
Date
2017
Authors
Rosenzweig, Steven, author
Schipanski, Meagan, advisor
Stromberger, Mary, committee member
Carolan, Michael, committee member
Davis, Jessica, committee member
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Abstract
A global transformation in semi-arid cropping systems is occurring as dryland (non-irrigated) farmers shift from crop rotations reliant on year-long periods of bare fallow to more intensively cropped systems. Bare fallow has reduced year-to-year variability in crop yields, but it has also constrained crop productivity and, therefore, reduced carbon (C) inputs to soils. Exposure to tillage and erosion, combined with C limitation, has gradually degraded dryland soils and reduced their capacity to capture water and supply plant nutrients, requiring dryland farmers to rely on external inputs to support plant growth. However, the emergence of no-till has enabled dryland farmers to save enough water to replace bare fallows with crops, a practice called cropping system intensification. Cropping intensification has potential implications for the environment and economy of dryland agriculture as it impacts every aspect of the agroecosystem – from soil health, to weed and nutrient management, to crop yields. This dissertation seeks to unravel the economic and environmental implications of cropping system intensification at both the field and landscape scale in the US High Plains, and to understand the social dynamics underpinning this revolution. I quantified the impacts of cropping system intensification on a range of soil health parameters on 96 dryland, no-till fields in the High Plains. Three levels of cropping system intensity – wheat-fallow, mid-intensity, and continuous – were represented along a potential evapotranspiration gradient that increases from northwestern Nebraska to southeastern Colorado. I conducted in-depth interviews with farmers to examine the motivations, perceptions, and social interactions that influence decisions about whether and how much to intensify, and to collect detailed field histories including input use and crop yields. To scale up the implications of these field-level analyses, and to assess the current extent of the cropping revolution in the High Plains, I conducted a spatial analysis using high-resolution satellite crop data to examine changes in cropping patterns over time at the landscape scale. I found that cropping system intensification was positively associated with soil organic carbon, aggregation, and fungal biomass, and these effects were robust amidst variability in environmental and management factors. I also found that intensified systems were associated with greater potentially mineralizable and total nitrogen (N), and arbuscular mycorrhizal fungal colonization of wheat roots, suggesting that cropping intensity enhances internal cycling of N and phosphorus (P). Continuous dryland farmers also achieved greater total crop production using fewer external inputs than wheat-fallow farmers, leading to enhanced profitability. To explain the social dynamics underpinning the cropping system revolution, I build on Carolan's application of Bourdieusian social fields to agriculture, and find several overlapping fields within Carolan's more general fields of sustainable and conventional agriculture, which are reflected in different degrees of intensification. I identify strategies for change, some of which would serve to reshape social fields, and others which leverage existing social positions and relationships, to enable farmers to overcome the barriers constraining cropping system intensification. Results from the spatial analysis suggest that, from 2008 to 2016, the High Plains witnessed a profound shift in cropping systems, as the historically dominant wheat-fallow system was replaced by intensified rotations as the dominant systems across the landscape. I estimated that these patterns over the 9-year study period increased annual grain production and annual net farm operating income, slightly reduced herbicide use, and increased C sequestration, contributing to greenhouse gas reductions. I projected each of these implications to a scenario of 100% continuous cropping adoption to estimate the potential environmental and economic impacts of cropping system intensification in the High Plains. Overall, my findings suggest that dryland cropping systems are gradually intensifying in the High Plains, and these trends are likely reversing historical negative environmental and economic trends to enhance the profitability and environmental sustainability of dryland agroecosystems.
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Subject
crop rotation
social fields
soil health
dryland agriculture
cropping system intensification
soil carbon