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Open Access Publications from the University of California

Scholarly Works (10 results)

  • Thesis
  • Peer Reviewed
UC Davis Electronic Theses and Dissertations (2022)

Agricultural stakeholders are becoming increasingly aware of the need for sustainable food systems. In a shift from an agricultural paradigm that prioritizes yield, there is now growing interest in multifunctional food systems that simultaneously promote environmental integrity while also providing adequate yield and nutrition. An investigation into how current and proposed alternatives to annual grain systems address the multifaceted objectives of food system sustainability is necessary given that annual grains currently comprise nearly 70% of earth’s cultivated land and provide majority of the world’s food. Furthermore, it is crucial to conduct research on these grain systems in highly productive and economically valuable agroecological regions, such as California (CA). As the largest and most diverse agricultural state in the U.S., CA provides market opportunities while also heralding what current and future food systems must overcome to maintain food supply amid projected water and weather extremes. Thus, we investigated the multifunctionality of a tilled annual wheat system and two proposed alternatives, no-till annual wheat production and novel perennial grain production, in the Mediterranean climate of California. We measured plant and soil parameters for three years in intermediate wheatgrass or IWG, no-till annual wheat, and tilled annual wheat at four nitrogen rates. IWG had significant fluctuations in aboveground biomass (AGB) and had the highest soil carbon (C) mineralization at each soil depth. No-till wheat had stable AGB and the highest soil C stabilization and microbial biomass in the topsoil, which suggests plant productivity and a lack of soil disturbance over time are key factors underpinning enhanced soil C stabilization and gains in microbial biomass in the topsoil. Yield stability, soil carbon storage and nitrogen use efficiency (NUE) were then compared among the three systems using a multifunctionality framework. IWG had large interannual fluctuations in grain yield and NUE, while annual wheat (till and no-till) had stable grain yields and higher NUE than IWG. Soil carbon gains in IWG and no-till wheat relative to the tilled wheat were contingent upon N fertilization and constrained to the topsoil, while tilled wheat stored carbon at depth and across the whole soil profile. Finally, soil microbial community composition and soil carbon were compared in tilled annual wheat and IWG during the final year of the experiment. Bacterial community composition was more sensitive to soil depth than crop type, whereas fungal community composition was influenced largely by crop type. While the fungi : bacteria ratio was higher in IWG at deep soil depths, tilled annual wheat had a higher abundance of fungal taxa known to positively correlate with soil carbon and higher soil carbon mass than IWG at the 60-90 cm soil depth. The two major takeaways from this three-year field experiment were: 1) sampling the subsoil (> 30 cm) is crucial when comparing system-level soil carbon storage potential of annual and perennial grain systems; and 2) plant productivity underpins numerous components of agricultural multifunctionality and thus plant adaption is of utmost importance when designing and implementing future food systems.

    Cover page: Comparative Multifunctionality of Dryland Annual and Perennial Grain Production Systems in a Mediterranean Climate
    • Article
    • Peer Reviewed
    UC Davis Previously Published Works (2023)
    Accurately detecting nitrogen (N) deficiency and determining the need for additional N fertilizer is a key challenge to achieving precise N management in many crops, including rice (Oryza sativa L.). Many remotely sensed vegetation indices (VIs) have shown promise in this regard; however, it is not well-known if VIs measured from different sensors can be used interchangeably. The objective of this study was to quantitatively test and compare the ability of VIs measured from an aerial and proximal sensor to predict the crop yield response to top-dress N fertilizer in rice. Nitrogen fertilizer response trials were established across two years (six site-years) throughout the Sacramento Valley rice-growing region of California. At panicle initiation (PI), unmanned aircraft system (UAS) Normalized Difference Red-Edge Index (NDREUAS) and GreenSeeker (GS) Normalized Difference Vegetation Index (NDVIGS) were measured and expressed as a sufficiency index (SI) (VI of N treatment divided by VI of adjacent N-enriched area). Following reflectance measurements, each plot was split into subplots with and without top-dress N fertilizer. All metrics evaluated in this study indicated that both NDREUAS and NDVIGS performed similarly with respect to predicting the rice yield response to top-dress N at PI. Utilizing SI measurements prior to top-dress N fertilizer application resulted in a 113% and 69% increase (for NDREUAS and NDVIGS, respectively) in the precision of the rice yield response differentiation compared to the effect of applying top-dress N without SI information considered. When the SI measured via NDREUAS and NDVIGS at PI was ≤0.97 and 0.96, top-dress N applications resulted in a significant (p < 0.05) increase in crop yield of 0.19 and 0.21 Mg ha-1, respectively. These results indicate that both aerial NDREUAS and proximal NDVIGS have the potential to accurately predict the rice yield response to PI top-dress N fertilizer in this system and could serve as the basis for developing a decision support tool for farmers that could potentially inform better N management and improve N use efficiency.
      Cover page: Reflectance Measurements from Aerial and Proximal Sensors Provide Similar Precision in Predicting the Rice Yield Response to Mid-Season N Applications.
      • Article
      • Peer Reviewed
      UC Davis Previously Published Works (2024)
        Cover page: Changes in soil N2O emissions and nitrogen use efficiency following long-term soil carbon storage: Evidence from a mesocosm experimentCreative Commons 'BY-NC' version 4.0 license
        • Article
        • Peer Reviewed
        UC Davis Previously Published Works (2015)
        Conservation agriculture (CA) has been promoted as a method of sustainable intensification and climate change mitigation and is being widely practiced and implemented globally. However, no-till (NT), a fundamental component of CA, has been shown to reduce yields in many cases. In order to maintain yields following adoption of CA, it has been recently suggested that fertilizer application should be an integral component of CA. To determine the contribution of nitrogen (N) fertilizer in minimizing yield declines following NT implementation, we assessed 2759 paired comparisons of NT and conventional tillage (CT) systems from 325 studies reported in the peer-reviewed literature between 1980 and 2013. Overall, we found that NT yields decreased −10.7% (−14.8% to −6.5%) and −3.7% (−5.3% to −2.2%) relative to CT in tropical/subtropical and temperate regions, respectively. Among management and environmental variables that included: the rate of N fertilization; the duration of the NT/CT comparison; residue, rotation, and irrigation practices; the crop type; and the site aridity, N rate was the most important explanatory variable for NT yield declines in tropical/subtropical regions. In temperate regions, N fertilization rates were relatively less important. NT yield declines were most consistently observed at low rates of N fertilization during the first 2 years of NT adoption in tropical/subtropical regions. Applications of N fertilizer at rates of up to 85±12kgNha−1yr−1 significantly reduced NT yield declines in these scenarios. While this result should not be viewed as a rate recommendation, it does suggest that farmers applying rates of N fertilizer that are low for their specific system will, on average, see higher NT yields if they increase application rates. In addition, when crop rotation was not practiced or residues were removed from the field, NT yield declines were magnified by low rates of N fertilization in tropical/subtropical regions. These results, based on a global data set and across a broad range of crops, highlight the importance of N fertilization in counteracting yield declines in NT systems, particularly in tropical/subtropical regions.
          • Article
          • Peer Reviewed
          UC Davis Previously Published Works (2022)
          Wheat (Triticum aestivum L.) is a major global commodity and the primary source for baked products in agri-food supply chains. Consumers are increasingly demanding more nutritious food products with less environmental degradation, particularly related to water and fertilizer nitrogen (N) inputs. While triticale (× Triticosecale) is often referenced as having superior abiotic stress tolerance compared to wheat, few studies have compared crop productivity and resource use efficiencies under a range of N-and water-limited conditions. Because previous work has shown that blending wheat with triticale in a 40:60 ratio can yield acceptable and more nutritious baked products, we tested the hypothesis that increasing the use of triticale grain in the baking supply chain would reduce the environmental footprint for water and N fertilizer use. Using a dataset comprised of 37 site-years encompassing normal and stress-induced environments in California, we assessed yield, yield stability, and the efficiency of water and fertilizer N use for 67 and 17 commercial varieties of wheat and triticale, respectively. By identifying environments that favor one crop type over the other, we then quantified the sustainability implications of producing a mixed triticale-wheat flour at the regional scale. Results indicate that triticale outyielded wheat by 11% (p < 0.05) and 19% (p < 0.05) under average and N-limited conditions, respectively. However, wheat was 3% (p < 0.05) more productive in water-limited environments. Overall, triticale had greater yield stability and produced more grain per unit of water and N fertilizer inputs, especially in high-yielding environments. We estimate these differences could translate to regional N fertilizer savings (up to 555 Mg N or 166 CO2-eq kg ha-1) in a 40:60 blending scenario when wheat is sourced from water-limited and low-yielding fields and triticale from N-limited and high-yielding areas. Results suggest that optimizing the agronomic and environmental benefits of triticale would increase the overall resource use efficiency and sustainability of the agri-food system, although such a transition would require fundamental changes to the current system spanning producers, processors, and consumers.
            Cover page: Optimizing water and nitrogen productivity of wheat and triticale across diverse production environments to improve the sustainability of baked products
            • Article
            • Peer Reviewed
            UC Davis Previously Published Works (2015)
            No-till agriculture represents a relatively widely adopted management system that aims to reduce soil erosion, decrease input costs, and sustain long-term crop productivity. However, its impacts on crop yields are variable, and an improved understanding of the factors limiting productivity is needed to support evidence-based management decisions. We conducted a global meta-analysis to evaluate the influence of various crop and environmental variables on no-till relative to conventional tillage yields using data obtained from peer-reviewed publications (678 studies with 6005 paired observations, representing 50 crops and 63 countries). Side-by-side yield comparisons were restricted to studies comparing conventional tillage to no-till practices in the absence of other cropping system modifications. Crop category was the most important factor influencing the overall yield response to no-till followed by aridity index, residue management, no-till duration, and N rate. No-till yields matched conventional tillage yields for oilseed, cotton, and legume crop categories. Among cereals, the negative impacts of no-till were smallest for wheat (−2.6%) and largest for rice (−7.5%) and maize (−7.6%). No-till performed best under rainfed conditions in dry climates, with yields often being equal to or higher than conventional tillage practices. Yields in the first 1–2 years following no-till implementation declined for all crops except oilseeds and cotton, but matched conventional tillage yields after 3–10 years except for maize and wheat in humid climates. Overall, no-till yields were reduced by 12% without N fertilizer addition and 4% with inorganic N addition. Our study highlights factors contributing to and/or decreasing no-till yield gaps and suggests that improved targeting and adaptation, possibly including additional system modifications, are necessary to optimize no-till performance and contribute to food production goals. In addition, our results provide a basis for conducting trade-off analyses to support the development of no-till crop management and international development strategies based on available scientific evidence.
              • Article
              • Peer Reviewed
              California Agriculture, Volume 76, Issue 2-3 (2022)

              Small-grain crop growers need to match their crops' nitrogen (N) needs with fertilizer applications. This can be challenging because small grains are grown under diverse conditions and their growing season interacts with unpredictable precipitation. Resulting conditions can lead to nitrate-N leaching and runoff losses. More widespread and accurate soil N testing could help growers improve N fertilizer use efficiency, reduce the risk of N loss, and fulfill regulatory requirements. Soil samples from across California small-grain growing regions were tested with a soil nitrate quick test as well as standard laboratory procedures. The quick test is inexpensive and easy to use, and it provides rapid results. A correction factor was developed to convert the quick test values to lab and fertilizer equivalents. The correction factor is based on site-specific soil bulk density and the extracting solution used. An interactive webtool was developed that integrates this information for users. The quick tests provide accurate, real-time estimates of soil nitrate-N in the field to help improve fertilizer use efficiency and reduce N losses.

                Cover page: Improvements to the soil nitrate quick test for California small grains
                • Article
                • Peer Reviewed
                UC Davis Previously Published Works (2022)
                Nitrate (NO3) leaching from agriculture represents the primary source of groundwater contamination and freshwater ecosystem degradation. At the field level, NO3 leaching is highly variable due to interactions among soil, weather and crop management factors, but the relative effects of these drivers have not been quantified on a global scale. Using a global database of 82 field studies in temperate rainfed cereal crops with 961 observations, our objectives were to (a) quantify the relative importance of environmental and management variables to identify key leverage points for NO3 mitigation and (b) determine associated changes in crop productivity and potential tradeoffs for high and low NO3 loss scenarios. Machine learning algorithms (XGboost) and feature importance analysis showed that the amount and intensity of rainfall explained the most variability in NO3 leaching (up to 24 kg N ha-1), followed by nitrogen (N) fertilizer rate and crop N removal. In contrast, other soil and management variables such as soil texture, crop type, tillage and N source, timing and placement had less importance. To reduce N losses from global agriculture under changing weather and climatic conditions, these results highlight the need for better targeting and increased adoption of science-based, locally adapted management practices for improving N use efficiency. Future policy discussions should support this transition through different instruments while also promoting more advanced weather prediction analytics, especially in areas susceptible to extreme climatic variation.
                  Cover page: Predicting nitrate leaching loss in temperate rainfed cereal crops: relative importance of management and environmental driversCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Davis Previously Published Works (2024)
                  Environmental indicators for nitrogen (N) use efficiency (NUE) based only on N inputs and N removal are becoming widely used in science, policy, and commercial supply chains to track the sustainability of food production. However, these indicators do not reflect the contribution of inherent soil productivity, which can supply half of crop demand and is therefore a core principle in determining how much fertilizer is needed and the corresponding risk of N losses. Using a global dataset of optimal N rates for crop production, we evaluated the performance of conventional (N recovery efficiency) and simplified NUE indicators to understand their relationship and respective limitations, helping inform policy efforts for simultaneously meeting food production and sustainability goals. A key finding is that conventional agronomic approaches designed to optimize crop productivity and profit related to N fertilizer inputs have tradeoffs for environmental performance, with only 35 and 31 % of observations (n=448) falling within sustainable ranges for NUE. Meanwhile, simplified NUE indicators such as N balance or the ratio of N outputs to inputs were unable to detect sites with inherently low N recovery efficiency and high risk of N losses, highlighting a weakness of neglecting soil N supply in their calculations. Together these results suggest the need for a combined approach that merges insights from locally available agronomic data on N recovery efficiency with global environmental thresholds for NUE. Using our findings as a case study, we propose new steps forward for evaluating NUE in different cropping systems and regions to enhance food security while mitigating N pollution.
                    Cover page: Critical assessment of nitrogen use efficiency indicators: Bridging new and old paradigms to improve sustainable nitrogen management
                    • Article
                    • Peer Reviewed
                    California Agriculture, Volume 78, Issue 2 (2024)

                    Agronomic crops, including grains, forages, and fiber, are central to California agriculture, yet face many challenges. To ensure resilience, a statewide survey was conducted by the University of California Cooperative Extension service (UCCE) to identify high-priority needs and inform extension programming based on grower, consultant, and allied industry input. The goal was to compare the importance of different topics with the level of satisfaction regarding UCCE’s delivery of information on these topics. Survey respondents identified integrated pest management, nutrient and irrigation management, and variety testing as high-priority needs, with overall high satisfaction regarding UCCE’s program delivery on these topics. Topics needing more focus (high priority but below-average level of satisfaction) included testing new products, soil health management, and water conservation and storage. Areas of low priority and low satisfaction included niche marketing, emerging crops, organic production, harvest/post-harvest technology, salinity management, compost and manure management, and greenhouse gas emission reductions. To address stakeholder challenges, results from this study suggest that research and extension efforts should prioritize issues directly impacting on-farm crop production. At the same time, areas of low interest reflect a need for more support to engage farmers on these topics, particularly those concerning state environmental regulations and challenges to local and global food production and security.

                    • 1 supplemental PDF
                    Cover page: Setting research and extension priorities for agronomic crops in California
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