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Evaluation of plant-grazer-soil interactions and agroecosystem carbon cycling properties in California perennial crop-livestock systems


One historically foundational, yet scientifically understudied, agroecological diversification strategy is the integration of animals and crops within the same production system. Integrated crop-livestock (ICL) systems, where animals provide various grazing-based services for crop production and the diversification of on-farm income, are common across much of the world’s subsistence farming communities. Global practitioners of ICL management rely on extensively developed local and indigenous knowledge systems, with regional specificity and complexity in the coordination of components. However, the industrial intensification of agriculture, especially throughout the last century, has increasingly resulted in highly specialized, mechanized, and de-coupled crop and animal production throughout much the world’s market-based agricultural systems. This is currently understood to contribute to poor nutrient (re)cycling within and between agricultural operations, higher rates of external input, reduced adaptive capacity, and an increased environmental footprint (GHG emissions, land-use conversion, etc.) of both crop and animal productions. However, much of these dynamics remain underexplored and the potential for SOC storage remains largely unknown. The goal of the research presented here was to narrow critical knowledge gaps specific to perennial ICL management outcomes, especially as they relate to soil nutrient cycling and carbon flux and storage dynamics within semi-arid climates. Collectively, the experiments in this study aim to observe perennial ICL outcomes across both spatial (i.e. bulk vs. rhizosphere soils) and temporal (i.e. long-term vs. short-term adoption; intra-seasonal vs. inter-seasonal monitoring) scales. The value of this information is in validating the potential usefulness of ICL for the purposes of energy and nutrient cycling, soil carbon sequestration, and the provisioning of other key soil ecosystem services. More specifically to the cycling of carbon, this research explored if and how these more complex and diversified ICL systems impact: (1) functional traits of the forage plant community; (2) soil physicochemical characteristics; (3) recycling and retention of carbon and nutrients; (4) soil biological activity and carbon substrate utilization; and (5) SOC biochemical and physical partitioning into storage pools. This information is critical for informing future integrated-systems research platforms and management applications; toward the goal of developing ICL production models with well-coordinated and strategically applied grazing practices. To carry forth this work, a comprehensive review of ICL literature was first conducted (Chapter 1). The broad aim of this review was to identify the status of ICL research, especially as it pertained to biogeochemical dynamics of carbon and nutrients within agroecosystems. Whereas the state of ICL literature is currently scarce, the aim of this review shifted toward providing a perspective and the development of working hypotheses and potential mechanistic frameworks. The goal was to appraise whole-system ICL interactions and their potential to support a suite of essential soil ecosystem functions. Next, in-situ and laboratory experimental approaches were utilized in a series of California-based integrated sheep-vineyard (ISV) research projects. This included a survey of paired vineyards with long-term management legacies (Chapter 2) and a short-term (2 year) field monitoring trial (Chapter 3), to assess agroecosystem dynamics within vineyards utilizing both sheep-integration (ISV) and conventional understory management techniques (CONV). This contributes novel ISV management data by quantifying field-scale shifts in a suite of soil physical, chemical, and biological properties over both short- and long-term temporal scales, as well as some first insights into the impact of perennial ICL management on above- and belowground understory plant community dynamics. This research contributes more mechanistic understanding of how perennial ICL management impacts specific plant-grazer-soil interactions and biogeochemical properties regulating SOC deposition, storage, and turnover. Collectively, the experiments and framework development in this study provide some of the first insights into the potential SOC storage benefits associated with perennial cropland grazing, particularly within subsoils. Our observations provide early evidence that high-density, short-duration rotational grazing management in perennial croplands holds significant potential to increase SOC storage, with proposed mechanisms related to the rate and efficiency of microbial carbon accrual. The paired survey experiment showed significant SOC storage increases within long-term ISV sites, including in the persistent mineral-associated organic carbon (MAOC) pool. Similar benefits were observed over the course of the two-year monitoring trial, which saw significant increases in SOC under ISV management relative to the vineyard utilizing mowing. Across both experiments, there was an extremely strong trend of larger labile SOC flux pools and increasing soil C flux rates in vineyards utilizing ISV grazing. In particular, vineyards using grazing had substantially larger soil microbial communities. Laboratory incubations showed higher rates of soil C mineralization under ISV management across spatial (i.e. bulk vs. rhizosphere soil) and temporal (i.e. long-term ICL legacy effects vs. short-term intra-seasonal monitoring) scales. However, the carbon use-efficiency of microbial communities varied across experimental scales. Microbial energy investment strategies and metabolic utilization patterns showed notable differences between grazed and ungrazed vineyards, but will require substantially more experimental investigation to accurately identify the underlying mechanisms and interpret their meaningful consequences. Importantly, results from multiple scales indicate that perennial ICL adoption is unlikely to provide substantial negative consequences for no-till perennial cropping systems. In the comparison of long-term managed ISV sites, soil structural indicators remained similar if not slightly improved. Both nutrient retention and plant-available nutrients also appeared to increase under ISV management. While intra-seasonal monitoring showed increased rates of soil CO2 efflux in grazed vineyards, it is unclear the extent to which this translates to long-term GHG emissions; especially when considering observed benefits for SOC storage. However, these outcomes are likely influenced by the intensity and periodicity (seasonality and frequency) of grazing events, as well as site-specific edaphic and climatic limitations. Soil biogeochemical outcomes may therefore be highly variable under different agroecosystem and grazing management regimes, and how these components are synchronized across time and space. Considerations of cropland co-management strategies, such as understory species composition and tillage regime, will interact to determine some soil habitat and resource conditions. The strategic application of grazing in coordination with knowledgeable shepherding practitioners is therefore necessary to optimize potential soil benefits. Where perennial cropland agroecosystem design and management may be easily altered to facilitate both spatial and temporal livestock re-integration, better understanding of the mechanistic pathways between grazing disturbances and cropland SOC cycling will be useful toward strategically improving the internal regulation of soil functions and increasing longer-term SOC storage. As such, my work highlights certain benefits of using updated soil carbon conceptual frameworks for linking SOC flux and storage indicators within applied agricultural research contexts. Future ICL research, design, and management should continue to develop across multiple spatial scales – the farm, landscape, and region – and explore the potential benefits for multifunctional landscape productivity, the efficient (re)cycling of energy and nutrients, and the support of various ecologically-based benefits that are critical for planetary regulation and societal prosperity. Meanwhile, this early research suggests that the adoption of perennial ICL practices should continue to expand, with strong potential of beneficial outcomes and preliminary assurance of minimal trade-offs.

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