Performing Department
(N/A)
Non Technical Summary
Phosphorus is an essential nutrient for crop growth and development, but in excess may exacerbate insect pest pressure in agroecosystems. While high levels of phosphorus is a soil characteristic of the vast majority of cropping systems, remarkably little is known about how surplus phosphorus alters agroecological communities via both bottom-up forces, such as plant defenses, and top-down forces, such as predation of insect herbivores. Filling this knowledge gap is critical for farmers to develop sustainable fertility management strategies while mitigating the environmental harms of phosphorus runoff into water bodies. This predoctoral integrated research and extension project includes two objectives to address agroecological consequences of surplus phosphorus: 1a) conduct a manipulative paired comparison of high and low phosphorus on four collaborating farms, measuring effects of phosphorus on top-down predation of herbivores, 1b) characterize how changes in soil quality and microbial diversity due to excess phosphorus application impact the bottom-up suppression of herbivores, and 2) engage and educate regional farmers about fertility and pest management with an on-farm field day and by developing a guidebook to common agricultural arthropods in the Southeast. Understanding the role of phosphorus in microbe-plant-insect interactions is crucial to both crop production and the development of sustainable agroecosystems. The outcomes of this project will help us identify pratical, organic fertility-management strategies that reduce phosphorus applications and enhance insect pest management while providing a hands-on educational environment for farmers.
Animal Health Component
0%
Research Effort Categories
Basic
50%
Applied
50%
Developmental
0%
Goals / Objectives
The long-term goals of this project are to improve crop production while reducing the environmental and human harms of agriculture by promoting fertility management strategies that enhance biocontrol, while expanding our basic knowledge of how soil nutrients mediate microbe-plant-insect interactions. This project was inspired by the insights and concerns of farmers in the southeastern U.S. with high soil phosphorus levels on their farms. The two main objectives of this project aim to provide farmers with evidence-based scientific information on how fertility strategies affect crop productivity and pest management:Objective 1a: Conduct a manipulative paired comparison of high and low phosphorus on four collaborating farms to determine the effects of phosphorus on top-down predation of herbivores.Objective 1b: Characterize how changes in soil quality and microbial diversity due to excess phosphorus application impact the bottom-up suppression of insect pests.Objective 2: Engage and educatefarmers about fertility and pest management with an on-farm field day, and by developing a guidebook to common cucurbit crop arthropods in Georgia.
Project Methods
During the proposed project, two major objectives will examine how surplus phosphorus impacts agroecological communities via both bottom-up forces such as plant defenses, and top-down forces, such as insect predators.Obj. 1a: Conduct a manipulative paired comparison of high and low phosphorus (P) on four collaborating farms, measuring effects of phosphorus on top-down predation of herbivores. Methods: PD Sharp will perform a randomized complete block design experiment on four organically-managed farms in Georgia. Three treatments will be applied: high P, low P, and a control (no fertilizer). Treatments will be applied across 30m rows with three replicates of each treatment. Ten zucchini plants, C. pepo, will be seeded in each plot. Before treatments are applied, soil will be tested to determine pre-existing N, P, and K nutrient levels. Visual arthropod surveys will be conducted biweekly with herbivores and natural enemies counted and identified to the family level. Fruit will be harvested four times per week and separated into two categories based on USDA standards, marketable and unmarketable. Two random zucchini plants from each plot will be destructively harvested (cut at the base of the stem) for analysis of plant biomass and nutrient content. Briefly, subsamples from each plot will be homogenized, dried, and milled to a fine powder. The samples will then be submitted for estimates of foliar nutrients. Analysis: PD Sharp will use structural equation models (SEMs) to map out interactions between the many covariates measured in this experiment. SEMs combine a series of linear regressions to test a web of hypothesized causal relationships. The "PiecewiseSEM" package in R will be used to determine best-fit combinations of these generalized linear mixed models, providing explanatory links between fixed effects (e.g., soil P, foliar nutrients, herbivore and predator abundance, crop biomass, and yield), while accounting for random effects of spatiotemporal variation. The best-fit combination will be determined using directed separation tests and Akaike Information Criteria (AICc).Obj. 1b: Characterize how changes in soil quality and microbial diversity due to excess phosphorus (P) application impact the bottom-up suppression of insect pests. Methods: PD Sharp will randomly collect three soil samples for each replicate noted in Obj. 1a from the 0-15 cm profile twice throughout the growing season. These three subsamples will be homogenized for each plot and microbial DNA will be extracted using the Qiagen MagAttract PowerSoil Pro DNA Kit for KingFisher (Qiagen, Carlsbad, CA). DNA concentration will be quantified using Qubit assay (Invitrogen, Carlsbad, CA) before samples are sent to Novogene (Sacramento, CA) for library preparation and sequencing on an Illumina MiSeq. DNA samples will be divided into two aliquots, one used for bacterial sequencing and one used for fungal sequencing. The remaining soil will be submitted to the UGA Agricultural & Environmental Services Laboratories for analysis of micronutrients, macronutrients and percent organic matter. In addition, we will collect leaf tissue to measure concentrations of cucurbitacins, the main defensive compounds in cucurbit plants. Samples will then be analyzed by liquid chromatography mass spectrometry (HPLC-MS/MS) at the following the methods of Theis et al. (2014). In brief, PD Sharp will lyophilize and grind leaf tissue, extract metabolites, separate them by liquid chromatography and identify compounds by mass spectrometry. Analysis: PD Sharp will normalize differences in sample sequencing depth and assess alpha diversity using diversity indices. PD Sharp will then use Principal Coordinates Analysis to examine sample dissimilarity according to treatment with Bray-Curtis distances. Functional diversity will be inferred using PICRUSt and FUNGuild. Also, PD Sharp will use permutational multivariate ANOVA (PERMANOVA) with farm and treatment as predictor variables to evaluate soil P effects on bacterial and fungal identity and function. In addition, PD Sharp will use generalized linear mixed models to evaluate effects of soil P on cucurbitacin concentration, herbivore performance, microbial biodiversity and plant biomass. Best-fit models will be chosen using an information theoretic approach, selecting models that include the fewest variables within 2 corrected AICc values.Obj. 2: Engage and educate farmers about fertility and pest management with an on-farm field day, and by developing a guidebook to common cucurbit crop arthropods in Georgia. Methods: PD Sharp will collaborate with four organically-managed farms, UGA Extension and Georgia Organics for engage and educate farmers. First, by participating in the on-farm experiment themselves, collaborating farmers will be able to spread the word about the results within their networks. Second, in collaboration with UGA Extension and Georgia Organics, PD Sharp will organize an on-farm field day during the growing season. The field day will be open to all farmers in the region, and the content of the field day will include education on soil health, fertility management strategies for reducing soil P, and conservation strategies to promote beneficial arthropods. Third, PD Sharp will co-present with collaborating farmer(s) on approaches to organic fertility management and pest-management at regional grower-focused conferences. Co-presenting will give collaborating farmers agency over the research and provide an opportunity for them to network with other farmers in the region. Lastly, PD Sharp will develop a guidebook to common cucurbit crop pests and natural enemies specific to Georgiawith pictures for identification, host plant information, life cycle information, scouting strategies, and conservation strategies for beneficial insects. This guidebook will tie this project together with a section on nutrient management strategies for enhancing crop productivity and pest management, while reaching growers that cannot attend the in-person field day.Evaluation: PD Sharp will present results of the project at the annual National Entomological Society of America meetings, and at the USDA NIFA EWD Project Directors Meeting. PD Sharp will co-present with collaborating farmers at regional grower-focused conferences. PD Sharp will evaluate the success of these presentations through attendance and feedback forms. PD Sharp will synthesize and share applied knowledge from this research through a field day and a guidebook (see 'Obj. 2') that will be published through the UGA Extension peer review system. The success of the field day will be evaluated via public interest, attendance, questions, and feedback at the conclusion of the field day. The influence of the guidebook will be assessed based on frequency of use and farmer evaluations. Finally, PD Sharp will submit three manuscripts to peer-reviewed journals at the end of the third year that will demonstrate my successful research training: (1) a review on managing soil phosphorus to promote arthropod pest suppression that will be submitted to Frontiers in Sustainable Food Systems; (2) a research paper on the effects of soil phosphorus on top-down predation of herbivores that will be submitted to Biological Control, and (3) a research paper on the effects of soil phosphorus on herbivore performance via altering plant chemical defenses that will be submitted to the Journal of Chemical Ecology.