Doctor of Philosophy / Agronomy / Nathan O. Nelson / Agricultural fields contribute phosphorus (P) to water bodies, which can degrade water quality. The P index (PI) is a tool to assess the risk of P-loss from agricultural fields. However, due to limited measured data, P indices have not been rigorously evaluated. The Agricultural Policy/Environmental Extender (APEX) model could be used to generate P-loss datasets for P index evaluation and revision. The objectives of the study were to i) determine effects of APEX calibration practices on P-loss estimates from diverse management systems, ii) determine fertilizer and poultry litter management effects on P-loss, iii) evaluate and update the Kansas PI using P-loss simulated by APEX and iv) determine appropriate adsorption isotherms with advection-dispersion equation with column leaching experiment. Runoff data from field studies in Franklin and Crawford counties were used to calibrate and validate APEX. Poultry litter and inorganic fertilizer application timing, rate, method, and soil test P concentration effects on P loss were analyzed using location-specific models. A column leaching laboratory study was also conducted to test the adsorption isotherms. Location-specific model satisfactorily simulated runoff, total P (TP) and dissolved P (DP) loss meeting minimum model performance criteria for 2/3 of the tests whereas management-specific models only met the criteria in 1/3 of the tests. Applying manure or fertilizer during late fall resulted in relatively lower TP loss compared to spring applications before planting. The Kansas-PI rating and the APEX simulated P-loss were correlated with r² of 0.40 (p<0.001). Adjusting the weighting factors for Prate, soil test P, and erosion improved the correlation (r² = 0.46; p<0.001. Using a component PI structure and determining the weighting factors by multiple linear regression substantially improved the correlation between the PI and TP loss (r² = 0.69; p<0.001). In the P-leaching experiment, both the linear and nonlinear adsorption isotherms did not fit the experimental data. A multi-reactional advection-dispersion model that better describes all the P processes and complexities in soils should be included in the future. These procedures can provide a roadmap for others interested P transport in soils and using computer models in evaluation, and modifying their PI.

Phosphorus (P) inputs into water reservoirs are the primary cause for accelerated eutrophication affecting water quality. Attempts are underway to regulate inputs originating from concentrated animal feeding operations (CAFOs). The purpose of this research was to relate runoff dissolved (DP) and total P (TP) losses to site-specific characteristics from plots in CAFOs and compare them to their corresponding risk assessment using the Texas Phosphorus Index (PI). Initial studies showed that soil test P (STP) methods used in Texas by inductively coupled plasma were highly reproducible regardless of manure source or application rate. However, NH4OAc-EDTA extraction efficiency was increased with respect to other methods as soil conditions became less acidic, probably due to dissolution of the greater portion of Ca-bound P resulting in STP values that could be three times greater than those of Mehlich III for the same soil. Surface application of dairy manure to high pH soils were positively correlated to STP at various soil-sampling depths down to 15 cm. First order linear relationships between STP values and DP concentrations in runoff were statistically significant for extraction methods and sampling depths but were different among different soils under neutral to calcareous conditions. Attempts to reproduce this relationship on fields that received periodic applications of manure or effluent with various incubation periods failed, although there was a single highly significant relationship between STP and runoff DP for different soils when soil conditions were acid ( pH<6.5) with various sampling depths. Analyses of NH4OAc-EDTA extractable soil elements showed Mg was significantly correlated to DP concentration across various management and soils, indicating that Mg-bound P is a major component controlling P release into runoff. Use of the Texas PI reflected vegetation type closely, with grass-covered sites averaging the lowest risk rating, and having the lowest DP and TP losses, while conversely tilled sites had the highest. However, overall relationship was poor when estimates for erosion rates were used due to experimental design limitations. Use of measured erosion rates for plots and inclusion of extractable Mg improved correlations between PI rating to DP and TP losses, with r2 ranging from 0.60 to 0.87.

Version 2.0 of the P Index for Virginia uses coefficients describing the risk of P losses for different manure sources applied to fescue pasture that have not been verified on Virginian soils. In the first experiment, four sources of manure (dairy slurry, piggery waste, beef solids, and poultry litter) and triple superphosphate (TSP) were applied iso-nitrogenously to pasture plots (1.5 m2, 10% slope) with 31 ppm Mehlich 1-P soil test. The P treatments were amended in spring at a rate of 62.7 kg P2O5/ha and compared against a no-P-amended control. Forage was cut and removed monthly (n=5). Five rainfall simulations (65-70 mm/h) were conducted at three occasions (June, August, and October); the soil moisture was below field capacity at two events. Continuous surface runoff was collected for 30 min from each plot in accordance with the protocol of the National P Research Project. Data were statistically analyzed using Proc Mixed of SAS with rain event or cutting used as the repeated measure. Runoff concentrations of total P (TP) and dissolved reactive P (DRP) did not vary by treatment. The control showed less TP (0.126 mg/l) and DRP (0.068 mg/l) concentration than all other treatments (ranges 0.190 to 0.249 mg TP/l and 0.129 to 0.182 mg DRP/l) in runoff during the first event (40 d after treatment). The control had the lowest (0.118 mg/l) and TSP the highest (0.248 mg/l) TP concentration during the second event 24 h later. Samples taken at 5-min intervals during the second simulation showed a significant decrease in TP and DRP concentrations over time for all treatments but the control. Treatments did not affect edge-of-the-field losses of TP, DRP, or TKN. Soil test P and water-extractable P measured after the fifth and final rainfall simulation did not correlate to P concentrations in runoff. Forage yields and their N and P concentrations were not impacted. Results indicated a decreasing impact of manure, spring-applied to fescue pasture, on runoff P concentrations throughout the season. Highest TP concentrations were found during the first pair of simulated rainfalls from the TSP treatment. In a second experiment, indoor runoff boxes were used to simulate management intensive rotational grazing. Commercial fertilizer TSP and manure application increased runoff TP concentration from 0.146 mg/l to 0.245 mg/l and DRP concentration from 0.105 mg/l to 0.183 mg/l. Runoff P did not differ between organic or inorganic P treatments, possibly due to the small area of the boxes. However, application of manure increased runoff TKN overall, with a linear decrease as the time increased between application and rain simulation. / Master of Science

The Lower Mississippi Alluvial Basin (LMAB) faces significant environmental challenges due to phosphorus (P) runoff from agricultural lands, contributing to eutrophication and aquatic ecosystem degradation. Excess nutrient runoff, particularly P, threatens water quality and contributes to hypoxia in the Gulf of Mexico. The current Mississippi Phosphorus Index (P-Index), a tool for assessing P loss vulnerability from agricultural fields, has shown limitations in its applicability across the diverse conditions of the Mississippi Delta, a sub-region of the LMAB. This research presents a comprehensive revision of the P-Index by employing a suite of analytical techniques and diverse data sources, including geospatial analysis, rainfall simulations, and extensive data from soil tests, agricultural censuses, and expert evaluations. The aim was to enhance the model's sensitivity and accuracy in predicting P loss vulnerability, thereby enabling more precise nutrient management recommendations tailored to the Mississippi Delta's unique agricultural and environmental conditions. The study identified a critical lack of variability in the P-Index's recommendations for different agricultural scenarios within the region, highlighting its inadequacy in accurately reflecting the specific vulnerabilities to soil P loss. Through a detailed sensitivity analysis and recalibration of the model, incorporating updated parameters and data sources, significant improvements were achieved. The revised P-Index now better distinguishes between various agricultural practices set in the environmental conditions of the MS Delta, offering differentiated recommendations that align closely with the region's real-world complexities. Furthermore, the research underscores the necessity for ongoing investigations into the equivalencies between different soil test P methods (Lancaster and Mehlich-III) and the impact of P levels in irrigation water on nutrient cycling and loss. The recalibrated P-Index represents a significant step forward in regional nutrient management strategies, promising enhanced environmental protection and agricultural sustainability through more informed and targeted recommendations. This work emphasizes the critical need for adapting nutrient management tools like the P-Index to regional conditions, ensuring they accurately address the environmental challenges and agricultural practices specific to areas like the Mississippi Delta. Keywords: Nutrient management, Phosphorus Index, Mississippi Delta, Agricultural runoff, Soil test phosphorus, Environmental sustainability, Sensitivity analysis, Trend analysis.