Southern phosphorus indices, water quality data, and modeling (APEX, APLE, and TBET) results: A comparison

Authors: OSMOND, DEANNA - North Carolina State University; Bolster, Carl; SHARPLEY, ANDREW - University Of Arkansas; CABRERA, MIGUEL - University Of Georgia; FEAGLEY, SAM - Texas A&M University; FORSBERG, ADAM - University Of Georgia; MITCHELL, C - Auburn University; MYLAVARAPU, R - University Of Florida; OLDHAM, J - Mississippi State University; RADCLIFFE, DAVID - University Of Georgia; RAMIREZ-AVILA, J - Mississippi State University; STORM, D - Oklahoma State University; WALKER, F - University Of Tennessee; ZHANG, H - Oklahoma State University

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/11/2016
Publication Date: 7/20/2017
Publication URL: https://handle.nal.usda.gov/10113/5763101
Citation: Osmond, D., Bolster, C.H., Sharpley, A., Cabrera, M., Feagley, S., Forsberg, A., Mitchell, C., Mylavarapu, R., Oldham, J.L., Radcliffe, D., Ramirez-Avila, J.J., Storm, D.E., Walker, F., Zhang, H. 2017. Southern phosphorus indices, water quality data, and Modeling (APEX, APLE, and TBET) results: A comparison. Journal of Environmental Quality. doi:10.2134/jeq2016.05.0200.

Interpretive Summary: In this study we compare output from twelve phosphorus (P) indices in the southern United States. Phosphorus indices are simple tools developed in each state to guide land owners and nutrient management planners on applying manure and fertilizer in a way that minimizes the loss of nutrients to surrounding waterways with the goal of protecting water quality. We compared the ratings from 12 southern phosphorus indices with 3 phosphorus loss models as well as measured field-scale phosphorus loss data collected from benchmark sites throughout the South. Large differences in output from the different phosphorus indices were observed when applied on the same field using the same land management practices. This work highlights the large differences in how each of these phosphorus indices assigns risk of phosphorus loss and suggests that a more coordinated approach among states in evaluating phosphorus loss risk from agricultural fields is warranted.

Technical Abstract: Phosphorus (P) indices in the southern United States frequently produce different recommendations for similar conditions. After assembling data from benchmark southern sites (Arkansas, Georgia, Mississippi, North Carolina, Oklahoma, and Texas), land treatment information was used in the 12 southern P loss tools (Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, and Texas). Phosphorus-Index ratings were developed using both measured erosion losses from each benchmark site and RUSLE2 predictions; mostly, there was no difference in P-index outcome. The derived loss ratings were then compared to measured P loads at the benchmark sites by converting them to USDA-NRCS P-Index rating equivalents: Low (< 2.2 kg ha-1 yr-1), Moderate (2.2 to 5.5 kg ha-1 yr-1), and High (> 5.5 kg ha-1 yr-1). The Southern P-Index ratings were also evaluated against three fate and transport water quality models (Annual P Loss Estimator, APLE; Agricultural Policy/Environmental eXtender, APEX; and, Texas Best management practice Evaluation Tool, TBET). Finally, State P Indices were compared against each other and USDA-NRCS loss ratings; there was 54% correspondence. Model estimate correspondence with USDA-NRCS loss ratings was 61% (APEX), 48% (APLE), and 52% (TBET). A Kendall modified Tau analysis suggested that correlations between measured and calculated P-loss ratings were similar or better for most P Indices than the models. However, it is unclear how to assign losses and potential risk since water resources have different sensitivities to P and typically this is not standardized in P-index loss ratings, including the national nutrient management standard.