Assessing and Adjusting a Nutrient Risk Assessment Tool to Improve Water Resources in the Lower Mississippi River Basin

The Mississippi (MS) Phosphorus Index (P-Index) frequently produces Low and Medium Phosphorus (P) loss vulnerability ratings for assessed fields, but these do not correlate with measured P losses in surface runoff (Osmond et al., 2017). A study was performed to assess the performance of the MS P-Index and to adjust it to more accurately rate P‐Loss vulnerability in the MS Delta region. Results indicate that the distance to water influenced the P‐index rating more than any other factor, explaining more than 50% of its variability (Figure 1). Inorganic fertilizer application rate contributed the least in explaining the variance in P‐Index output. Analysis of results indicated that the weights assigned to organic fertilizer application might be reduced due to limited reported use in the study region. Based on the premise of no‐fertilization and a trend analysis showing an increase in STP levels in the region from 2001 to 2021 (median: 100-ppm in 2021), the influence of soil test P (STP) on agricultural runoff P concentrations was corroborated by rainfall simulation tests and collection of runoff irrigation samples. Further analysis provides insight for assigning weights to parameters and perhaps including additional parameters (e.g. groundwater quality used for irrigation) to the risk assessment tool. The latter might provide better guidance to achieve reduction goals defined by the Mississippi Nutrient Management Plans and Nutrient Reduction Strategies (Ramirez-Avila et al., 2012).