Do plot studies generate “directionally” correct assessments of field-level phosphorus losses?

Abstract

The National Phosphorus Research Project coordinated a tremendous amount of research at the plot scale to assess the influence of nutrient management on phosphorus (P) transport at the fields scale. The objective of this research was to determine if plot-scale rainfall simulations could be used to assess P transport from two fields that were managed using no-tillage or rotational tillage. Plots were constructed within the management zone but adjacent to monitored fields. Phosphorus transport at the field scale from throughout the growing season was compared to confidence limits established by the rainfall simulations, and a secondary analysis compared values from individual storms to the rainfall simulations. Soluble phosphorus (SP) loads from the no-tillage AS1 field (75 g ha⁻¹ [0.066 lb ac⁻¹]) were greater than from the rotationally tilled AS2 field (11g ha⁻¹ [0.010 lb ac⁻¹]) in 2004. Stratification of P in the uppermost portion of the soil profile is a known contributor to SP loading in long-term no-tillage fields. This trend was reversed in 2006 though, as SP loads were 16 g ha⁻¹ (0.014 lb ac⁻¹) from the AS1 field and 55 g ha⁻¹ (0.049 lb ac⁻¹) from the AS2 field. The greater loads from the AS2 field were due to greater discharge and a greater P application rate, compared to the AS1 field. Soluble P and total P loads were generally directionally correct, but the values obtained from plots were not necessarily similar to those observed at the field scale. Precipitation normalized loads for SP and total P were the most similar metric when comparing values from the plot to the field scale (i.e., more field scale values fell within the 95% confidence limits set by the plot data than the other metrics). Using cumulative field-scale data from each year or the mean values from storms by year did not appear to change the results of this study. This study would appear to confirm that the management decisions based on the National Phosphorus Research Project are likely to be sound and will probably lead to improved quality of runoff water from fields. Precipitation normalized loads appear to be a metric that may provide additional insight into P transport at various scales.