Predicting the changes in environmentally and agronomically significant phosphorus forms following the cessation of phosphorus fertilizer applications to grassland

Phosphorus (P) loss from soil can impair surface water quality. Losses from soil are related to soil P concentrations, but agronomic measures such as Olsen P do not in many cases predict the potential for P loss. One possible strategy to decrease P loss is to stop applying P fertilizers. We examined the changes in both agronomic (Olsen P) and environmental [water-extractable P (WEP) and calcium chloride-extractable P (CaCl2-P)] P tests, and the potential implications following a halt to P fertilizer application to four long-term grassland field trials on different soil types. Exponential decreases in Olsen P and WEP concentration over time were observed in three of the four trials, but only in one trial for CaCl2-P. The rates of decrease in Olsen P (OP) and WEP concentration were best correlated with initial WEP (WEPi) concentration and the quotient of Olsen Pi/P retention (PR, a measure of Al- and Fe-oxides), respectively. The equation t = 1/(-0.035 X ln OPi/PR - 0.0455) X (ln WEPt - ln WEPi) was used to predict the time (t) taken for WEP concentrations at the sites to decrease to 0.02 mg/L (WEPt), which is proposed as a limit for dissolved reactive phosphorus in overland flow, and the result was 2344 yr. Results from a similar equation for Olsen P predicted a quicker rate of WEP. A significant decline in dry matter (DM) yield was observed at one trial site. For this site, the rate of decline in DM yield was of a similar magnitude to the rate of decline in WEP concentration. This suggests that halting P fertilizer application to decrease P loss as measured by WEP concentration may decrease farm productivity. An alternative, more financially acceptable, strategy is required, such as a negative P balance while maintaining yields with N fertilizer, but further work is required to assess both the agronomic and environmental implications of this strategy.