Long‐term Changes in Grassland Soil Phosphorus with Fertilizer Application and Withdrawal

Long‐term phosphorus (P) applications can increase soil P concentrations in excess of agronomic optima, posing a risk to water quality. Once fertilization stops, however, it may take time for soil P concentrations to decline. Whereas P fertilization adds orthophosphate, little is known about changes in other soil P forms during P buildup and drawdown. This study examined changes in P pools (total P, Olsen P, Mehlich P, and water‐extractable P) and P forms determined by 31 P‐nuclear magnetic resonance spectroscopy (P‐NMR) in grazed grassland plots from Northern Ireland. Between 1994 and 1999, all plots received 8.3 kg P ha −1 yr −1 with variable rates of nitrogen (100–500 kg N ha −1 yr −1 ). From 2000 to 2005, plots received 0, 20, 40, or 80 kg P ha −1 yr −1 and 250 kg N ha −1 yr −1 ; from 2005 to 2010, no P fertilizer was applied to any plots. In 2005, soil P pool concentrations at the highest P fertilization rates were significantly elevated compared with those in 2000 but had decreased to 2000 concentrations by 2010. In soils receiving no P, soil P pool concentrations were significantly lower than those in 1994 only in 2010. There were few changes in P forms determined by P‐NMR. Orthophosphate followed the same trend observed for the soil P pools; total organic P, total inositol phosphates, and total orthophosphate monoesters and diesters were highest in 2010 in the soil receiving no P fertilizer for 10 yr. For these soils, fertilizer application and cessation influenced inorganic P more than organic P. Core Ideas Investigated soil P forms with P fertilization and then without P fertilization. Adding inorganic P in fertilizer increased inorganic P in soil. Inorganic P concentrations decreased in soil after fertilizer cessation. There were few changes in organic P during fertilization and after fertilizer cessation.