Changes in Grassland Soil P Forms Under a Zero P Application Regime.

Despite regulations controlling fertiliser use in agriculture, there has been a significant time lag in the decline in soil P to agronomic optimum levels. Between 2000 and 2005 0, 10, 20, 40, 80 kg P ha^-1 was applied to five grazed grassland plots, building Olsen soil P to 19, 24, 28, 38 and 67 mg P L^-1, respectively. From 2005 to 2010, the plots received zero P applications, except for dung from grazing animals. Changes in Olsen P were monitored weekly during this period. Selected soil samples from 2000, 2005 and 2010 were analysed for P forms using solution ³¹P NMR spectroscopy and P fraction using sequential extraction techniques. Soil P decline was linearly related to Olsen P in 2005 (R² 0.99), requiring up to 13 years for soil at 67 mg Olsen P L^-1  to decline to the agronomic optimum.  In 2005, inorganic P dominated total soil P (53-63%) in all the plots, except for the plot with the lowest Olsen P concentration (49%). By 2010 this trend had been reversed with inorganic P accounting for only 38-42% of total P in all plots except for the plot with the highest Olsen P concentration in 2005 (52%). Extraction with NaOH-EDTA for ³¹P-NMR recovered 73% of total soil P, of which 53% was organic P forms. These were dominated by myo-inositol hexakisphosphate (phytate) and its stereoisomers. The extractability of P with sequential P fractionation varied significantly under a zero P fertiliser regime, the biggest impact being on P associated with NH₄Cl and NH₄F extractions, both of which are the most available to plants and for loss to water. This study demonstrates the challenges in balancing agronomic and environmental soil P targets, particularly in intensive dairy systems where a high proportion of soils are above the agronomic optimum.