Relationship between Soil Phosphorus and Phosphorus Budget in Grass Sward with Varying Nitrogen Applications

Changes in soil available P usually relate to P budgets, but the sensitivity of the relationship to N fertilization is poorly understood. Four P rates (0, 15, 30, and 45 kg ha(-1)) as main plots and four N rates (0, 60, 120, and 180 kg ha(-1)) as subplots were applied each year from 1999 to 2006 to a timothy (Phleum pratense L.) sward established in 1998 on a gravelly sandy loam soil (Typic Fragihumods). Soil available P was assessed using water (Cp) and Mehlich-3 (P-M3) extractions. Annual P budgets were calculated as the difference between applied P and P offtake and were then summed over the duration of the experiment to determine the cumulative P budget (Bcum). Over the 8-yr study, Bcum (-148 to 286 kg P ha(-1)), Cp (0.13 to 0.94 mg L-1), and P-M3 (22.3 to 111.7 mg kg(-1)) were affected by both P and N fertilization. The Cp and P-M3 were linearly related to Bcum and the relationship was similar among N rates [Cp = 0.0016Bcum + 0.3 (r(2) = 0.86, P < 0.001) and P-M3 = 0.20Bcum + 47.7 (r(2) = 0.90, P < 0.001)]. A positive or negative Bcum of 100 kg P ha(-1) would then raise or deplete Cp by 0.16 mg L-1 and P-M3 by 20 mg kg(-1). The relationship between Bcum and soil available P was not affected by N applications in this gravelly sandy loam soil.