Soil Nutrients and Other Major Properties in Grassland Fertilized with Nitrogen and Phosphorus

Understanding how nutrient distribution relates to soil depth is essential for improving fertilization practices for grasslands. This study evaluated the distribution of P, other elements, and selected soil properties at 0- to 5-cm and 5- to 15-cm layers in a grass sward following several years of N and P fertilization. Soil samples were collected in the spring of 2010 from a timothy grass sward (Phleum pratense L.) established in 1998 on a gravely-sandy loam soil. Sixteen combinations of P (0, 15, 30, and 45 kg ha−1) as triple superphosphate and N (0, 60, 120, and 180 kg ha−1) as calcic ammonium nitrate were broadcast annually from 1999 to 2006 in a split-plot design with P as main plots and N as subplots. Concentration of three soil P indicators [water (CP), Mehlich-3 (PM3), and acid oxalate (POx) extractable P] and total phosphorus (PT) were greater in the 0- to 5-cm than in the 5- to 15-cm layer. Differences in concentration of CP, PM3, POx, and PT between the two soil layers increased with increasing P applications. The NH4OAc extractable Ca and Mg concentrations were significantly greater in the 0- to 5-cm soil layer compared with the 5- to 15-cm soil layer. The NH4OAc extractable K was significantly decreased by N application being greater in the 5- to 15-cm soil layer than in the 0- to 5-cm soil layer. Soil pH decreased and the concentration of Mehlich-3 Al increased with increasing N applications only in the 0- to 5-cm layer. This study demonstrated that several years of P fertilizer in grasslands could increase soil available P in the 0- to 5-cm layer, possibly due to lack of mixing and the low mobility of P. Our results also suggest that changes in soil chemical properties induced by N fertilizer in grasslands could be heightened at the 0- to 5-cm layer.