4.2. Topsoil P balance
A unique characteristic of phosphorus is its immobility in soil. When P fertilizer is applied to the soil it remains in place and very little phosphorus moves into or through the subsoil (Holanda et al., 1998). Therefore, topsoil P balance may reflect soil P absorption and transformation in crop soil (Ding and Yu, 2008). In terms of P budget, P accumulated in both treatments in both growing seasons with more P left in 2009 compared to that in 2010. Yet, soil total P or inorganic P balance in topsoil was negative in 2009 and positive in 2010. Therefore, the contradiction between P budget and P balance suggests that more soil P was transformed to biomass P in spring wheat roots in 2009 than 2010, which was excluded from soil P according to the soil P test method used in this study. Wheat root distribution in different soil layers may vary due to different
rainfall patterns in the two growing seasons according to the results reported by Asseng et al. (1998), which may further affect P distribution in roots.