Phosphorus efficiency, soil phosphorus dynamics and critical phosphorus level under long-term fertilization for single and double cropping systems

Wise phosphorus (P) management is paramount to ensure food and environmental security and prolong the life of P resources. Long-term experiments under winter wheat–summer maize (with irrigation) and winter wheat–summer fallow (rain-fed) cropping systems were undertaken to investigate P efficiency, soil P availability and P downward movement in the deep profile of a loess soil in China. The four nutrient management regimes in each experiment were: a control without any nutrient input; P and K combined (PK); N, P and K combined (NPK), and organic manure (M) combined with NPK (MNPK). The P efficiency ranged from 20 to 54% under the double cropping system, and from 13 to 33% in the single cropping system over the 25 years studied, and the highest P efficiency was in the NPK treatment in both systems. Application of P significantly increased soil total-P, Olsen-P and CaCl2-P over the years, especially in the MNPK treatment. Two segment regression analyses indicated that Olsen-P was about 14 mg kg−1 or 16 mg kg−1 for optimal crop yield under double cropping or single cropping. In addition, Olsen-P content above 37 mg kg−1 or 57 mg kg−1 led to a significant increase in CaCl2-P content under double cropping or single cropping, which might indicate risk of P leaching. After 24 years of P applications, total P accumulated at various depths depending on treatment under double cropping, but there was no accumulation under single cropping. While Olsen-P leached down to a depth of 300 cm in the MNPK treatments under both cropping systems, and higher concentrations were recorded in the deeper layers in single cropping than in double cropping systems. The Olsen-P leached to a shallower depth in the PK and NPK treatments under single cropping compared to double cropping. We conclude that P supply in excess of the crop’s requirement (e.g. MNPK) or an unbalanced nutrient supply (e.g. PK) resulted in not only low P efficiency and massive accumulation of P in the topsoil but also leaching out of root zone under both irrigated and rain-fed conditions in the loess soil. Thus, P fertilization recommendations need to be adjusted based on changes in soil P over time.