| In order to maintain high crop yields and decrease the risk of contaminating environment, the five experimental sites with various soils and climate conditions were chosen to investigate the change of phosphorus (P) use efficiency, develop models describing soil P fertility, and determine the critical value of soil available P under wheat-maize cropping systems by examining changes in crop biomass and P concentrations in crops and soils (0-20 cm) based on a long-term fertilization (15 years) study of the National Long-term Monitoring Network of Soil Fertility and Fertilizer Effects. The main results and conclusions are:1. The 15-year mean wheat grain yields was 1312 kg ha-1without P (CK, N and NK treatments) and 4212 kg ha-1 with P fertilization (NP, NPK, NPKM and NPKSt treatments) across the five experiment sites. Similarly, the 15-year mean maize grain yields were 3066 kg ha-1 without P and 5789 kg ha-1 with P fertilization across the five experiment sites. The mean wheat and maize grain yields with P fertilization were 221% and 88.8% higher than those without P application.2. The contribution of P fertilizers to wheat yield increase ranged from 97 to 321% (average 171%) and from 14 to 204% (average 56%) for maize. The contribution of P fertilizers to wheat yield increase was greater than maize. Meanwhile, the supplying ability of soil P to maize production was 42.3% higher than that to wheat production due to the high temperature in the maize growing season. Thus a greater portion of P fertilizer should be applied to wheat than maize in a wheat-maize rotation. The mean harvest index of wheat and maize in treatments with P fertilizers across all experimental sites was greater than 0.4 and 0.5, respectively, suggesting it may potentially be a soil P fertility index.3. In soils without P addition, the concentration of Olsen-P decreased over time until reach about 3mg/kg, remained constant afterword in maize-wheat cropping systems. The decreasing trend of in Olsen-P in soils without P addition over time could be described by an exponential function. Applying P fertilizer at rate of 29.2 to 82.2 kg ha-1 per year causes Olsen-P accumulating in soils. For soil with P application, the model development requires the critical concentration of Olsen-P, P application rate, crop yield and soil pH. The models describing the rate of depletion and accumulation of Olsen-P could be validated independently and used for the accurate predicting the rate of Olsen-P changes in maize-wheat cropping system. The Olsen-P accumulated at 1.21 mg kg-1 per year in maize-wheat cropping system with the fertilization rate used in this long-term study.4. Application of 1 kg fertilizer P produce 104, 112, 47.7 and 96 kg (average 90 kg) of wheat grain yields or 151, 162, 173 and 177 kg (average 166 kg) of maize grain yields in NP, NPK, NPKM and NPKSt treatments. But 1 kg P from soil mineralization produces 268 kg wheat grain and 325 kg maize grain in soils without P fertilizer addition. Crops uptake 1 kg P produce 233 kg of wheat grain and 285 kg of maize grain from soil without P addition and produce 233 kg wheat grain and 285 kg maize grain from soil with P addition, representing12.3% and 13.1% reductions.5. In the NP, NPK, NPKM and NPKSt treatments, the P recovery efficiency over 15 years were 51.3, 56.0, 35.5 and 47.9% across all experimental sites. There was a net P loss of 149, 162 and 185 kg ha-1 (average 11.0 kg ha-1 per year) in CK, N and NK treatments of 15 years, but a net P gain of 455, 410, 1333 and 549 kg ha-1 (average 45.8 kg ha-1 per year) in the NP, NPK, NPKM and NPKSt treatments over 15 years.6. The mean critical values of soil Olsen-P for wheat were 12.5, 14.6 and 19.0 mg kg-1 (average 16.3 mg kg-1) and 12.0, 13.2 and 20.7 mg kg-1 (average 15.3 mg kg-1) for maize among study sites using linear-linear, linear-plateau and Mitscherlich model, respectively.Our study suggests that in order to achieve high crop yields, the long-term P input-output balance, soil P supplying capacity, and yield targets should be considered when making P fertilizer recommendations and developing strategies for intensively managed wheat-maize cropping systems.
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