Response of alkaline soil Olsen-P to phosphorous budget under different long-term fertilization treatments

Objectives  Phosphorous is easily immobilized in soil, especially in alkaline soil, which is one of the main reasons for the low phosphorous efficiency in crop production. This paper studied the response of soil Olsen-P contents and the P budget under different long-term P fertilization treatments in the alkaline soils of Northern China.

Methods  The data used in this research were from the long-term experiments located in Hebei, Beijing, Shandong, Tianjin, Henan and Shanxi. All the six experiments were conducted from 1991 to 2011 under winter wheat-summer maize rotation, and in soils of pH > 7. The selected treatments included no P input control (P0), chemical P fertilizer (P), chemical P and straw (P+S), pure manure (M), chemical P and manure (P+M). The crop yield was recorded, and soil Olsen-P and organic matter (SOM) contents were determined. The soil P budget was calculated, the relationships of soil Olsen-P with crop yield and soil P budget, and the P use efficiency (PUE) were calculated. The main factors influencing soil Olsen-P efficiency were discussed.

Results  Without P input (P0), the annual soil P was in deficit of −357.73–−28.21 kg/hm², and the soil Olsen-P content kept decreasing with experimental years in rate of 0.14 mg/(kg·year); the crop yields were low, with wheat yield lower than 2000 kg/hm² and maize yield lower than 4000 kg/hm². Under the four P fertilization treatments, the soil P were in a surplus of 23.65–860.93 kg/hm², with the surplus amount in order of P+M > P > P+S > M. The soil Olsen-P content increased with experimental years and the annual increase range was in order of P+M 4.85 mg/(kg·year) > M 1.87 mg/(kg·year) > P+S 0.65 mg/(kg·year) > P 0.63 mg/(kg·year). The wheat yield was in a range of 3399–7880 kg/hm², and there was no obvious difference among the four treatments. The maize yield was in a range of 4186–9176 kg/hm², which was highest under P+M and lowest under P. The wheat and maize yields kept increasing until the soil Olsen-P reached 22.47 mg/kg for wheat and 20.68 mg/kg for maize. The P use efficiency (PUE) of wheat was the highest under P+S treatment (16.17%), and that of maize was the highest under M treatment (16.45%). With the increase of soil Olsen-P, the PUE of wheat and maize improved, with the fastest rise under P+S treatment and slowest under P+M treatment. The soil Olsen-P had an extremely and positively significant (P < 0.01) relationship with soil P budget. With every 100 kg/hm² of P deficit, the soil Olsen-P decreased by 0.90 mg/kg. And with every 100 kg/hm² of P surplus, the soil Olsen-P increased by 22.10 mg/kg in M, 10.60 mg/kg in P+M, 3.90 mg/kg in P+S and 2.60 mg/kg in P fertilization treatment. Analyzed by RDA method, soil organic matter (SOM) was the main factor influencing soil Olsen-P efficiency, and could explain 85.0% of soil Olsen-P efficiency variance.

Conclusions  With the increase of Olsen-P in soil, the PUE of wheat and maize improved, and there was a striking increase of PUE under P+S. Long-term application of M or P+M has effectively increased the efficiency of soil Olsen-P as the increment of SOM. But under P+M, the P use efficiency was low, and the soil P was easily to be leached or immobilized. So, the total rate of P under P+M should be reduced, and the proportion of organic fertilizer should be increased, so as to maximize both agronomic and environmental benefits.