Effects of long-term fertilizations on soil phosphorus and its supply to rice in red paddy soil

Objectives Rational soil phosphorus (P) management is important to crop production and environment protection. There has been few studies focusing on soil phosphorus and its absorption in double cropping rice in southern China. This paper analyzed soil phosphorus evolution, phosphorus activation coefficient, plant P uptake and P budget based on a long-term paddy soil quality evolution monitor experiment (since 1984) in Jiangxi Province, China to provide important scientific basis for guiding rational application of phosphate fertilizer.

Methods The long-term experiment started in 1984 in Nanchang, Jiangxi Province, China. It included eight fertilization treatments with triplicates: no nutrient input (CK), chemical PK, chemical NP, chemical NK, chemical NPK, 70% mineral N plus 30% organic fertilizer N (70F + 30M), 50% mineral N plus 50% organic fertilizer N (50F + 50M), and 30% mineral N plus 70% organic fertilizer N (30F + 70M). The chemical fertilizers were urea, SSP and potash chloride. For early rice, the rates were N 150, P₂O₅ 60 and K₂O 150 kg/hm², and those for late rice were N 180, P₂O₅ 60 and K₂O 150 kg/hm², respectively. The test organic fertilizers were milk vetch with N 0.30%, P₂O₅ 0.08% and K₂O 0.23%, and rotten pig manure with N 0.45%, P₂O₅ 0.19% and K₂O 0.60%, respectively. The application rates of organic manures were based on N equivalent and the deficits of P and K were supplemented by chemical fertilizers, except the supplemental K in 30F+70M treatment exceeded the designed quantity. Growth and grain yields of rice were measured each year (1984–2012), and the P content was analyzed. Soil samples were collected from 0–20 cm layer for soil total-P and Olsen-P determination. The cumulative P budget was discussed.

Results After 29 years’ experiment, the soil total-P content decreased by 4.6 mg/(kg·a) in chemical NK treatment, while that was increased by 3.3–19.4 mg/(kg·a) in P containing treatments, and was averagely increased by 16.1 mg/(kg·a) in treatments of 70F + 30M, 50F + 50M and 30F + 70M, being 4.89 times of that in NPK treatment. The total P content increased to 1.07 g/kg averagely (mean value of 2010–2012), 1.18 times higher than that of initial value. The soil phosphorus activation coefficient (PAC) decreased from 4.24% to 2.5% in no P treatment, but increased significantly in P containing treatments. The average PAC was increased to 8.51% in three organic-inorganic fertilization treatments, which was 2.89 times of that in NPK treatment. The amounts of phosphorus uptake by both early and late rice plants in fertilizer treatments were significantly higher than those by CK, with increment ranges of 29.9%–124.2% or 28.6%–103.2%, respectively. Phosphorus uptake in balanced fertilization treatments (NPK, 70F + 30M, 50F + 50M and 30F + 70M) were significantly higher than that in unbalanced fertilization treatments (PK and NP), with an average increments of 38.7% and 32.9%, respectively. There was a very significant positive linear correlation between yield and phosphorus uptake both in early and late rice. The early and late rice yields would be increased by 115 kg/hm² and 106 kg/hm² per kilogram phosphorus uptake, respectively. No significant correlation was observed between total soil P content and cumulative P budget in CK. The total P content would decrease 6 mg/kg for every P deficit of 100 kg/hm² in NK. The total soil P content would increase 9.3 mg/kg for every soil P surplus of 100 kg/hm² in NP, PK and NPK treatments, and increase 63.3 mg/kg in organic-inorganic fertilizer treatments, being 6.78 times of that of NPK treatment.

Conclusions Application of both chemical P and organic fertilizers could effectively increase soil phosphorus storage and availability. Combining application of organic and chemical fertilizers performs better than chemical phosphorus fertilizer alone in the same amount of phosphorus inputs level. Therefore, decreasing the amount of total P input and increasing the organic fertilizer portion should be recommended to improve food production and protect environment in red paddy soil in southern China.