Phosphorus Accumulation Characteristics And Environmental Risks Of Purple Soil Under Long-term Fertilization

Phosphorus (P) is one of the major essential element for plant growth and metabolism. Phosphatic fertilizers applied to the soil to increase crop yield, excessive application of P fertilizers to cropland can cause P accumulate, the risk of P loss potential increased when much P accumulate, then it will cause various ecological problems such as entrophication. Study the rules of P accumulation and evaluation of soil P loss potential have agreat significance for reduce and control the environmental risk of phosphorus and agriculture production. This study based on the National Purple Soil Fertility and Fertilizer Efficiency Monitoring Base as a platform and the data of soil physicochemical properties and crop yield of purple soil in 22 years long-term fertilization as the research object. Through the model simulation and indoor simulation, explore the P accumulation characteristics and P loss risks of purple soil under long-term fertilization, it aimed to provide basis and support for purple soil P optimization management and environment protection.The main results were as follows:The results of 22 years long-term experiment showed that applied 120～180 kg/hm2 phosphate fertilizer (P2O5) to paddy-wheat rotation cropland, which increase the content of soil effective phosphorus(P) (Olsen-P) and total phosphorus in surface soil, the available P increase 26.4～63.6 mg/kg than beginning, it increased 0.80～2.32 mg/kg every year; Straw returned or organic fertilizer and chemical phosphorus fertilizer mixed application can effectively activate P in the soil, accelerate the accumulation of soil effective P, the surface soil effective P of treatment applied chemical phosphate fertilizer alone (NPK) increase 0.8 mg/kg every year, when the surface soil effective P of treatment applied chemical phosphate fertilizer and straw returned or organic fertilizer increase 1.10～1.24 mg/kg every year.The changes of plough layer Olsen-P was highly significant to soil P balance. Treatments of long-term P fertilizers applied, soil Olsen-P increased by 4.76 mg/kg in average when per 100 kg/hm2 P surplus; Treatments without long-term P fertilizers applied, soil Olsen-P decreased by 0.44 mg/kg in average when per 100 kg/hm2 P deficit. The Olsen-P of NPK increased 4.72 mg/kg of per 100 kg/hm2 P surplus, when treatments applied chemical phosphate fertilizer and straw returned or organic fertilizer (NPKM、NPKS) increase 6.19�'�3.86 mg/kg. Quantity of P surplus increase with the long-term phosphate fertilizer application, when120 kg/hm2 (P2O5) phosphate fertilizer applied every year, the quantity P surplus was 361.9 kg/hm2, and treatments applied chemical phosphate fertilizer and straw returned or organic fertilizer (NPKM and NPKS) P surplus were 392.0 and 706.9 kg/hm2,1.5 times chemical phosphate fertilizer applied and straw returned((NPK)1.5S) P surplus was 1057.2 kg/hm2. P activation coefficient (PAC) of treatments without phosphate fertilizers applied were 0.47～0.56%, PAC of NPK was 2.52%, PAC of NPKM,NPKS and (NPK)ClS were 2.73～2.96%.The accumulation rates and changes of the amount of accumulated of purple soil Olsen-P after long-term P fertilizer applied can be simulated by Olsen-P= Olsen-Pi × exp(A t)-B and Olsen-P=Olsen-Pi+(D/pH)×(Pm-CmYm) ×t. The accumulation rates and changes of the amount of accumulated of soil Olsen-P were affected by the amount of P applied、the amount of P taked from soil and pH. The annual average accumulation rates was 0.90-3.90 mg/kg, and (NPK)1.5S>NPKM> PK>NPKS>NPK; Accumulation rates of the first 10 years were higher than the last 12 years. According to the mean accumulation rates of the fiest 22 years can predict the time of soil Olsen-P increase to runoff threshold (40mg/kg) and leaching threshold (60mg/kg), runoff threshold and leaching threshold of 1.5NPKM、 NPKM、PK、NPKS and NPK were 13、20、22、46、65 and 21、32、36、80、 118 years respectively.Four kinds of model were used to simulate the critical values of soil Olsen-P for wheat and rice, the average ritical values of soil Olsen-P for wheat was 10.44 mg/kg, relative yield is 84.06%; the average ritical values of soil Olsen-P for rice was 10.73 mg/kg, relative yield is 87.36%.Long-term phorsphate fertilizers applied in plough layer has a tendency to move down. The content of soil effective P of P applied treatments were higher than that without P applied treatments 0.23～9.84 mg/kg in 20～40 cm layer, then 40～60cm layer and 60-80cm layer were 0.45～7.13 mg/kg and 1.53～7.82 mg/kg respectively. The P has leached to the 80 cm depth in wheat-riace rotation, which increase the risks of P leaching and ground water pollution. P moving downwards was influenced by amount of P fertilizer, kind of organic manure and planting methods. The quantity of P moving downwards increased with the increase of P application rate. P moving was easier in soil of chemical P application combined with organic fertilizer than combined with straw on the condition of equivalent nutrient.There were a large amount of P accumulated in soil and it will cause P in soil moves with the surface water flow in rice season. P moving downwards water bodies mainly happened in first 10 days after paddy flooded water, P concentration of surface water higher in 10 days and TP concentration of surface water were 0.32-28.39 mg/L. It was forbidden to drainage water in the first 10 days after paddy transplanted to avoid water pollution. TP concentrations were dfferent for dfferent fertilization treatments, (NPK) 1.5S was the highest, the NPKM and partial application nitrogen-phosphorus fertilizer and phosphated potash fertilizer was second,treatments without P applied the lowest, it showed that excessive fertilization and chemical fertilizers applied with organic fertilizers increase the risk of soil P runoff.Readily Desorbable Phosphorus(RDP), Degree of Phosphorus Sorption Saturation (DPSS), Maximun Buftfering Capacity (MBC), Qm and Phosphorus Sorption Index(PSI) were the important indexes of evaluating P moving ability of soil-water interface. The RDP, DPSS and PSI of (NPK)1.5S、NPKS treatments were higher than NPK treatmen, then (NPK)1.5S、NPKS treatments soil firming and buffering capacity decreased, P moving ability increased and the risk of P move to environment increased. DPSS、PSI、RDP have significant positive correlation with total P, Olsen-P and soil organic matter, and hanve significant negative correlation with pH.Linear segmentation model (the Split-line) was used on soil Olsen-P and CaCl2-P fitting to get soil phosphorus leaching critical point, the result showed that phosphorus leaching environment threshold for (Olsen-P 27.45, CaC2-P 0.70), When Olsen-P content in soil is more than 17.34 mg/kg, CaCl2-P in the soil rapidly increasing, the risk of phosphorus runoff incresed.Combined with soil Olsen-p critical values, the suitable soil Olsen-P content range that can both obtain satisfy the high yield and the reduction of soil P runoff was 10.73～27.45 mg/kg.It can make use of these indicators of purple soil region of soil phosphorus on agricultural efficiency and environmental risk assessment, and determine the optimal management strategy of regional phosphate fertilizer.