Relationship Between Available Phosphorus And Phosphorus Balance And Its Mechanism Under Different Long-term Fertilizations In Black Soil

Phosphorus(P) is one of the necessary nutrients for plant growth, and it involved in a variety of physiological processes in plants. Available P in the soil, which is the most effective P sink for plants,has been considered an important indicator for evaluating the capacity of the soil to supply P and for influencing crop yield and P loss risk from runoff. Rational control of soil available P not merely has a dual meaning of agriculture and the environment but also can predict the future of P rate from the angle of soil fertility. Changes in soil available P are driven by the P balance(P input minus P output), and their relationship value is important index to P rate prediction. In this study, the two investigated long-term experiments(Gongzhuling and Harbin) were established on black soils in the northeast region of China. Six fertilization treatments were selected:(1) unfertilized(CK);(2) nitrogen only(N);(3)nitrogen and potassium(NK);(4) nitrogen and phosphorus(NP);(5) nitrogen, phosphorus, and potassium(NPK); and(6) nitrogen, phosphorus, potassium and manure(NPKM). We developed a dataset of the changes in the available P and P balance, meanwhile determined soil P adsorption and desorption properties and soil P fraction characteristics. The aim is to explore the change of available P,P balance and their relationships. At the same time, effect of soil properties(p H value and soil organic matter), soil P adsorption and desorption properties and soil P fraction characteristics on the relationship between change of available P and P balance were analyzed. The main results are as follows:1. Without the application of P fertilizer(CK, N and NK), the available P concentration declined,and the available P concentration increased over time at the two sites, and the correlation coefficients were significant(P<0.01).The available P concentrations for CK, N and NK in the black soils at Gongzhuling, decreased by 0.34, 0.29 and 0.29 mg/(kg a), respectively, without P addition and increased by 1.15, 1.24 and 8.16 mg/(kg a), respectively, with P fertilization for NP, NPK and NPKM.After the 21 fertilization periods, the P contents of the Gongzhuling were depleted by 336 kg/ha in the CK, 676 kg/ha in the N, and 773 kg/ha in the NK treatments, respectively. The P balances were-55,-91,and 844 kg/ha for the NP, NPK and NPKM treatments, respectively. The change in soil available P concentration was significantly(P<0.05) correlated with the P balances except for the NP and NPK treatments of Gongzhuling. With an average deficit of 100 kg/ha P, the soil available P concentration at both sites decreased by 0.70~2.14 mg/kg in the treatments without P addition and increased by 19.63mg/kg in the NPKM treatments with 100 kg/ha of P accumulation. There are significant differences for CK, N, NK and NPKM. The change in available P by each 100 kg/ha P balance indicates the P activating ability from sparingly P to available P, and it defined as soil available P efficiency(mg/kg).2. Effect of soil properties, soil P adsorption and desorption properties and soil P fraction characteristics on the soil available P efficiency:(1) Fertilizations changed the black soil p H value and organic matter; thereby it changed the soil available P efficiency. Fertilization for many years, the p H values for N, NK, NP and NPK treatments reduced 0.53, 0.95, 0.95 and 1.08 in comparison with CK at the Gongzhuling site. The soil organic matter significantly increased 10g/kg for the NPKM treatment than CK and NK at Gongzhuling. Theaccumulated P is easier to transfer into available P in which soil has the characteristics of low p H and organic matter. The available P increase 2.15 mg/kg by each 100 kg/ha P balance when soil organic matter improved 1g/kg.(2) Fertilizations changed the soil P adsorption and desorption properties; thereby it changed the soil available P efficiency. Soil readily desorbable P(RDP) of NPK is 9 and 3 times than CK and NK,and NPKM is 761% higher than NPK treatment. Degree of P saturation(DPS) was 23 and 22 times higher for NPK than CK and NK, and and NPKM is 381% higher than NPK treatment. The constant of adsorption energy(K) of CK, NK and NPK are 3, 3 and 4 times higher than NPKM. The maximal absorptive capacity(Qm) for NPK and NPKM reduced 48% and 52% than NK treatment. That is to say inorganic P fertilizer plus manure reduced the amount of adsorption site and colloid affinity to P, and increased soil readily desorbable P than CK and NK.(3) Fertilizations changed the soil P fraction characteristics; thereby it changed the soil available P efficiency. There was a significant positive correlation between soil available P efficiency and highly active P, e.g. Resin-P(r=0.97, P<0.01), Na HCO3-Pi(r=0.95, P<0.01), Na OH-Pi(r=0.95, P<0.01).The content of labile P(Resin-P + Na HCO3-Pi + Na HCO3-Po) improved 19% and 16% when inorganic P fertilizer, especially inorganic P fertilizer plus manure applied into soil than no P treatment. The fertilization which has higher ratio of labile P to total P has higher soil available P efficiency.3. The transformation process of soil P fractions varied among long-term fertilizations. The proportion of labile P to total P decreased with no P applied. Compared to 1990, the labile P decreased5% and 2% at CK and NK treatment, and has all increased 10% at NPK and NPKM treatment in 2010.The decrease of labile P was 19.64 mg/kg, the moderately labile P was 13.94 mg/kg, and the decrease of stable P was 16.93 mg/kg at CK treatment. For NK treatment, there were 10.06 mg/kg labile P and31.18 mg/kg stable P reduce, and these P transformed to 5.75 mg/kg moderately labile P, and others were taken in by maize.The increase of labile P was 45.22 mg/kg, the moderately labile P was43.51mg/kg, and the decrease of stable P was 117.56 mg/kg at NPK treatment. P chemical fertilizer combined organic improve soil total P by 647.39 mg/kg after 21-year fertilization. Of all, 42.29% of the fertilizer of inorganic P and manure transferred into Dil HCl-Pi, 16.60%, 14.74% and 16.41% transferred into Resin-P, Na HCO3-Pi and Na OH-Pi.In short, the available P increase 2.15 mg/kg by each 100 kg/ha P balance when soil organic matter improved 1g/kg. When applied nitrogen fertilizer, the cooperative utilization of P and soil acidification promote the process of dissolution for stable P. And P application increased the proportion of labile P to total P, reduced the amount of adsorption site and colloid affinity to P, increased soil readily desorbable P. Thus, the available P efficiency is higher for NPKM than CK and NK. Study the effect of fertilizations for available P efficiency can provide scientific basis for rational control of P rate of black soil, thus ensure the friendly and sustainable development of agriculture.