| Phosphorus (P) is a major nutrient limiting plant growth in many soils. The problems of phosphate fertilizer industry China is facing today include1) limited resources of phosphate minerals;2) low use efficiency;3) Environmental pollution caused by overuse of phosphate fertilizer. Phosphorus availability to crops can be increased by application of organic or inorganic amendments such as manures, composts, crop residues, rock phosphates or various commercial inorganic P fertilizers. Little is known about the effects of long-term application of organic and inorganic P fertilization on P fixation, organic P mineralization and crop yields. The objectives of this study are1) to investigate the effects of additions of relatively small amounts of superphosphate (SP) combined with various rates of pig manure (PM) on crop yields, organic and inorganic P fractions, accumulation of microbial biomass carbon and phosphorus (MB-C, P) and the correlation between MB and yields.2) to determine the P fate by using32P tracing technology.3) to investigate the effect of combined application of SP and PM inoculated with phosphate-solubilizing bacteria (PSB).The pot experiment with different ratios of inorganic to organic fertilizer was conducted to evaluate the best proportion of SP to PM. The treatments included0:0,100:0,90:10,85:15,80:20,70:30, and0:100of inorganic to organic P fertilizer ratios. Another pot experiment used SP and manure in combination and in the same time inoculated with phosphate-solubilizing bacteria (PSB) K3. The treatments included the90:10K3(SP manure+K3),85:15K3,80:20K3,70:30K3,0:100K3. The main results obtained were as follows.The100:0treatment gave the highest yield and shoot biomass, which was similar to the yield from80:20treatment in the first wheat season.The yield of rice grain was the highest in the100:0treatment, which was much higher than any other treatments. The80:20treatment gave the highest content of soil available P, which was similar to the yield from the third wheat in the70:30treatment. The grain yield from80:20treatment was significantly higher than that from100:0treatment. There was no difference between the yields from70:30and0:100treatments. The yield of soybean was the highest in the100:0treatment, which was significantly higher than that from the100:0treatment, increasing by5.7%. The result indicated that the combined organic and inorganic phosphorous fertilizer had a significant synergistic effect in long time application.As for various forms of organic P under different fertilization in paddy soil, labile organic P(LOP) in90:10,85:15,80:20and70:30treatments was higher than that of the100:0treatment, increasing by40.0%,53.3%,60.0%and64.4%, respectively. In the treatment of no P application, the contents of moderately labile organic P (MLOP) decreased. The decreasing rate was in order of0:100>70:30=80:20>85:15>90:10>100:0>0:0. The combination of organic and inorganic P fertilizers had stronger effects on iron phosphates (Fe-P), calcium phosphates (Ca-P) and occluded P (O-P) than on aluminium phosphates (Al-P) in paddy soil. Compared with the100:0treatment, soil inorganic phosphorous (Pi) decreased by12.4%,16.1%,17.8%and19.3%when the ratios of TSP/PM fertilizer were90:10,80:20,70:30, and0:100, respectively. When combining chemical fertilizer and organic manure application to the calcareous soil, the accumulated phosphorus was transformed mainly into readily available forms of Ca2-P and Ca8-P, which increased the inorganic phosphorus pool of Ca8-P、A1-P、Fe-P and Ca2-P by combine use of chemical fertilizer and organic manure, and increased the contents of these forms of inorganic phosphorus as well.The phytase activity of the80:20treatment was higher than that of the100:0treatment, increasing by14.9%(the first wheat season)、12.7%(the second rice season)、49.6%(the third wheat season) and28.1%(the fourth soybean season), respectively. MB-C and MB-P of the80:20treatment was higher than that of the100:0treatment at soybean different stages, and P accumulation of the80:20treatment was also higher than that of the100:0treatment, increasing by43.1%(the seeding stage),42.7%(the flowering stage),64.7%(the seed filling stage),24.1%(the harvest stage), respectively. Under the combined fertilization, the dynamics of soybean phosphorous uptake was well regulated by MB-P, which promoted the soil available P and phosphorous accumulation in plant. We found a remarkably close relationship between MB-P and yield, which was y=23.413Ln(x)-40.254(r2=0.9584).We conducted a32P-labeled experiment to investigate the mechanisms affecting P availability in the paddy soil and calcareous soil. Sorption isotherms of the treated paddy soils were different.32P in the80:20treatment moved faster than the other two treatments in the paddy soil. The leachate volumes collected from the paddy soil before32P first appeared were13.1(80:20),19.0(100:0) and19.8(0:0) mL. Total32P recovery for the100:0and the80:20treatments increased significantly after about45mL of leachate were collected. When the amount of leachate was50mL, only1.6%,8.4%and9.8%of the total32P added leached out from the0:0,100:0and80:20treated soils, respectively. The other result indicated that the phosphorus isothermal adsorption curves of the3experimental calcareous soils all properly fitted the Langmuir adsorption equation. The affinity constant of P in the80:20treatment was lower than in the100:0treatment.32P in the80:20treatment also moved faster than the other two treatments in the calcareous soil. The leachate volumes collected from the calcareous soil before32P first appeared were40.8(80:20) and70.3(100:0) mLThe result showed the80:20K3treatment gave the higher yield than the100:0treatment in every season. The80:20K3treatment was not significantly different from the70:30K3treatment and higher than the100:0treatment in shoot biomass. The combination of TSP and manure inoculated with PSB could increase quantities of bacteria, decrease counts of fungi remarkably and affect on soil microbial communities through denaturing gradient gel electrophoresis (DGGE) and change the average well color development (AWCD).Our results indicated that a ratio of80:20resulted in the best grain yield, which was a more suitable ratio than the other ratios. Mechanisms of combined application of organic and inorganic phosphorous fertilizers improving the phosphorous use efficiency were that1) Fe-P and O-P were decreased in paddy soil, and Ca2-P, Ca8-P were increased meanwhile Ca10-P, O-P were decreased in calcareous soil.2) The phytase activity had been promoted by the combining organic and inorganic fertilization. LOP and MROP of the combined organic and inorganic fertilizer treatments were higher than that of the inorganic fertilizer treatment.3) The manure could block P-absorbing sites effectively, thus decreasing Pi from fixation.4) Plant phosphorous uptake was well regulated by MB-P, which increased fertilizer use efficiency. Inoculation with phosphorus solubilizing bacteria can activated soil P and observably improve the P use efficiency in the first and second seasons, which has a positive significance for keeping high and stable yields. |
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