Effects of long-term synergistic use of organic and inorganic exogenous P on phosphorus availability in fluvo-aquic soil

Objectives Application of exogenous phosphorus is an important way to increase phosphorus availability of soil. The forms and their chemical behaviors of different phosphorus sources in soil play a decisive role in the effectiveness of soil phosphorus. Understanding of the synergistic relationship between long-term combined use of organic and inorganic exogenous phosphorus and its effect on phosphorus availability in fluvo-aquic soil can provide a theoretical basis for rationally reducing the application rate of phosphate fertilizers and increasing their utilization rate, and scientifically managing soil phosphorus resources.

Methods A 25 year in situ fertilization experiment was used. Four fertilization treatments were investigated: no phosphorus fertilizer (P0), chemical fertilizer only (FP), chemical fertilizer plus organic manure (FP+M) and chemical fertilizer plus maize straw (FP+S). The effects of long-term combined use of chemical phosphate, organic manure, and straw as major exogenous phosphorus on sustainable production capacity, phosphorus fertilizer use efficiency(PUE), fertilizer after-effect, soil phosphorus forms and availability were analyzed.

Results Continuous application of phosphorus for 25 years, the coefficient of variation of P0, FP, FP+M and FP+S treatments for wheat yield were 49.0%, 14.8%, 17.2%, 15.4% respectively, and for maize yield were 28.7%, 27.1%, 24.4%, 23.2% respectively, the stability of yield was higher than that of no phosphorus fertilizer treatment, especially the increase in wheat yield stability was significant; sustainable yield index (SYI) of P0, FP, FP+M and FP+S treatments for wheat were 0.23, 0.62, 0.60, 0.64 respectively, and for maize were 0.45, 0.47, 0.53, 0.52 respectively, the effect of phosphorus application on SYI of wheat yield was greater than that of maize. The accumulative phosphorus physiology efficiency(APPE) of FP, FP+M and FP+S treatments were188.3 kg/kg, 163.2 kg/kg and 177.6 kg/kg respectively. Chemical phosphate had the highest APPE, when combined with organic fertilizer or straw, the APPE was reduced. The average after-effects of FP, FP+M and FP+S treatments were 1.30%, 0.71% and 1.16% respectively, application of inorganic phosphate fertilizer alone had the highest after-effect. Long-term no application of phosphorus fertilizer, the content of soil inorganic phosphorus decreased, and mainly consumed invalid O-P, the reduction of O-P in P0 treatment accounted for 41.3% of the inorganic phosphorus reduction. Long-term application of chemical phosphate fertilizer reduced the proportion of invalid phosphorus, but the fixation of chemical phosphorus was still strong, by FP treatment the content of Ca₈-P increased from 12.0% to 21.1%, the content of invalid O-P and Ca₁₀-P decreased from 77.7% to 62.9% (still more than 60%) and the content of Olsen-P increased from 6.4 mg/kg to 20.7 mg/kg. The synergistic use of chemical phosphorus and organic phosphorus could promote the conversion of invalid phosphorus into available phosphorus and slow-effective phosphorus, which could improve soil phosphorus efficiency, by FP+M treatment the proportion of Ca₂-P was increased from 2.0% to 5.4%, the proportion of slow-effective Al-P, Ca₈-P and Fe-P increased by 25.6 percentage points, the proportion of invalid O-P and Ca₁₀-P decreased by 29.0 percentage points, and the content of Olsen-P increased from 6.3 mg/kg to 51.8 mg/kg. The application of straw phosphorus increased the content of slow-effective Ca₈-P, reduced the content of invalid phosphorus, and increased the potential phosphorus supply capacity of soil. Compared with FP the proportion of Ca₈-P by FP+S treatment increased by 7.9 percentage points, the proportion of Ca₁₀-P decreased by 6.5 percentage points, and the content of Olsen-P increased from 6.3 mg/kg to 21.7 mg/kg.

Conclusions The input of exogenous phosphorus significantly increased crop yield, yield stability and sustainable yield index. FP+S treatment maintained the best sustainable production capacity in the wheat−maize rotation system. Using inorganic phosphorus alone had the highest fertilizer after-effect and accumulative phosphorus physiology efficiency, but the fixation of chemical phosphorus in soil was high. The synergistic use of inorganic phosphorus and organic fertilizer could rapidly improve the efficiency of soil phosphorus, and had a significant effect on maintaining the composition of phosphorus fractions and soil quality. The synergistic use of inorganic phosphorus and straw phosphorus has a strong excitation effect on soil phosphorus, and weakens the fixation of phosphorus in soil, and enhances the potential phosphorus supply capacity of soil.