Meta-analysis on the effects of low molecular weight organic acids on increasing availability of soil phosphorus

Objectives Long term phosphorus fertilization has resulted in accumulation of phosphorus in soil. However, most of the phosphorous is immobilized to become difficult for plant use. Low molecular weight organic acids (LMOA) have been found as effective “P activator”, which has become one of the research hotspot at present. In this paper, literature was searched and analyzed for the rational application of low molecular weight organic acids to improve the availability of phosphorus in soil.

Methods 831 published studies in the past 30 years (1990–2018) were found using the key words containing “available P”, and a database was established based on the papers related to activation of soil fixed P by LMOA. Using Meta-analysis method, the activation effect of LMOA was analyzed under different soil pH, total P content and available P content, incubation types and time, and organic acid kind and their concentrations.

Results The LMOA concentration was 0–1 mol/L in the literatures. Comparing to controls without adding LMOA, LMOA decreased the content of calcium-P (Ca-P), aluminum-P (Al-P), iron-P (Fe-P), Occluded-P (O-P), and organic-P by 27.1%, 21.3%, 15.5%, 8.22% and 5.42%, respectively, and increased the content of available P by 213%. In calcareous soils, LMOA transformed the octacalcium P (Ca₈-P) and decandin P (Ca₁₀-P) to labile dicalcium P (Ca₂-P), with the content of Ca₈-P and Ca₁₀-P decreased by 8.36% and 11.8%, and the content of Ca₂-P increased by 7.90%. In the soil with total P content less than 1 g/kg or available P content less than 20 mg/kg, the LMOA increased the available P content by 331% and 343%, respectively, and the increasing effects on total P and available P were 107% and 189% higher than those obtained in soils with total P greater than 1 g/kg and available P higher than 20 mg/kg, respectively. In acid (pH < 6) and neutral (pH 6–8) soils, LMOA addition increased the available P by 329% and 320%, which were much higher than the value of 56.9% in calcareous soils (pH > 8). The P activation effect was influenced by incubation period greatly. The soil available P contents were increased by 257% in the 1st day of incubation, and by 372% during the incubation period of 10–20 days, but decreased after 20 days. The activation effect of P was also affected by incubation method. The soil available P contents were higher in shaking culture than those in normal culture. The P activation effect of oxalic and citric acids was better than others, and when their concentration was below 90 mmol/L, the available P contents were increased by 288% and 185%, respectively.

Conclusions The activation effect of LMOA depends on soil pH, total P, and available P content, as well as the adding acid types and adding time. Generally speaking the activation effect of LMOA on soil insoluble P is better in acid and neutral soils than in calcareous soils, and more effective in soils with lower total P. Moreover, the activation effect on soil insoluble P normally lasts for 10 to 20 days, and decreases afterwards. It is concluded that oxalic acid and citric acid perform better in activiting soil insoluble P than other LMOA.