The Role of Organic Acids on the Release of Phosphorus and Zinc in a Calcareous Soil

Phosphorus (P) and zinc (Zn) fixation by soil minerals and their precipitation is one of the major constraints for crop production in calcareous soils. Recent Studies show that root exudates are effective for the extraction of the large amounts of nutrients in calcareous soils. A part of the root exudations are Low Molecular Weight Organic Acids (LMWOAs). LMWOAs are involved in the nutrients availability and uptake by plants, nutrients detoxification, minerals weathering and microbial proliferation in the soil. At nutrients deficiency conditions citric and oxalic acids are released by plants root in large quantities and increase nutrient solubility like P, Zn, Fe, Mn and Cu in the rhizosphere. These components are the large portion of the carbon source in the soil after exudations are mineralized by microorganisms, quickly. In addition, soil surface sorption can affect their half-life and other behaviors in the soil. In order to study the effect of oxalic and citric organic acids on the extraction of phosphorus and zinc from a calcareous soil, an experiment was conducted.
Studied soil was calcareous and had P and Zn deficiency. Soil sample was collected from A horizon (0-30 cm) of Damavand region. 3 g of dried soil sample was extracted with 30 ml of oxalic and citric acids extraction solutions at different concentrations (0.1, 1 and 10 mM) and different time periods (10, 60, 180 and 360 minutes) on an orbital shaker at 200 rev min-1.The soil extracts then centrifuged for 10 minutes (16000g). After filtering, the pH of the extractions was recorded and then phosphorus, calcium and zinc amounts were determined. Soil extraction with distilled water was used as control. Each treatment was performed in 3 replications. Statistical analysis was performed with ANOVA test followed by the Bonferroni method significant level adjustments due to multiple comparisons.
The results of variance analysis showed that the effect of different concentrations of organic acids at various times on the pH of extractions was significant at 1% level. Low concentrations of organic acids at various times had no effect on the pH of soil extractions compared to control, but organic acids at 10 mM concentration increased the pH of extractions. This high pH amount was related to the organic acid mineralization and consumption of H by microorganisms in the soil. The results of variance analysis also showed that the effect of different concentrations of organic acids at various times in the extraction of Ca from the soil was significant at 1% level .Citric acid extracted higher Ca from the soil compared to oxalic acid. The maximum extracted Ca was observed at 10 mM concentration of citric acid at 10 minutes of shaking time. Extracted Ca at 0.1 and 1 mM concentrations of both organic acids was almost the same at all the times. The higher extraction of Ca with citric acid was due to the Ca precipitation as oxalate. The analysis of variance for P showed that the effect of different concentrations of organic acids at various times was significant at the 1% level. Extracted P by oxalic acid was more than citric acid .The highest amount of P was obtained by 10 mM concentration of oxalic acid at 360 minutes. The amounts of extracted P by both organic acids at 0.1 and 1 mM concentrations were similar to control. Citric acid at 10 mM concentration also released lower P compared to other concentrations and control. More P extraction of oxalate than citrate was due to the Ca-oxalate formation and P release from calcium phosphate in calcareous soil. Different concentrations of organic acids at different time periods had no effect on Zn extraction from the soil and the amount of extracted Zn by organic acids was lower than control.
Organic acids at 10 mM concentration were effective in Ca and P extraction from the soil but had no significant effect on the Zn extraction. It seems that organic acid more than 10 mM concentrations is required for Zn extractionfrom from the calcareous soils. The type and concentration of Organic acids, soil type, shaking time and experimental conditions are important factors that affect the obtained results.