Organic acid regulation of inorganic phosphorus release from Mollisols with different organic matter contents

Phosphorus (P) is used to enhance agricultural productivity. Organic acids increase the release of inorganic phosphorus (P-i) into soil solutions and thereby improve phosphorus utilization in soils; however, the effects of both organic acids and the soil organic matter contents on P-i release are poorly understood. To assess the importance of these factors in determining phosphorus availability, the release of soil P-i fractions (i.e. aluminium-bound phosphorus (Al-P), iron-bound phosphorus (Fe-P), occluded phosphorus (O-P) and calcium-bound phosphorus (Ca-P)) was evaluated on addition of organic acids (citric, oxalic and malic acids) to soils from Mollisols region that had different organic matter contents and received phosphorus fertilizer. Biochemically produced humic acid was used to modify soil organic matter contents. Among P-i fractions, the Fe-P fraction had the highest concentration in all the soils with different organic matter contents. In the presence of organic acids, the release of P-i fractions decreased as the soil organic matter increased. Among the fractions, Al-P had the highest release, followed by Fe-P. Comparison among various organic acids revealed that oxalic acid was the most effective for P-i release. Path analysis showed that Al-P release had larger direct-path and indirect-path coefficients, which implied that Al-P release had the largest contribution to total P-i (TPi) release. We concluded that the effectiveness of organic acids in improving soil phosphorus availability was most pronounced in soils with low organic matter contents, but further field-scale research is needed to confirm this. Al-P was the most effective source of inorganic phosphorus in Mollisols.