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 (TP_i) 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.

中文翻译：

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有机酸对不同有机质含量软土中无机磷释放的调控

磷 (P) 用于提高农业生产力。有机酸增加无机磷 (P _i ) 释放到土壤溶液中，从而提高土壤中磷的利用率；然而，人们对有机酸和土壤有机质含量对 P _i释放的影响知之甚少。为了评估这些因素在确定磷有效性中的重要性，土壤 P _i的释放在添加有机酸（柠檬酸、草酸和苹果酸）对来自Mollisols地区的土壤具有不同的有机质含量并接受磷肥。生化产生的腐植酸用于改变土壤有机质含量。在P _i组分中，Fe-P组分在不同有机质含量的所有土壤中含量最高。在有机酸存在下，P _i的释放随着土壤有机质的增加，组分减少。在这些馏分中，Al-P 的释放量最高，其次是 Fe-P。各种有机酸的比较表明，草酸对P _i的释放最为有效。路径分析表明，Al-P释放具有较大的直接路径和间接路径系数，这表明Al-P释放对总P _i (TP _i )释放的贡献最大。我们得出结论，有机酸在提高土壤磷利用率方面的有效性在有机质含量低的土壤中最为明显，但需要进一步的田间规模研究来证实这一点。Al-P 是 Mollisols 中最有效的无机磷来源。