Studies on the solubilizing mechanisms by crops for the unavailable soil nutrients and its application to the cropping systems

在土壤中的Norg中，有效Norg被认为是易矿化Norg的一部分。当土壤在稻田和/或高地条件下在适当的温度下培养时，可以根据释放的无机N的量来估计该Norg。然而，培养方法很耗时，因此许多国家开发了几种化学提取方法来替代培养方法。利用根系分泌物中常见的有机酸对土壤中有机氮的增溶作用进行了研究。根据有机酸对土壤样品中Norg的增溶能力，将有机酸分为两组。草酸、丙二酸、酒石酸、柠檬酸和苹果酸可通过增加其浓度溶解Norg，而琥珀酸和戊二酸即使增加其浓度也不能溶解Norg。草酸、丙二酸、酒石酸、柠檬酸和苹果酸提取的有机氮量与提取物中铝和铁的量之和表现出很强的相关性。因此，有人认为，增溶能力可能是由于这些有机酸的结构，可以形成稳定的螯合物与铝和铁（草酸，丙二酸，酒石酸，柠檬酸和苹果酸）。此外，20 mM的柠檬酸可提取的Norg（CEON），这是最高的有机酸测试中，表现出很强的相关性与67 mM的磷酸盐缓冲液，这是现在被广泛使用的一个指标，用于估计土壤氮的有效性在日本，和N矿化使用培养方法在46个土壤样品属于不同的土壤类型。这些结果表明，CEON可能是土壤中可矿化Norg的重要组成部分。因此，作物分泌的有机酸对作物根际土壤中有效态氮的增溶起一定作用。

Within the Norg in soil, the available Norg is considered to be a fraction of easily mineralizable Norg. This Norg can be estimated based on the amount of released inorganic N when a soil is incubated under paddy and/or upland conditions at an appropriate temperature. However, incubation methods are time-consuming, and several methods of chemical extraction have thus been developed as substitutes for the incubation methods in many countries. The solubilization of organic nitrogen (Norg) in soil was examined by using organic acids often present in root exudates. The organic acids tested were classified into two groups depending on their solubilizing ability for Norg in a soil sample. Oxalate, malonate, tartarate, citrate and malate could dissolve Norg by increasing their concentrations, while succinate and glutarate could not dissolve Norg even when their concentrations were increased. The amount of organic N extracted with oxalate, malonate, tartarate, citrate and malate showed a strong correlation with the sum of the amounts of Al and Fe in the extracts. Therefore, it was assumed that the solubilizing ability may be attributed to the structure of these organic acids that can form stable chelate compounds with Al and Fe (oxalate, malonate, tartarate, citrate and malate). Furthermore, the amount of 20 mM citrate-extractable Norg (CEON), which was the highest among the organic acids tested, showed a strong correlation with both that of 67mM phosphate buffer, which is now being widely used as an index for estimating soil N availability in Japan, and N mineralized using an incubation method in 46 soil samples belonging to different soil types. These results suggested that CEON might be an important constituent of mineralizable Norg in soil. Therefore, organic acids secreted by crops play a role in the solubilization of available Norg surrounding the crop rhizosphere.