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

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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.