Relationships between labile organic matter and nitrogen mineralization in Japanese agricultural soils with reference to land use and soil type

The relationship between the amount of organic matter determined using various extraction methods and the nitrogen mineralization potential (N₀) in soil was examined with reference to land use (upland and paddy soils) and soil type (non-volcanic and volcanic soils). Surface soils were collected from agricultural lands all over Japan (n = 72). Seven methods of extracting labile organic matter were used: water extraction (WE), hot water extraction (HE), extraction using an autoclave (AU), phosphate buffer extraction (PB), sulfuric acid extraction (SA), separation of light fraction (LF) and permanganate oxidation (PM). Significant relationships were observed between N₀ and the content of organic N (OrgN) when the WE, AU, PB and LF methods were applied for all soil samples. Higher correlation coefficients were obtained for groups of similar soil samples in terms of land use and soil type. Namely, for the upland non-volcanic soils, PB-OrgN (r = 0.66**, **P < 0.01) and SA-OrgN (r = 0.69**, **P < 0.01) showed significant relationships and close values to N₀. For the upland volcanic soils, PB-OrgN (r = 0.72**, **P < 0.01) showed significant relationships and the closest value to N₀. Therefore, for the upland soils, PB was considered to be the most suitable method for predicting N mineralization potential. For the paddy non-volcanic soils, AU was considered to be the most suitable method for predicting the mineralization potential because of the significant relationship and the result was closest to N₀. The content of labile organic N regulated the soil N mineralization pattern. In the upland soils, the source of N mineralization differed between short-term and long-term mineralization and, in general, mineralization was regulated by the amount of labile organic N to be mineralized because there was a limited amount of labile organic N. In the paddy soils, the source of N mineralization was similar, regardless of the duration of mineralization, and mineralization could be regulated by a high amount of labile organic C instead of N because of the high content of labile organic N. These findings may provide useful information not only for investigations of the source of mineralized organic N, but also for enhancing the productivity of agricultural lands.