Evaluation of soil nitrogen status in Japanese agricultural lands with reference to land use and soil types

The nitrogen content of agricultural soils collected from throughout Japan was characterized according to the form and availability, and with reference to land use and soil types. In total, 147 plow layer soil samples were collected-80 from paddy fields and 67 from upland fields to include various soil types. Soil N was separated into four fractions: inorganic extractable-N (Iex-N), fixed NH4 sup(+)-N (Ifix-N), organic mineralizable-N (Omin-N), and organic stable-N (Osta-N). In the analysis, the total-N content was determined by the dry combustion method. The Iex-N content was determined by extraction with a 2 mol L sup (-1) KCl solution and the Ifix-N content by extraction with a HF-HCl solution after removal of organic-N. The Omin-N content was evaluated as the potentially mineralizable N based on a long-term incubation method and the Osta-N content was calculated as the difference between the contents of total-N and the above-mentioned three fractions. The average total-N content was 2,593 mg kg sup(-1), of which the contents of Iex-N, Ifix-N, Omin-N, and Osta-N were 37, 124, 159, and 2,273 mg kg sup(-1), respectively. Thus the available fraction (Iex-N and Omin-N) accounted for 7.6%, and the stable fraction (Ifix-N and Osta-N) for 92.4% of total-N on the average. Assuming that the plow layer depth was 15 cm and the bulk density was 1.0 Mg m sup(-3), the amounts of total-N, Iex-N, Ifix-N, Omin-N, and Osta-N were 3,890, 56, 186, 239, and 3,411 kg ha sup(-1), respectively. The amount of the available fraction (295 kg ha sup(-1)) was comparable to the total annual N input (266 kg ha sup(-1)), whereas the amount of the stable fraction (3,597 kg ha sup(-1)) was more than ten times larger. In relation to land use, the Iex-N content was lower (both absolute and relative contents) in paddy soils (32 mg kg sup(-1), 1.4%) than in upland soils (43 mg kg sup(-1), 1.8%), whereas the Omin-N content was higher in paddy soils (200 mg kg sup(-1), 8.9%) than in upland soils (108 mg kg sup(-1) 4.4%). No differences due to land use were observed in the stable N fraction. In contrast, differences due to soil types which were not observed in the available fraction were clearly observed in the stable fraction (Ifix-N and Osta-N), i.e. the Ifix-N content was higher in non-volcanic soils (156 mg kg sup(-1), 10.0%) than in volcanic soils (54 mg kg sup(-1), 1.6%) whereas the Osta-N content was higher in volcanic soils (3,639 mg kg sup(-1), 93.0%) than in non-volcanic soils (1,632 mg kg sup(-1), 80.1%). Accordingly, the 42.4% of variance of the Ifix-N content was explained by the clay and Alo contents and 48.1% of variance of the Osta-N content by the Alo content. These findings showing that, in Japanese agricultural soils, the available N fraction was strongly affected by the land use, whereas the stable N fraction was mainly determined by the soil types, should be taken into account in the management of soil N in agricultural practices and N dynamics in agricultural fields.