Changes in gross rates of nitrogen transformations in soil caused by conversion of paddy fields to upland fields

We investigated the differences in N transformations and CO₂production between paddy soils and upland soils to analyze the effect of conversion of paddy fields to upland fields on N transformations in soils. A portion of a paddy field consisting of Fine-Textured Strong-Gley soils was converted to an upland field, and the gross rates of N transformations were determined for the soils collected from both fields by using the ¹⁵N-NH₄⁺isotope dilution method. The rates in both soils were determined under submerged and moist conditions respectively, and the quantitative characteristics of N transformations under each condition were analyzed. The rates of mineralization and immobilization under the moist conditions were higher than those under the submerged conditions, especially large differences were noted in the immobilization rate. Mineralization rate was faster than the immobilization rate under submerged conditions, whereas the order of the rate was opposite under moist conditions. For the same temperature and moisture content, the gross N transformation rates and CO₂production in the soils from the paddy field exceeded those from the upland field. These differences were attributed to the decrease in the amount of readily decomposable organic matter caused by the conversion of the field.