Loss of nitrate nitrogen in the soil surface layer in Japanese grassland during the heavy rainfall season and its enhancement by the root mat

Abstract

Measurement of nitrate and chloride in the soil profile have been made in a grassland field at the National Grassland Research Institute, Tochigi, which received heavy applications at the rate of 60 kg/ha of nitrogen and chloride as potassium nitrate and calcium chloride respectively, at 5 days intervals in the summer wet season. The chloride anion was utilized as a tracer of the nitrate anion. Compared with the plots without a root-mat zone (0–2.5 cm), those with a root-mat zone had a lower NO3−N/Cl ratio in the soil surface layer (0–15 cm). Under intensive rainfall (17th July to 5th Sept., 519 mm) and high temperature (23°C), the surface root zone of the grass sward was assumed to be in a significantly reducing condition. From the change of NO3−N/Cl ratio in the soil profile and the plant uptake of nitrogen and chloride, considerable reduction of NO3−N and its loss from the root-mat zone was suggested.