Dual-isotope analysis of rapid decline of nitrate concentration in subsurface drainage from clayey ex-paddy fields during rainfall events

Paddy fields are important for producing rice and other upland crops in Japan. Clayey ex-paddy fields have specific nitrate (NO3−) dynamics characterized by the low hydraulic conductivity of the soil matrix and by macropores. A dual-isotope approach, which measures changes in stable isotope ratios of the nitrogen and oxygen in NO3− by isotopic fractionation (15N and 18O), is a powerful tool for exploring denitrification and its role in the N cycle. We investigated NO3− discharge in clayey ex-paddy fields planted with soybeans during rainfall events by this technique. Water and NO3− flowed rapidly through macropores, bypassing the soil matrix. The NO3− concentrations rapidly decreased during each event. The stable isotope ratios of N and O in NO3− ranged from −1.47‰ to +29.6‰ δ15N and from −8.17‰ to +10.2‰ δ18O. The ratio of enrichment factors for 15N (15ε) and 18O (18ε), 18ε/15ε, was 0.475, with a 95% confidence interval of 0.412–0.537 (n = 119). We conclude that denitrification is the major process causing the rapid decline of NO3− in subsurface drainage during each rainfall event in clayey ex-paddy fields.