Application of membrane inlet mass spectrometry to directly quantify denitrification in flooded rice paddy soil

Abstract

Denitrification has long been considered a major mechanism of N loss when N fertilizer is applied to flooded rice paddies. However, the direct determination of denitrification in soils is almost impossible because of the high atmospheric background of dinitrogen (N2). Dissolved N2 in a small water sample can be rapidly and precisely measured through membrane inlet mass spectrometry (MIMS). This study is the first to directly measure N2 flux through MIMS in flooded rice paddy plots that received different amounts of urea. Ammonia (NH3) volatilization was measured simultaneously to verify whether NH3 volatilization and denitrification are complementary loss mechanisms. The average cumulative N2–N loss measured by MIMS 21 days after fertilization was 4.7 ± 1.7 % of the applied N, which was within the range of the reported values obtained by cumulative recovery of (N2 + N2O)–15N and 15N-balance technique. Underestimation or overestimation of denitrification can be prevented in MIMS given that N2 can be measured directly without 15N-labeled fertilizer. A good positive correlation was found between the dissolved in situ N2 concentrations of floodwater and the denitrification rates of intact soil cores. Urea incorporation reduced NH3 volatilization unlike surface broadcasting. However, urea incorporation significantly increased cumulative N2–N loss during the 21 days after fertilization. Correlation analysis showed that nitrate (NO3 −–N) concentration in floodwater could be the primary restricting factor for soil denitrification in the experimental field. Results suggest that MIMS is a promising technique for the measurement of denitrification in a flooded rice paddy.