Sadao Eguchi

Denitrification during vertical upwelling at an alluvium-diluvium interface below the upland perimeter of a riparian paddy.

Denitrification hotspots in riparian aquifers often develop in a relatively narrow zone at the upland-riparian interface, where nitrate-rich ground water of upland origin interacts with available soil organic carbon. In riparian paddy fields, denitrification in the aquifer has received less attention than that in the surface water and soil. This study aimed to determine the in situ activity of the denitrification hotspot formed at the vertical interface between the organic alluvial and the nitrate-rich diluvial aquifers around the depth of 2.0 m below the upland perimeter of riparian paddy, where vertical upwelling dominates the ground water recharge. The mass balances of water and solutes were approximately calculated from the one-dimensional vertical pressure head and water quality profiles with help of the stable isotopes analyses of water. The confined ground water of adjacent diluvial upland origin, with a high nitrate concentration of 1.72+/-0.42 mmol L(-1), mixed with the nitrate-deficient unconfined ground water at the alluvium-diluvium interface, and 63% of nitrate was removed by denitrification at a rate of 33 mg N m(-2) d(-1) and a nitrogen isotope fractionation factor of 0.988. The increase in bicarbonate concentration with the decrease in nitrate concentration suggested a heterotrophic denitrification with a stoichiometry of C:N=5:4. These results are the first to demonstrate the quantitative importance of denitrification in the aquifer below a riparian paddy in the removal of nitrate from the ground water of upland origin and emphasize the necessity of including this process in models for predicting watershed-scale surface water and ground water qualities.

Nitrogen and oxygen isotope enrichment factors of nitrate at different denitrification rates in an agricultural soil

ABSTRACT Quantitative evaluation of denitrification by the dual isotope approach, which measures the stable isotope ratios of nitrogen (δ15N) and oxygen (δ18O) in nitrate, has been hampered by the wide range of values reported for the ratio of enrichment factors for 15N and 18O (15ε and 18ε, respectively) during denitrification. The objectives of this study were to determine 15ε and 18ε values at different denitrification rates under controlled conditions, and to infer possible mechanisms by which the 18ε/15ε ratio is influenced under different conditions. Column experiments were conducted at 25, 15, and 10°C, which enabled determination of 15ε and 18ε at different denitrification rates, in the absence of nitrate replenishment from ammonium oxidation and other sources. The values of 15ε and 18ε ranged from −11.8 to −14.9‰ and from −8.4 to −15.9‰, respectively, with 15ε less sensitive to changes in the denitrification rates. The resultant 18ε/15ε ratio, ranging from 0.70 to 1.17, was close to the values reported for sediment incubations, and larger than those for groundwater systems. These results are consistent with the explanations that 18ε/15ε value itself is close to unity during denitrification, and that at smaller denitrification rates, concurrent reactions including re-oxidation of nitrite to nitrate lead to smaller apparent fractionation of 18O and smaller 18ε/15ε ratios. This suggests that while linear relationships between δ18O and δ15N give a strong evidence of denitrification, apparent 18ε/15ε values are site specific and depend on the ambient conditions. In evaluating denitrification in such systems, we suggest the use of 15ε in preference to 18ε because 15ε is less sensitive to denitrification rates.

Procedure for rapid determination of δ15N and δ18O values of nitrate: development and application to an irrigated rice paddy watershed

The dual isotope approach using the stable isotope ratios of nitrate nitrogen (δ(15)N(NO3)) and oxygen (δ(18)O(NO3)) is a strong tool for identifying the history of nitrate in various environments. Basically, a rapid procedure for determining δ(15)N(NO3) and δ(18)O(NO3) values is required to analyze many more samples quickly and thus save on the operational costs of isotope-ratio mass spectrometry (IRMS). We developed a new rapid procedure to save time by pre-treating consecutive samples of nitrous oxide microbially converted from nitrate before IRMS determination. By controlling two six-port valves of the pre-treatment system separately, IRMS determination of the current sample and backflush during the next sample pre-treatment period could be conducted simultaneously. A set of 89 samples was analyzed precisely during a 25-h continuous run (17 min per sample), giving the fastest reported processing time, and simultaneously reducing liquid nitrogen and carrier helium gas consumption by 35%. Application of the procedure to an irrigated rice paddy watershed suggested that nitrate concentrations in river waters decreased in a downstream direction, mainly because of the mixing of nitrate from different sources, without distinct evidence of denitrification. Our procedure should help with more detailed studies of nitrate formation processes in watersheds.