Abstract
Nitrous oxide (N2O) emission-suppressing activity of some electron-transport inhibitors of the Hill reaction system was investigated. The Hill reaction inhibitors—paraquat, isouron, bromacil, diquat, and simazine—all of which have been or are currently being used as herbicides in farming activity are expected to inhibit the electron-transporting pathways of nitrate respiration in denitrifying bacteria. Using N2O-emitting soil bed (5.0 g of fresh weight) from a continuously manured Andisol corn farmland in Hokkaido, Japan, which was autoclaved and further supplemented with an active N2O-emitter, Pseudomonas sp. 5CFM15-6D, and 1 mL of 100 mM NH4NO3 or (NH4)2SO4 solution as the sole nitrogen source (final concentration, 0.2 mM) in a 30 mL gas-chromatography vial, the effects of the five herbicides on N2O emission were examined. Paraquat and isouron (each at 50 µM) showed a statistically significant suppression of N2O emission in both the nitrification and the denitrification processes after a 7-day-incubation, whereas diquat at the same concentration accelerated N2O emission in the presence of NO3−. These results suggest that paraquat and isouron inhibited both the nitrification and the denitrification processes for N2O generation, or its upstream stages, whereas diquat specifically inhibited N2O reductase, an enzyme that catalyzes the reduction of N2O to N2 gas. Incomplete denitrifiers are the key players in the potent emission of N2O from Andisol corn farmland soil because of the missing nosZ gene. The electron relay system-inhibiting herbicides—paraquat and isouron—possibly contribute to the prevention of denitrification-induced nitrogen loss from the farming soil.
Highlights
Nitrogen, an essential element required for plants and other living creatures, is mainly provided in the form of amino acids, ammonium (NH4 +, stable in soil), or nitrate (NO3 −, lost from soil through leaching and denitrification) [1,2]
As the Winogradsky’s mineral solution-based gellan gum bed, supplementation of nitrogen substrates, with 2 mM NO3 −, and carbon source such as 0.2% sucrose resulted in the active acceleration of N2 O emission from the soil bed in the gas-chromatography vial (Figure 2) [28]
0.01–1.0% sucrose content was tested in parallel, N2 O emission from the soil bed culture was unstable, including control (0% sucrose), showing that the soil is far from a homogeneous material
Summary
An essential element required for plants and other living creatures, is mainly provided in the form of amino acids (as degraded protein in soil organic matters), ammonium (NH4 + , stable in soil), or nitrate (NO3 − , lost from soil through leaching and denitrification) [1,2]. Brachiaria humidicola and Sorghum bicolor of the family Poaceae ( known as Gramineae), release diterpene (brachialactone) and paraquinone derivatives (soligoquinones), respectively, from the plant roots as a BNI mechanism [5,6]. This mechanism involves the suppression of nitrogen loss from soil by the selective inhibition of ammonia oxidase in nitrifiers [4,6].
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