Abstract
The microbial reduction of nitrate, via nitrite into gaseous di-nitrogen (denitrification) plays a major role in nitrogen removal from aquatic ecosystems. Natural abundance stable isotope measurements can reveal insights into the dynamics of production and consumption of nitrite during denitrification. In this study, batch experiments with environmental bacterial communities were used to investigate variations of concentrations and isotope compositions of both nitrite and nitrate under anoxic conditions. To this end, denitrification experiments were carried out with nitrite or nitrate as sole electron acceptors at two substrate levels respectively. For experiments with nitrate as substrate, where the intermediate compound nitrite is both substrate and product of denitrification, calculations of the extent of isotope fractionation were conducted using a non-steady state model capable of tracing chemical and isotope kinetics during denitrification. This study showed that nitrogen isotope fractionation was lower during the use of nitrite as substrate (ε = −4.2 and −4.5‰ for both treatments) as compared to experiments where nitrite was produced as an intermediate during nitrate reduction (ε = −10 and −15‰ for both treatments). This discrepancy might be due to isotopic fractionation within the membrane of denitrifiers. Moreover, our results confirmed previously observed rapid biotic oxygen isotope exchange between nitrite and water.
Highlights
The growing world population requires an increased food production, necessitating the intensive use of nitrogen-containing synthetic fertilizers, which has led to nitrate pollution in many surface water and groundwater bodies[1,2,3,4,5]
Nitrite has been detected at concentrations exceeding the European Water Framework Directive (EU WFD) of 0.009 mg N L−1 in urbanized rivers[19,20,21,22,23]
The nitrogen isotope fractionation factors calculated in the current study for denitrification of nitrate of −21.1 and −24.4‰ are in good agreement with those found in the literature under similar conditions where denitrification occurs[58]
Summary
The growing world population requires an increased food production, necessitating the intensive use of nitrogen-containing synthetic fertilizers, which has led to nitrate pollution in many surface water and groundwater bodies[1,2,3,4,5]. The nitrogen and oxygen isotope ratios of NO3− (δ15N and δ18O) have been used since several decades to better distinguish the origin of NO3− 33–36 It provides information regarding the processes involved in its transformation, e.g. denitrification and nitrification in lakes, groundwater, riparian zones, rivers, soils, and marine environments[37,38,39,40,41,42,43]. Since during denitrification and nitrate consumption nitrite is both substrate and product, it is not possible to use the Rayleigh equation to calculate concentrations and isotope compositions of the intermediate compound nitrite For this reason, we developed the Isonitrite numerical model that simulates the evolution of oxygen and nitrogen isotope compositions of NO3− and NO2− during simultaneous nitrite production and consumption. This study is the first focusing on freshwater benthic denitrifying communities with two distinct sources of dissolved inorganic nitrogen, i.e, nitrite or nitrate
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