Greenhouse gas production in nitrogen removal wetlands

Abstract: Nitrogen removal wetlands are constructed in agricultural areas as a measure to reduce the amount of non-point source nitrogen that reaches coastal areas. Methane and nitrous oxide, two greenhouse gases, are produced under anaerobic conditions and an increased amount of waterlogged areas will therefore increase the potential emission of these gases. The question has arisen: Are we solving one environmental problem with measures that render another? In a study in pristine New Zealand wetlands, I found that plant phosphorus and nitrate in soil best describe the variation in anaerobic soil respiration to carbon dioxide and methane. Plant phosphorus, indicating wetland productivity, was positively correlated with methane production, whereas nitrate in soil was negatively correlated with methane production. Swedish constructed nitrogen removal wetlands are considered to be highly productive and they have high concentrations of nitrate in the water. Will the Swedish nitrogen removal wetlands produce a lot of methane or will the production be hampered? In a field study I found that temperature explains most of the seasonal variation in methane emission. When the temperature effect was taken into account I found a negative effect of nitrate on methane emission. In laboratory experiments I also found that temperature was important for methane production, and that this was due to low substrate availability at lower temperatures, i.e. temperature had an indirect effect on methane producers. At higher nitrate additions more nitrous oxide was produced and methane was to some extent restricted by high nitrate concentrations. Low carbon availability in a meadow soil compared to in sediment from a recently restored wetland resulted in lower nitrous oxide production from meadow soil than from wetland sediment. A faster conversion of nitrous oxide to nitrogen gas also suggests that nitrogen removal is more efficient in a permanently inundated wetland than in a meadow soil that is only occasionally wet. In another study I compared the benefit (nitrogen removal) with the risk (methane emission) in 36 nitrogen removal wetlands in southwestern Sweden. There was no positive relationship between nitrogen removal and methane emission, which means that a wetland can be optimized for nitrogen removal without an increased risk of methane emission. Nitrate concentration in the water was positively correlated to nitrogen removal and negatively correlated to methane concentration in the water (at constant temperature). The results therefore suggest that nitrogen removal wetlands should be located in highly nitrate-loaded areas. Overall I have shown that nitrate concentrations occurring in the constructed nitrogen removal ponds, can have a negative impact on methane development.