Abstract
Nitrous oxide (N2O) is a greenhouse gas with a global warming potential of 296 CO2 equivalents and is involved in the depletion of the ozone layer. Through studies on emission sources it was revealed that natural and agricultural soils are important sources of N2O emissions and are responsible for about
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50 % of the total N2O emitted to the atmosphere. Nitrification and denitrification were shown to be processes responsible for N2O production in and emission from soils. Remarkably, N2O emission patterns from soils often show typical hotspot behaviour at different spatial scales. Hotspots are soil spots which show a disproportionately high emission relative to the surrounding area. Riparian buffer zones (as a whole) are often hotspot ecosystems with high N2O emission in the landscape. Within riparian buffer zones a hotspot pattern of N2O emission has been observed. These hotspots can be responsible for a large fraction of the N2O emission from the entire ecosystem or landscape, while only covering a small fraction of the area . Integration of hotspots in mechanistic models is a big challenge and impossible without knowing the underlying mechanisms and the influencing biotic and biotic characteristics. This study aimed at finding the underlying mechanisms and required conditions for the occurrence of N2O emission hotspot behavior within a riparian buffer zone. The study especially focussed on processes and conditions which are distinct between hotspots and non-hotspots. Therefore, nitrogen transformations were studied to identify the responsible N2O producing process(es). It was found that denitrification is the source of N2O production and emission in the riparian buffer zone. A decreased N2O reduction rather than an increased production was shown to be responsible for the enhanced emissions. The imbalance between production and consumption could not be explained by nitrate and oxygen levels. pH, however appeared to be a major controlling factor of net N2O production in laboratory incubations. Field measurements of pH and N2O revealed that also under heterogeneous soil conditions pH is a controlling factor for N2O emission variability and it was observed that hotspot N2O emission only appears from acidic soil spots. The source of the large variability in soil pH is yet unknown. The possibility of microbial soil denitrification at pH 4 was investigated using a bioreactor, in order to enrich a single microbial species. Rhodanobacter sp. was found to be able to denitrify at pH 4 using soil derived electron donors, implying that soil microbial denitrification at pH 4 is well possible. Discussed is the possibility to extrapolate the mechanism behind N2O emission hotspots to other ecosystems. It seems likely that in a system where denitrification thrives, variability in a single impairing factor (like low pH) may lead to N2O emission hotspots. This knowledge should be used in N2O emission models and riparian buffer strip creation.
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