Low water level drives high nitrous oxide emissions from treatment wetland.

Wetlands that are restored for carbon sequestration or created for water treatment are an important sources of greenhouse gases, especially methane. The emission of nitrous oxide (N2O) from these systems is often considered negligible due to the inundation and anerobic conditions that support complete denitrification. We used closed chamber method to analyze N2O fluxes over a long-term period across heterogeneous wetland ecosystem constructed for treating nitrate-rich agricultural runoff. Our results showed that the water depth and temperature were most important factors affecting high N2O emissions. The shallow areas where water depth was less than 9 cm created N2O hot spots that emitted 48.8% of the total wetlands annual emission while only covering 6% of the total area. The annual emission from shallow-water hot spots with dense helophytic vegetation was 4.85 ± 0.5 g N2O-N m-2 y-1 while it was only 0.37 ± 0.01 g N2O-N m-2 y-1 in deeper zones. While the water depth was the main factor for high N2O emissions, the temperatures increased the magnitude of the flux and therefore summer droughts and water drawdown created even larger hot spots. These results also suggest that IPCC benchmarks could underestimate N2O emission from shallow waterbodies. Thus, it is important that the shallow zones and water level drawdown in the created or restored wetlands is avoided to minimize the N2O flux.

Environmental factors affecting greenhouse gas fluxes of green roofs in temperate zone.

This paper investigates the full seasonal greenhouse gas (GHG) dynamics of fluxes from three green roof systems (lightweight clay aggregate-based green roof - LR; grass roof - GR; sod roof - SR) and natural control site on shallow Leptosol (NC), using closed static chambers in the period April 2014 to December 2015. CO2, CH4 and N2O fluxes are measured and their relationships to meteorological parameters and substrate physicochemical characteristics are quantified. Median CO2 flux values were 21 (LR), 38 (GR), 62 (SR), and 82 (NC) mg CO2-C m-2 h-1. The results show ecosystem respiration (Reco) clearly increased until July and then decreased until November. Net ecosystem CO2 exchange (NEE) was more variable than Reco, depending on plant growth phase and weather conditions. Median NEE values for study period (from April to November 2015) were -7 (LR), -17 (GR), -136 (SR), and -82 (NC) mg CO2-C m-2 h-1. The percentage of autotrophic respiration (Ra) in Reco showed clear rise from LR (35%) to NC (62%). CH4 consumption dominated resulting in median fluxes as follows: -2 (LR), -1 (GR), -15 (SR), and -23 (NC) µg CH4-C m-2 h-1. N2O flux was low and highly variable in time, with median values varying from -0.07 (GR) to 2.18 (NC) µg N2O-N m-2 h-1. During the maximum vegetation growth, NEE exceeded Reco value. Green roofs are effective CH4 sinks, but they do not significantly affect N2O flux. The main environmental factors determining GHG fluxes in linear models were parameters describing moisture regime, meteorological parameters and soil physical characteristics. These models can be used to predict GHG fluxes from similar green roof systems in analogous climatic conditions. We conclude that green roof technology may be used to mitigate excessive ambient GHG levels in urban areas.

Author Correction: Nitrogen-rich organic soils under warm well-drained conditions are global nitrous oxide emission hotspots.

The original version of this Article contained an error in the first sentence of the Acknowledgements section, which incorrectly referred to the Estonian Research Council grant identifier as "PUTJD618". The correct version replaces the grant identifier with "PUTJD619". This has been corrected in both the PDF and HTML versions of the Article.

Nitrogen-rich organic soils under warm well-drained conditions are global nitrous oxide emission hotspots.

Nitrous oxide (N2O) is a powerful greenhouse gas and the main driver of stratospheric ozone depletion. Since soils are the largest source of N2O, predicting soil response to changes in climate or land use is central to understanding and managing N2O. Here we find that N2O flux can be predicted by models incorporating soil nitrate concentration (NO3-), water content and temperature using a global field survey of N2O emissions and potential driving factors across a wide range of organic soils. N2O emissions increase with NO3- and follow a bell-shaped distribution with water content. Combining the two functions explains 72% of N2O emission from all organic soils. Above 5 mg NO3--N kg-1, either draining wet soils or irrigating well-drained soils increases N2O emission by orders of magnitude. As soil temperature together with NO3- explains 69% of N2O emission, tropical wetlands should be a priority for N2O management.

The impact of a pulsing groundwater table on greenhouse gas emissions in riparian grey alder stands.

Floods control greenhouse gas (GHG) emissions in floodplains; however, there is a lack of data on the impact of short-term events on emissions. We studied the short-term effect of changing groundwater (GW) depth on the emission of (GHG) carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in two riparian grey alder (Alnus incana) stands of different age in Kambja, southern Estonia, using the opaque static chamber (five replicates in each site) and gas chromatography methods. The average carbon and total nitrogen content in the soil of the old alder (OA) stand was significantly higher than in the young alder (YA) stand. In both stands, one part was chosen for water table manipulation (Manip) and another remained unchanged with a stable and deeper GW table. Groundwater table manipulation (flooding) significantly increases CH4 emission (average: YA-Dry 468, YA-Manip 8,374, OA-Dry 468, OA-Manip 4,187 µg C m(-2) h(-1)) and decreases both CO2 (average: OA-Dry 138, OA-Manip 80 mg C m(-2) h(-1)) and N2O emissions (average: OA-Dry 23.1, OA-Manip 11.8 µg N m(-2) h(-1)) in OA sites. There was no significant difference in CO2 and CH4 emissions between the OA and YA sites, whereas in OA sites with higher N concentration in the soil, the N2O emission was significantly higher than at the YA sites. The relative CO2 and CH4 emissions (the soil C stock-related share of gaseous losses) were higher in manipulated plots showing the highest values in the YA-Manip plot (0.03 and 0.0030 % C day(-1), respectively). The soil N stock-related N2O emission was very low achieving 0.000019 % N day(-1) in the OA-Dry plot. Methane emission shows a negative correlation with GW, whereas the 20 cm depth is a significant limit below which most of the produced CH4 is oxidized. In terms of CO2 and N2O, the deeper GW table significantly increases emission. In riparian zones of headwater streams, the short-term floods (e.g. those driven by extreme climate events) may significantly enhance methane emission whereas the long-term lowering of the groundwater table is a more important initiator of N2O fluxes from riparian gley soils than flood pulses.