Elberling, Bo

Jørgensen, Christian Juncher

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u n i ve r s i t y  o f  co pe n h ag e n  Københavns UniversitetFlooding-induced N2O emissions following extreme weather flooding of a non-managed wetland soilJørgensen, Christian Juncher; Elberling, BoPublication date:2012Document versionEarly version, also known as pre-printCitation for published version (APA):Jørgensen, C. J., & Elberling, B. (2012). Flooding-induced N2O emissions following extreme weather flooding ofa non-managed wetland soil.Download date: 02. Feb. 2020F A C U L T Y   O F   S C I E N C E U N I V E R S I T Y   O F   C O P E N H A G E N  Flooding-induced N2O emissions following extreme weather flooding of a non-managed wetland soil Introduction  Climate change is predicted to cause major changes in precipitation patterns with increased frequency and intensity of large rainfall events. In natural wetland ecosystems, changes in precipitation patterns will have a direct influence on the seasonal position of the free standing water level (WL) as well as on the relative frequency of complete ecosystem flooding.   Rapid shifts from naturally drained to fully flooded soil conditions in wetland soil alter the subsurface biogeochemistry by limiting subsurface oxygen (O2) availability and providing improved conditions for N2O production via denitrification and potential emission from the soil to the atmosphere. N2O emissions can occur in direct response to both a falling and rising WL. However, the contribution of either a falling or rising WL to net ecosystem emission of N2O on the field scale is unclear and the potential importance of flooding induced N2O emission pulses on the annual emission budget is currently unknown in natural wetlands.   How future changes in precipitation patterns and fluctuating positions of the free standing water level (WL) will modify the current N2O emission dynamics from wetlands is currently an open question. Without an appropriate spatiotemporal understanding of the current state of these dynamic processes, a resulting underestimation or misrepresentation of net N2O surface emission is likely to occur; especially if these potential N2O pulses are of significant proportions in relation to net annual surface emissions              Study site & methods  The Maglemosen experimental site is a non-managed minerotrophic wetland located approximately 20 km north of Copenhagen, Denmark (55°51´N, 12°32´E). At the selected study site, Phalaris arundinacea is the dominant vegetation cover.               Field investigations: Annual N2O fluxes were determined by near-continuous automated closed static chambers (n=5) with a 30 minute sampling frequency.   Subsurface N2O concentrations (5 cm resolution) were determined weekly by buried silicone probes. Subsurface O2 concentrations (5 cm res.) were measured every 10 min using buried O2 optodes.   Laboratory investigations: High resolution N2O and O2 concentration profiles (500µm increments) were measured using micro-electrodes                                  Christian Juncher Jørgensen & Bo Elberling  University of Copenhagen, Department of Geography & Geology Øster Voldgade 10, 1350 Copenhagen, +45 35322500, cjj@geo.ku.dk 5 cm10 cm        -90-60-30030Water level (cm)-500501001502002500255075100O2 (atm. sat. %)N2O flux (µg N2O-N m-2 hr-1) 13-08 14-08 15-08 16-08 17-08 18-082010cd60050040030020010000 5 10 15 20 30 4025 35 45Time after flooding (hr)N2O (nmol cm-3)5 mm12.5 mm22 mmStudy aims  (1) to relate temporal water level variations to belowground N2O concentrations and net annual N2O surface emission patterns  (2) evaluate the temporal effects of changes in subsurface N2O production and consumption on net N2O emissions  (3) to evaluate the temporal nature and total contribution of flooding induced N2O emissions to net annual N2O emissions from a non-managed Danish wetland.         0204060No data0321-1       -2Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct2009/2010129 mmPrecipitation (mm d-1) Daily flux (mg N2O-N m-2d-1 ) & WL (m)   Figure 2 (top left) – a short-lived flooding-induced N2O emission pulse (top graph, N2O: black dots; flux sum: 1.83 mg N2O-N m-2) was observed following extreme weather flooding, which resulted in rapid water level rise and O2 depletion in the top soil (lower graph).  Figure 3 (bottom left) – microsensor profiles of N2O (open dots) and O2 (solid dots) at time 15 and 27 hours after laboratory flooding together with modeled net N2O production and consumption profiles.  Figure 4 (bottom right) – three depth-specific N2O concentration profiles from the laboratory flooding experiment indicating the sequential progression from net N2O production to net N2O consumption         Abstract Flooding of wetland soil promotes subsurface N2O production in the soil and potential emission to the atmosphere in distinctive emission pulses. Rapid flooding of the wetland was observed twice during the growing season of 2010 in response to high precipitation events, but a flooding-induced N2O emission pulse was only observed when the soil conditions had been oxidized to soil depths below approximately 30 cm in more than 2-3 weeks before the flooding. The flooding events observed in this study are likely examples of the pattern of future climatic conditions, where prolonged drainage during dry summers are combined with late season precipitation events with increasing intensity and frequency. We conclude that unless the future balance between the inherent N2O consumption capacity in the peat soil and the rates of NO3- formation and net availability is markedly altered, a future increase in extreme weather flooding events is not expected to increase net N2O emissions significantly or play a major role in the annual N2O emission budget from this type of non-managed natural wetland.                Figure 1 –N2O fluxes, precipitation and position of the water level (WL) in the period October 2009 to October 2010. Daily average N2O fluxes (left y-axis; mg N2O-N m-2 d-1) are shown in the upper bar diagram (positive flux = emission and negative flux = uptake). The position of the WL (left y-axis; cm. below the surface) is shown by the solid line. Daily cumulative precipitation (right y-axis; mm d-1) is shown in the lower bar diagram (Note: Daily cumulative precipitation was of the scale with 129 mm on the 14th August 2010 and indicated by a dashed bar).        0 100 200 300200 400 600b (t27)00 100 200 300-20-15-10-5050 200 400 600a (t15)Depth (mm)O2 (µmol L-1)N2O (µmol L-1) N2O (µmol L-1)O2 (µmol L-1) Net N2O production (x10-3 nmol cm-3 s-1)(t15)(t27)0 10 20-10cConclusions:  Two flooding events occurred during the measurement period, but only one flooding-induced emission pulse  Flooding-induced N2O pulse when the top soil had been oxidized  for more than 2-3 weeks prior to flooding   N2O pulse accounted for ~ 2.5% of the net annual N2O emission budget of  ~ 0.74 kg N2O-N ha-1 yr-1  Main emission periods occurred during the growing season of Phalaris arundinacea and when the maximum subsurface N2O concentration where in the root zone of P. arundinacea   Microsensor N2O  concentration profiles show rapid response in N2O production/consumption following flooding  N2O production and consumption profiles show sequential N2O production and consumption capacities of more than 500 nmol cm-3 in less than 24 hours.  Under current climatic conditions and nitrification rates in the soil, flooding-induced N2O emission pulses at the studied field site constitute only a small percentage of the net annual N2O emission budget and is most likely to remain low unless the future balance between the inherent N2O consumption capacity in the peat soil and the rates of NO3- formation and net availability is markedly altered A future increase in extreme weather flooding events is not expected to increase net N2O emissions significantly or play a major role in the annual N2O emission budget from the studied non-managed natural wetland. Reference: Jørgensen C. J.,  Elberling, B., 2012. Effects of flooding-induced N2O production, consumption and emission dynamics on the annual N2O emission budget in wetland soil. Soil Biology and Biochemistry, 53, 9-17.  Picture 1 (top) – automated N2O flux chamber Picture 2 (bottom) - O2 and N2O microsensors  

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