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Estimates of regional and global freshwater N 2 O emissions have remained inaccurate due to scarce data and complexity of the multiple processes driving N 2 O fluxes the focus predominantly being on summer time measurements from emission hot spots, agricultural streams. Here, we present four-season data of N 2 O concentrations in the water columns of randomly selected boreal lakes covering a large variation in latitude, lake type, area, depth, water chemistry, and land use cover. Nitrate was the key driver for N 2 O dynamics, explaining as much as 78% of the variation of the seasonal mean N 2 O concentrations across all lakes. Nitrate concentrations varied among seasons being highest in winter and lowest in summer. Of the surface water samples, 71% were oversaturated with N 2 O relative to the atmosphere. Largest oversaturation was measured in winter and lowest in summer stressing the importance to include full year N 2 O measurements in annual emission estimates. Including winter data resulted in fourfold annual N 2 O emission estimates compared to summer only measurements. Nutrient-rich calcareous and large humic lakes had the highest annual N 2 O emissions. Our emission estimates for Finnish and boreal lakes are 0.6 and 29 Gg N 2 O-N/year, respectively. The global warming potential of N 2 O from lakes cannot be neglected in the boreal landscape, being 35% of that of diffusive CH 4 emission in Finnish lakes., (© 2019 The Authors. Global Change Biology published by John Wiley & Sons Ltd.)

We have examined how some major catchment disturbances may affect the aquatic greenhouse gas fluxes in the boreal zone, using gas flux data from studies made in 1994-1999 in the pelagic regions of seven lakes and two reservoirs in Finland. The highest pelagic seasonal average methane (CH(4)) emissions were up to 12 mmol x m(-2) x d(-1) from eutrophied lakes with agricultural catchments. Nutrient loading increases autochthonous primary production in lakes, promoting oxygen consumption and anaerobic decomposition in the sediments and this can lead to increased CH(4) release from lakes to the atmosphere. The carbon dioxide (CO(2)) fluxes were higher from reservoirs and lakes whose catchment areas were rich in peatlands or managed forests, and from eutrophied lakes in comparison to oligotrophic and mesotrophic sites. However, all these sites were net sources of CO(2) to the atmosphere. The pelagic CH(4) emissions were generally lower than those from the littoral zone. The fluxes of nitrous oxide (N(2)O) were negligible in the pelagic regions, apparently due to low nitrate inputs and/or low nitrification activity. However, the littoral zone, acting as a buffer for leached nitrogen, did release N(2)O. Anthropogenic disturbances of boreal lakes, such as increasing eutrophication, can change the aquatic greenhouse gas balance, but also the gas exchange in the littoral zone should be included in any assessment of the overall effect. It seems that autochthonous and allochthonous carbon sources, which contribute to the CH(4) and CO(2) production in lakes, also have importance in the greenhouse gas emissions from reservoirs.

Peatland buffer zones with sedimentation ponds are established with the intention of capturing solids and nutrients liberated in drained forestry catchments. As noted in earlier fertilization experiments, added nitrogen (N) immediately increases nitrous oxide (N2O) emissions in such buffers, and we expected the same to happen after disturbances in the catchment caused by clear-cutting, soil preparation, and ditch cleaning. We measured N2O fluxes, water table dynamics, and vegetation cover from a wetland one year before and two years after the clear-cut and buffer establishment. The low pre-harvest emissions did not increase, but N2O emissions from the sedimentation pond exceeded those from humic lakes with a high N load. In the soil profile, N2O concentrations were high, indicating a potential to produce N2O in the buffer. In one sub-site the soil N2O concentration was below the atmospheric level, which was in accordance with the high concentrations of carbon dioxide (CO2) and methane (CH4). The change in vegetation along the overland flow paths could be explained by a shift in the species thriving in wet conditions but not in those requiring higher nutrient levels. In spite of the apparent potential of soil to produce N2O, the fluxes to the atmosphere remained low. Transformation of N2O to unobserved N2 may explain some of the low N emissions, together with the low concentrations entering the buffer. [ABSTRACT FROM AUTHOR]

Forest clear-cutting followed by soil preparation means disturbance for soil microorganisms and disruption of N and C cycles. We measured fluxes of N2O and dissolved organic carbon (DOC) in upland soil (podzol) and adjacent peat within a clear-cut forest catchment. Both soil types behaved in a similar way, showing net uptake of N2O in the first year after the clear-cutting, and turning to net release in the second. The N2O flux dynamics were similar to those of N content in logging residues, as reported from a nearby site. As organic matter is used in the food web of the decomposers, we attempted to explain the dynamics of N2O uptake and release by measuring the concurrent dynamics of the low molecular weight (LMW) fraction and the aromaticity of DOC in a soil solution. The labile and most readily available LMW fractions of DOC were nearly absent in the year following the clear-cutting, but rose after two years. The more refractory high molecular weight (HMW) fraction of DOC decreased two years after the clear-cutting. The first year’s net uptake of N2O could be accounted for by the growth of decomposer biomass in the logging residues and detritus from the degenerating ground vegetation, resulting in immobilization of nitrogen. Simultaneously, the labile, LMW fraction of DOC became almost completely exhausted. The low availability of the LMW fraction could retard the growth and cause the accumulated decomposer biomass to collapse. During the following winter and summer the fraction of LMW clearly increased, followed by increased N2O emissions. The presence of LMW DOC fractions, not the concentration of DOC, seems to be an important controller for N2O liberation after a major disturbance such as clear-cutting and site preparation. The complex connection between DOC characteristics, nitrification or denitrification merits further studies. [ABSTRACT FROM AUTHOR]

SUMMARY 1. The effects of increasing CO2 and nitrogen loading and of a change in water table and temperature on littoral CH4 , N2 O and CO2 fluxes were studied in a glasshouse experiment with intact sediment cores including vegetation (mainly sedges), taken from a boreal eutrophic lake in Finland. Sediments with the water table held at a level of 0 or at -15 cm were incubated in an atmosphere of 360 or 720 p.p.m. CO2 for 18 weeks. The experiment included fertilisation with NO3 – and NH4 + (to a total 3 g N m-2 ). 2. Changes in the water table and temperature strongly regulated sediment CH4 and cCO2 fluxes (community CO2 release), but did not affect N2 O emissions. Increase in the water table increased CH4 emissions but reduced cCO2 release, while increase in temperature increased emissions of both CO2 and CH4 . 3. The raised CO2 increased carbon turnover in the sediments, such that cCO2 release was increased by 16–26%. However, CH4 fluxes were not significantly affected by raised CO2 , although CH4 production potential (at 22 °C) of the sediments incubated at high CO2 was increased. In the boreal region, littoral CH4 production is more likely to be limited by temperature than by the availability of carbon. Raised CO2 did not affect N2 O production by denitrification, indicating that this process was not carbon limited. 4. A low availability of NO3 – did severely limit N2 O production. The NO3 – addition caused up to a 100-fold increase in the fluxes of N2 O. The NH4 + addition did not increase N2 O fluxes, indicating low nitrification... [ABSTRACT FROM AUTHOR]

Concentrations of dissolved methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) were measured in the water columns of non‐oxygenated and artificially oxygenated, ice‐covered eutrophied lakes in the mid‐boreal zone in Finland during late winter 1997 and 1999. Sampling was conducted during winter stratification, the critical period for oxygen (O2) deficiency in seasonally ice‐covered, thermally stratified lakes. Oxygen concentrations were maintained at least at a moderate level throughout the oxygenated water columns, whereas the non‐oxygenated columns suffered anoxic hypolimnia. The mean concentrations of dissolved CH4 exceeding the atmospheric equilibrium were greater in the non‐oxygenated water columns (20.6–154 μM) than in the oxygenated ones (0.01–1.41 μM). In contrast, the mean excess CO2 concentrations varied less between the non‐oxygenated and oxygenated sites (0.28–0.47 and 0.25–0.31 mM, respectively). Oxygenated water columns had greater mean excess concentrations of N2O (0.018–0.032 μM) than the non‐oxygenated ones (0.005–0.024 μM). If the accumulated greenhouse gas stores in the water columns during winter are assumed to be released to the atmosphere during the spring overturn, the global warming potentials (GWP, time horizon 100 yr) of these potential emissions at the non‐oxygenated, eutrophic study sites ranged from 177 to 654 g CO2 equivalent (CO2–e) m−2 compared with 144 to 173 g CO2–e m−2 at the oxygenated sites. The increase in the accumulation of CH4 was the main reason for the higher GWP of the non‐oxygenated sites. Anthropogenic eutrophication of lake ecosystems can generate increased CH4 emissions due to associated O2 depletion of their sediment and water column. [ABSTRACT FROM AUTHOR]