Forest streams are important sources for nitrous oxide emissions - Nitrous oxide emissions from Swedish streams

Streams and river networks are increasingly recognized as significant sources for the greenhouse gas nitrous oxide (N2O). N2O is a transformation product of nitrogenous compounds in soil, sediment and water. Agricultural areas are considered a particular hotspot for emissions because of the large input of nitrogen (N) fertilizers applied on arable land. However, there is little information on N2O emissions from forest streams although they constitute a major part of the total stream network globally. Here, we compiled N2O concentration data from low-order streams (~1,000 observations from 172 stream sites) covering a large geographical gradient in Sweden from the temperate to the boreal zone and representing catchments with various degrees of agriculture and forest coverage. Our results showed that agricultural and forest streams had comparable N2O concentrations of 1.6 +/- 2.1 and 1.3 +/- 1.8 mu g N/L, respectively (mean +/- SD) despite higher total N (TN) concentrations in agricultural streams (1,520 +/- 1,640 vs. 780 +/- 600 mu g N/L). Although clear patterns linking N2O concentrations and environmental variables were difficult to discern, the percent saturation of N2O in the streams was positively correlated with stream concentration of TN and negatively correlated with pH. We speculate that the apparent contradiction between lower TN concentration but similar N2O concentrations in forest streams than in agricultural streams is due to the low pH (<6) in forest soils and streams which affects denitrification and yields higher N2O emissions. An estimate of the N2O emission from low-order streams at the national scale revealed that ~1.8 x 10(9) g N2O-N are emitted annually in Sweden, with forest streams contributing about 80% of the total stream emission. Hence, our results provide evidence that forest streams can act as substantial N2O sources in the landscape with 800 x 10(9) g CO2-eq emitted annually in Sweden, equivalent to 25% of the total N2O emissions from the Swedish agricultural sector

Metan- och koldioxidflöden från sötvattensmiljöer : Variation i tid och rum samt en integrerad bedömning av emissioner från sjöar och vattendrag i ett avrinningsområde

Freshwater bodies such as lakes and streams release the greenhouse gases methane (CH4) and carbon dioxide (CO2) into the atmosphere. Global freshwater CH4 and CO2 emissions have been estimated to be of a similar magnitude to the global land or ocean carbon sink, and are thus significant components of global carbon budgets. However, the data supporting global estimates frequently lacks information regarding spatial and temporal variability and are thus highly inaccurate. In this thesis, detailed studies of the spatio-temporal variability of CH4 and CO2 fluxes were conducted in the open water areas of lakes and streams within a whole catchment in Sweden. One aim was also to evaluate the importance of spatio-temporal variability in lake and stream fluxes when making whole catchment aquatic or large scale assessments. Apart from the expected large spatio-temporal variability in lake fluxes, interactions between spatial and temporal variability in CH4 fluxes were found. Shallow lakes and shallow areas of lakes were observed to emit more CH4 as compared to their deeper counterparts. This spatial variability interacted with the temporal variability driven by an exponential temperature response of the fluxes, which meant that shallow waters were more sensitive to warming than deeper ones. Such interactions may be important for climate feedbacks. Surface water CO2 in lakes showed significant spatio-temporal variability and, when considering variability in both space and time, CO2 fluxes were largely controlled by concentrations, rather than gas transfer velocities. Stream fluxes were also highly variable in space and time and in particular, stream CH4 fluxes were surprisingly large and more variable than CO2 fluxes. Fluxes were large from stream areas with steep slopes and periods of high discharge which occupied a small fraction of the total stream area and the total measurement period, respectively, and a failure to account for these spatially distinct or episodic high fluxes could lead to underestimates. The total aquatic fluxes from the whole catchment were estimated by combining the measurements in open waters of lakes and streams. Using our data, recommendations on improved study designs for representative measurements in lakes and streams were provided for future studies. Thus, this thesis presents findings relating to flux regulation in lakes and streams, and urges forthcoming studies to better consider spatio-temporal variability so as to achieve unbiased large-scale estimates.Sötvatten som sjöar och vattendrag är källor till växthusgaserna metan (CH4) och koldioxid (CO2) i atmosfären. De globala utsläppen av CH4 och CO2 från sötvatten har uppskattats vara av samma storleksordning som den globala land- eller havskolsänkan och är därmed viktiga delar av jordens växthusgasbudget. De globala uppskattningarna saknar ofta information om variation i tid och rum och är därmed mycket osäkra. Denna avhandling behandlar hur CH4- och CO2-flöden från öppet vatten i sjöar och vattendrag i ett avrinningsområde varierar rumsligt och tidsmässigt. Ett syfte var också att utvärdera betydelsen av dessa variationer när data extrapoleras för att göra storskaliga uppskattningar av växthusgasflöden från vattenmiljöer. Förutom de förväntade stora rumsliga och tidsmässiga variationerna i sjöars gasflöden identifierades interaktioner mellan rumsliga och tidsmässiga variation för CH4-flöden. Den rumsliga variabiliteten med högre CH4-flöden från grunda vatten interagerade med tidsvariationen, som i sin tur drevs av en exponentiell temperaturrespons av gasflödena. Det betyder att grunda vattenområden var mer känsliga för uppvärmning än djupare vatten och därmed att vattendjupet har betydelse för hur sjöars CH4-utsläpp påverkas av klimatet. Koncentrationer av CO2 i sjöars ytvatten uppvisade också en betydande rumslig och tidsmässig variation som tillsammans visar att CO2-flöden över längre perioder till stor del styrs av koncentrationer snarare än av gasutbyteshastigheter. Vattendragens gasflöden varierade också mycket i tid och rum. Detta gällde i synnerhet CH4-flödena vilka var förvånansvärt stora och mer varierande än CO2-flödena. Gasflödena var höga från områden i vattendrag med högre lutning och då det var höga vattenflöden, trots att dessa områden och tidsperioder utgjorde en bråkdel av den totala arean och mätperioden. Att inte räkna med dessa gasflöden från bäcksektioner med höga vattenhastigheter eller korta perioder med höga flöden, leder till underskattningar. De totala CH4- och CO2-flödena från öppet vatten i hela avrinningsområdet uppskattades genom att kombinera mätningar i sjöar och vattendrag. Denna avhandling visar att rumslig och tidsmässig variabilitet har stor betydelse, och den ger information om hur denna variation kan beaktas för bättre framtida mätningar och storskaliga uppskattningar av växthusgasflöden från sjöar och vattendrag

Influence of weather variables on methane and carbon dioxide flux from a shallow pond

Freshwaters are important sources of the greenhouse gases methane (CH4) and carbon dioxide (CO2) to the atmosphere. Knowledge about temporal variability in these fluxes is very limited, yet critical for proper study design and evaluating flux data. Further, to understand the reasons for the variability and allow predictive modeling, the temporal variability has to be related to relevant environmental variables. Here we analyzed the effect of weather variables on CH4 and CO2 flux from a small shallow pond during a period of 4 months. Mean CH4 flux and surface water CH4 concentration were 8.0 [3.3-15.1] +/- A 3.1 mmol m(-2) day(-1) (mean [range] +/- A 1 SD) and 1.3 [0.3-3.5] +/- A 0.9 A mu M respectively. Mean CO2 flux was 1.1 [-9.8 to 16.0] +/- A 6.9 mmol m(-2) day(-1). Substantial diel changes in CO2 flux and surface water CH4 concentration were observed during detailed measurements over a 24 h cycle. Thus diel patterns need to be accounted for in future measurements. Significant positive correlations of CH4 emissions with temperature were found and could include both direct temperature effects as well as indirect effects (e.g. related to the growth season and macrophyte primary productivity providing organic substrates). CO2 flux on the other hand was negatively correlated to temperature and solar radiation, presumably because CO2 consumption by plants was higher relative to CO2 production by respiration during warm sunny days. Interestingly, CH4 fluxes were comparable to ponds with similar morphometry and macrophyte abundance in the tropics. We therefore hypothesize that CH4 and CO2 summer emissions from ponds could be more related to the morphometry and dominating primary producers rather than latitude per se. Data indicate that CH4 emissions, given the system characteristic frameworks, is positively affected by increased temperatures or prolonged growth seasons

Photochemical mineralisation in a boreal brown water lake : considerable temporal variability and minor contribution to carbon dioxide production

Sunlight induces photochemical mineralisation of chromophoric dissolved organic matter (CDOM) to dissolved inorganic carbon (DIC) in inland waters, resulting in carbon dioxide (CO2) emissions to the atmosphere. Photochemical rate modelling is used to determine sunlight-induced CO2 emissions on large spatial and temporal scales. A sensitive model parameter is the wavelength-specific photochemical CDOM reactivity, the apparent quantum yield (AQY). The modelling studies so far assume that AQY spectra determined for single lakes and on single occasions represent larger spatial and temporal scales. Here, we studied a humic boreal lake in Sweden. We measured AQY spectra for photochemical DIC production monthly between June and November 2014 and parameterised a photochemical rate model. Photochemical reactivity increased slightly during the open water period, likely due to a high rainfall event with consecutive mixing in autumn that increased availability of highly photoreactive CDOM. However, the variability in AQY spectra over time was much smaller than previously reported variability in AQY spectra across lakes. Yet, using either the AQY spectrum from the least or from the most photoreactive water sample resulted in a 5-fold difference in simulated annual DIC photoproduction (2.0 ± 0.1 and 10.3 ± 0.7 g C m−2 yr−1, respectively). Using the monthly measured AQY spectrum to simulate DIC photoproduction for month-long time periods resulted in an apparent time lag between irradiance and DIC photoproduction. This suggested that temporal variability in AQY spectra occurs on shorter time scales. Therefore, we parameterised the model with the pooled AQY spectrum of six monthly measurements. Simulated DIC photoproduction for three years (2012–2014) averaged 4.5 ± 0.2 g C m−2 yr−1, which represented 3 % of the mean CO2 emissions from this lake. We conclude that (1) it may be recommendable to conduct repeated AQY measurements across the season for more accurate simulation of annual photochemical DIC production in lakes and (2), in agreement with previous studies, direct CDOM photomineralisation makes only a minor contribution to mean CO2 emissions from Swedish humic lakes

Technical Note: Cost-efficient approaches to measure carbon dioxide (CO2) fluxes and concentrations in terrestrial and aquatic environments using mini loggers

Fluxes of CO2 are important for our understanding of the global carbon cycle and greenhouse gas balances. Several significant CO2 fluxes in nature may still be unknown as illustrated by recent findings of high CO2 emissions from aquatic environments, previously not recognized in global carbon balances. Therefore, it is important to develop convenient and affordable ways to measure CO2 in many types of environments. At present, direct measurements of CO2 fluxes from soil or water, or CO2 concentrations in surface water, are typically labor intensive or require costly equipment. We here present an approach with measurement units based on small inexpensive CO2 loggers, originally made for indoor air quality monitoring, that were tested and adapted for field use. Measurements of soil-atmosphere and lake-atmosphere fluxes, as well as of spatiotemporal dynamics of water CO2 concentrations (expressed as the equivalent partial pressure, pCO(2aq)) in lakes and a stream network are provided as examples. Results from all these examples indicate that this approach can provide a cost- and labor-efficient alternative for direct measurements and monitoring of CO2 flux and pCO(2aq) in terrestrial and aquatic environments.Funding Agencies|Linkoping University; Swedish Research Council VR</p

Spatio-temporal patterns of stream methane and carbon dioxide emissions in a hemiboreal catchment in Southwest Swedend

Global stream and river greenhouse gas emissions seem to be as large as the oceanic C uptake. However, stream and river emissions are uncertain until both spatial and temporal variability have been quantified. Here we investigated in detail the stream CH4 and CO2 emissions within a hemiboreal catchment in Southwest Sweden primarily covered by coniferous forest. Gas transfer velocities (k(600)), CH4 and CO2 concentrations were measured with multiple methods. Our data supported modelling approaches accounting for various stream slopes, water velocities and discharge. The results revealed large but partially predictable spatio-temporal variabilities in k(600), dissolved gas concentrations, and emissions. The variability in CO2 emission was best explained by the variability in k, while dissolved CH4 concentrations explained most of the variability in CH4 emission, having implications for future measurements. There were disproportionately large emissions from high slope stream reaches including waterfalls, and from high discharge events. In the catchment, stream reaches with low slope and time periods of moderate discharge dominated (90% of area and 69% of time). Measurements in these stream areas and time periods only accounted for amp;lt;36% of the total estimated emissions. Hence, not accounting for local or episodic high emissions can lead to substantially underestimated emissions.Funding Agencies|Swedish Research Councils FORMAS [2009-872]; VR [2012-48]; Swedish Nuclear Fuel and Waste Management Company (Svensk Karnbranslehantering AB) [15443]</p

Technical Note: Cost-efficient approaches to measure carbon dioxide (CO2) fluxes and concentrations in terrestrial and aquatic environments using mini loggers

Fluxes of CO2 are important for our understanding of the global carbon cycle and greenhouse gas balances. Several significant CO2 fluxes in nature may still be unknown as illustrated by recent findings of high CO2 emissions from aquatic environments, previously not recognized in global carbon balances. Therefore, it is important to develop convenient and affordable ways to measure CO2 in many types of environments. At present, direct measurements of CO2 fluxes from soil or water, or CO2 concentrations in surface water, are typically labor intensive or require costly equipment. We here present an approach with measurement units based on small inexpensive CO2 loggers, originally made for indoor air quality monitoring, that were tested and adapted for field use. Measurements of soil-atmosphere and lake-atmosphere fluxes, as well as of spatiotemporal dynamics of water CO2 concentrations (expressed as the equivalent partial pressure, pCO(2aq)) in lakes and a stream network are provided as examples. Results from all these examples indicate that this approach can provide a cost- and labor-efficient alternative for direct measurements and monitoring of CO2 flux and pCO(2aq) in terrestrial and aquatic environments.Funding Agencies|Linkoping University; Swedish Research Council VR</p