The boreal biome exchanges large amounts of carbon (C) and greenhouse gases (GHGs) with the atmosphere and thus significantly affects the global climate. A managed boreal landscape typically consists of various sinks and sources of carbon dioxide (CO2), methane (CH4), and dissolved organic and inorganic carbon (DOC and DIC) across forests, mires, lakes, and streams. Due to the spatial heterogeneity, large uncertainties exist regarding the net landscape carbon balance (NLCB) and its spatiotemporal variability. In this study, we compile terrestrial and aquatic fluxes of CO2, CH4, DOC, DIC, and harvested C obtained from tall-tower eddy covariance measurements, stream monitoring, and remote sensing of biomass stocks for an entire boreal catchment (~68 km2) in Sweden to estimate the NLCB across the land-water-atmosphere continuum and to investigate its inter-annual variability during a five-year study period (2016-2020). Our earlier results showed that this managed boreal forest landscape was a net C sink during 2016-2017 with the landscape-atmosphere CO2 exchange being the dominant component, followed by the C export via harvest and streams. We further found that abiotic controls (e.g., air temperature and incoming radiation) regulated the difference in the NLCB between these initial two years, whereas land cover types (e.g., mire vs. forest) and management practices (e.g., clear-cutting) determined its spatial variability. Here we extend our investigations to gain further insights into the multi-year inter-annual variability of the NLCB and its regulating drivers. Overall our study advocates the need for integrating terrestrial and aquatic fluxes at the landscape scale based on long-term tall-tower eddy covariance measurements combined with biomass stock and stream monitoring to develop a holistic understanding of the inter-annual variations in NLCB of managed boreal forest landscapes and to better evaluate their potential for mitigating climate change.

Climatic changes are thawing and mobilizing vast amounts of organic carbon (OC) previously stored for millennia in permafrost soils of northern circumpolar landscapes. Climate-driven increases in fire and thermokarst disturbance may play a key role in accelerating OC mobilization by exposing permafrost soils and liberating stored OC, yet the extent of OC mobilization, and mechanisms controlling land-water transfer are unclear. Here, we combine extensive dissolved OC (DOC) radiocarbon sampling with a hydrologic end-member mixing model for 11 headwater catchments throughout interior Alaska, demonstrating that patterns of permafrost soil C delivery to headwater streams are extremely heterogeneous. Mean age of DOC (from modern, to ~2000 yr b.p.) was dependent on seasonal thaw, local surface geology, and to a lesser extent, disturbance history due to catchment fire. Non-linear general additive models showed the DOC 14C signature and mean age were inversely proportional to hydrologic contributions from the solute- and ice-rich permafrost end-member, which dominated in catchments with silt-rich soils. Seasonal warming, soil composition, and the proportion of permafrost end-member contribution were of primary importance in explaining stream DOC 14C patterns, while the explanatory power of catchment burn extent was minimal, and of secondary importance. Together, these findings demonstrate that cross-catchment heterogeneity in ancient soil-OC export is a result of seasonal, geologic, hydrologic, and disturbance effects. As these effects remained poorly-integrated our new conceptual model of permafrost DOC export will help refine our understanding of land-water C exchanges in changing northern landscapes.

Dissolved organic matter (DOM) degradation in streams and rivers plays an important role in the global carbon cycle, but little is known about how the source and composition of riverine DOM regulate its microbial degradation and production of greenhouse gases. Using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we determined the composition of riverine DOM at a molecular level to improve our understanding on the quality, quantity, and microbial degradability of DOM in pristine subarctic rivers of Finnish Lapland. In spring and fall 2018, 21-day incubation studies were conducted with water samples taken from two rivers that represent contrasting types of catchment characteristics (e.g., vegetation and soil type). The changes in the DOM concentration and molecular composition, as well as in the carbon dioxide (CO2) and methane (CH4) production, were followed. Our results indicate that microbial utilization of lipids resulted in extensive degradation of DOM in the clearwater river. Peatland-derived DOM in the brown-water river had lower degradability compared to DOM in the clearwater river. This was due to a higher relative abundance of less biodegradable compounds such as lignin-like molecules in the brown-water river. Our study emphasizes the importance of lipids and the contribution of clearwater systems in DOM processing at catchment scales.

Unprecedented warming across the circumpolar north has changed the amount and timing of precipitation, increased mean annual air and subsurface temperatures, and enhanced permafrost degradation. Subsequently, dissolved organic matter (DOM) sources, subsurface processing times, and flow pathways to nearby surface waters may continue to change. However, determining the degree of connection between the terrestrial-aquatic environment is difficult, particularly in Canada’s subarctic where the permafrost, low summer precipitation, and a lack of visible surface flow make it difficult to directly quantify this link. We paired stable water isotopes with DOM characterization techniques to determine the strength of the terrestrial-aquatic linkage across various lakes near Yellowknife, Northwest Territories, Canada. The two-year field campaign serendipitously sampled a wet and a dry year, providing a comparison of lake water and carbon chemistry over variable hydrologic conditions. DOM concentration, compositions (based on UV-absorbance parameters), and stable water isotopes (δ2H and δ18-H2O) varied with hydrologic regime, indicating strong and rapid linkages between the terrestrial-aquatic interface. During the ‘wet’ year, lake water isotopes representative of precipitation aligned with increases to DOM concentration and increases to overall DOM molecular weight. Conversely, ‘dry’ year lake water isotopes indicated significant evaporation with little change to DOM concentration and a shift towards smaller DOM molecules. Different measures of DOM composition are important depending on lake size and hydrological condition (wet or dry year). Further, an isotope mass balance was used to derive an estimate of water residence time (WRT), with shorter WRT lakes, containing terrestrial-like DOM characteristics, being more susceptible to changes in DOM concentration. Thus, hydrology and carbon chemistry across subarctic taiga shield lakes, underrepresented in the literature, are intricately linked and can be used to assess the sensitivity in the amount and form of DOM exported to these lakes under a warming climate.

Understanding the mechanisms responsible for the delivery of carbon across terrestrial to aquatic (T-A) interfaces is critical for constraining T-A fluxes from headwater catchments. This is particularly important for boreal forests where organic carbon stocks are high and increasingly vulnerable to hydrologic change. Dissolved organic carbon (DOC) can represent a significant fraction of terrestrial carbon exported to boreal streams. Hydrologic pathways responsible for the largest export of DOC from the landscape are generally connected to streams during the most intense periods of water delivery to the landscape (e.g., snowmelt or storm events). However, the timing and magnitude of DOC export to headwater streams are further complicated by variability in landscape geomorphological, hydrometeorological, and biogeochemical factors. To better assess mechanisms controlling the delivery of DOC from boreal landscapes, DOC was investigated in two morphologically distinct regions of an experimental watershed in western Newfoundland. The watershed is dominated by low relief wetlands in the upper catchment and steep forested hillslopes in the lower catchment. The response of stream DOC was compared during two fall storms of similar magnitude but different moisture conditions during the autumn transition. Concentration-discharge relationships of DOC with stream hydrographic separation analysis in the hillslope dominated lower catchment suggests preferential flowpaths through shallow mineral soil horizons was the dominant transport pathway with lateral shallow subsurface flow becoming important with increasing landscape saturation. By contrast, a rapid response to infiltrating water into near-stream wetlands in the upper catchment resulted in an early pulsing of DOC to the stream during the storm event. These results indicate watershed scale responses of DOC in boreal headwater systems are heterogeneic and controlled by spatially distinct landscape units and moisture-dependent hydrobiogeochemical responses. Furthermore, the ongoing shifts in boreal hydrology will have important implications for the release of DOC and T-A carbon fluxes with climate change.

Wetlands are an integral part of the boreal landscape; wetland type and hydrologic status are important for characterizing local to regional landscape ecology, biology, and surface hydrology, making accurate wetland mapping and characterization vital for purposes of carbon accounting and habitat assessment. Because boreal wetland ecosystems are diverse and the landscape hydrology is extremely dynamic, wetlands are some of the most difficult landscapes to accurately map. Additionally, the northern region is experiencing rapid change as the region warms. The research we present includes the use of field and remote sensing collections conducted in conjunction with the NASA ABoVE program in the areas around the Great Slave Lake and Peace-Athabascan Delta in Canada, where we are working to advance the ability to characterize wetlands (type, structure, function, and hydrology). Methods exploit the unique capability of synthetic aperture radar (SAR) in conjunction with multi-spectral electro-optical remote sensing for mapping both wetland type and status. Results include maps of wetland type relevant to waterfowl habitat assessment, wetland extent and inundation status, and hydroperiod, a measure of the flooding frequency and pattern of the region. The use of these products for waterfowl distribution and abundance modeling will be described, along with preliminary results of these research efforts. While this work is focused on waterfowl habitat modeling, developing robust methods to map and monitor wetlands for these regions will provide an important resource for a variety of needs, and allow for valuable tools for monitoring the region’s wetlands into the future.

Atmospheric CO2 and CH4 concentrations have greatly increased since pre-industrial times due to anthropogenic emissions, leading to a climate change crisis. Developing effective strategies to mitigate climate change requires an improved understanding of the different sources and sinks of atmospheric carbon across the globe. The carbon exchange between the continents and the atmosphere is considered the most uncertain component of regional and global carbon budgets. One of the main sources of this uncertainty is that the continental landscape is made up of a heterogeneous mosaic of elements (e.g., forests, wetlands, and inland waters) that each has its own set of ecosystem properties and processes. This heterogeneity has led to the compartmentalization of the landscape carbon budget with each element treated independently from the other. Nevertheless, this compartmentalized perspective tends to overlook aspects of the landscape structure and functioning that link these elements together, such as lateral carbon exchanges. To address this issue, we are working towards a more holistic framework that not only integrates the main landscape elements but also the exchange of carbon between them: The Net Watershed Exchange (NWE) Framework. The framework is based on a whole-landscape mass balance and effectively integrates the origin, lateral movement, and fate of carbon across the landscape, using the watershed as a natural, directional spatial unit. Based on this framework, we propose an indirect approach to estimate the watershed-atmosphere carbon exchange based on constraining the net accumulation of carbon in the watershed with the lateral export to the ocean through rivers. This approach provides a complementary perspective to landscape-atmosphere carbon exchange estimates provided by top-down and bottom approaches and may contribute to constraining these estimates across scales. Furthermore, the proposed framework offers a platform to increase communication and synergy between the different terrestrial, aquatic and atmospheric research communities.

Understanding variability in source, timing and magnitude of water transport in headwaters is important to unravelling climate change impacts on terrestrial to aquatic (TA) delivery of solutes particularly in wet boreal forest watersheds where soil C stocks are high and vulnerable to climate change. In maritime eastern Canada, both spring snowmelt and fall peak discharge (Q) decrease with increasing latitude in concert with decreasing mobilization of forest soil dissolved organic carbon (DOC). However, decadal records indicate significant trends: snowmelt peak Q is decreasing but fall peak Q is increasing, especially in lower latitude watersheds. To understand the ramification of these Q trends on TA fluxes, seasonal and interannual variations (2014-2019) in water fluxes and sources were determined for an experimental watershed in western Newfoundland, Canada. Results indicate that reductions in the magnitude of the spring melt events reduces annual runoff ratios. This response is due to decreases in the forest hillslope runoff; no change in runoff was observed for the upper low-relief catchment across study years with contrasting snowmelt dynamics. Hydrographic separation analysis for spring melt and fall events indicate that shallow wetland solute sources likely increase relative to hillslope sources of water with decreasing spring snow melt and increasing fall event magnitude. However, our monitoring of dissolved organic carbon (DOC) indicates that, relative to spring melt, the water flux normalized yield of forest soil DOC in fall is enhanced at a time when hillslope delivery of DOC evolves with the number hydrologic events. These results provide a hypothesis and mechanism for climate driven increases in TA delivery of DOC in these mesic boreal forest watersheds.

With rising demand for renewable energy, removal of logging residues after clearfelling occurs more frequently. The reported environmental effects vary. Extraction of logging residues may reduce the site productivity, but decomposition of treetops and branches may cause leaching of nutrients to nearby waterbodies. Therefore, regionally specific science-based information on nutrient cycling in forest ecosystems is needed.

To evaluate environmental effects of different intensity regeneration fellings three experimental sites were established in 2012 in typical forest site types in Latvia – Myrtillosa and Hylocomiosa (oligotrophic and mesotrophic mineral soil sites, dominated by Pinus sylvestris), and Oxalidosa turf. mel. (eutrophic drained peatland site, dominated by Picea abies). Each site contains two felling plots: whole-tree harvesting (WTH); stem only harvesting (SOH) plot and an unharvested control plot (C). Below the plots a riparian zone for protection of waterbody was left. Soil solution was sampled monthly in all plots and riparian zone during vegetation season for eight years.

After the felling pH of soil solution decreased both in felled and riparian zone plots in all sites. In the eutrophic site more distinguished acidification was observed in WTH plot and its riparian zone. In oligotrophic site more pronounced acidification was observed in SOH plot, but no significant differences were observed in any riparian zone plot. A spike of N-NO3- concentrations was observed in WTH plot of the eutrophic site three years after felling, and concentrations of N-NO3- remained elevated for most of the observation period in its riparian zone. In mesotrophic site spikes of N-NO3- concentrations were present in both felling plots, while in oligotrophic site only in SOH felling plot. In riparian zones of both (mesotrophic and oligotrophic) sites N-NO3- concentrations remained stable and low. P-PO43- concentrations were generally low, and no explicit differences were observed between the felling intensities and in their riparian zones.

Mercury (Hg) is a global priority pollutant, and its organic form, methylmercury (MeHg), has harmful effect on biota and human health. In Northern environments numerous ecosystems can be described as Hg-sensitive because they biomagnify MeHg along food webs, yielding contaminant levels in top fish predator species and/or other aquatic food resources including aquatic mammals and efficiently transform total Hg into bioassimilable MeHg which has toxic effects on the neurological, cardiovascular, and reproductive systems of biota. Methylation processes are controlled by a network of complex microbial and abiotic interactions, and they depend on a range of environmental factors, including land use and management. Due to high costs and complexity of sampling and analyses, data on mercury and methylmercury are scarce in many countries, including Latvia.

The aim of a study conducted from 2018 to 2020 was to gain initial insight into chemical, landscape- and disturbance-related variables controlling methylation processes in watercourses located in a typical peatland forest. We sought to clarify the trends in the spatial and temporal distribution of mercury (total Hg and MeHg) in natural watercourses and drainage ditches under varying levels of anthropogenic disturbance. Total Hg, MeHg and general chemistry parameters were analysed in water and watercourse bottom sediments.

In water total Hg concentration was < 0.1 µg L-1, but annual mean MeHg concentration in water samples ranges up to 1.36 ± 0.91 ng L-1, displaying no significant differences between disturbed and undisturbed sites. In sediment the concentrations of both total Hg and MeHg were significantly higher in undisturbed sites (139 ± 10 µg kg-1 dw and 14 ± 5 µg kg-1 dw, respectively) characterized by high content of organic matter and nutrients if compared to disturbed sites.

Study was carried out within project “Interaction of microbial diversity with methane turnover and mercury methylation in organic soils, lzp-2018/1-0434” funded by Latvian Council of Science.

In the boreal biome of North America, aquatic habitats generated by beaver activity are an important component of the landscape. Over the last century the beaver population has been increasing exponentially, and consequently the extent of beaver habitat through the landscape is expected to increase. Despite that beaver ponds are consider as a hotspot of carbon dioxide (CO2) and methane (CH4) production, the carbon (C) emissions associated to beaver habitat have rarely been incorporated in estimates of inland water C budgets. This is hampered by a lack of robust estimates of the extent and size distribution of these habitats within the landscape. Recent developments of platforms for large-scale geospatial analysis as Google Earth Engine and high resolution satellite images vastly improve our ability to identify and map different types of waterbodies. Here we combine a large-scale geospatial analysis of the beaver habitat within a watershed of boreal Québec, Canada, and measurements of CO2 and CH4 emissions from sampled beaver ponds. Our preliminary results show that within the selected watershed (13,105 km2), beaver ponds comprise 106 km2 representing 10% of the total aquatic area. Given that these small waterbodies tend to develop high C emission rates per square meter of water basis, beaver ponds could play a major role in the C cycle of the boreal biome

The boreal biome is a net terrestrial carbon sink that has accumulated a significant amount of the Earth’s soil organic carbon. Increasingly, the importance of aquatic systems as a major component of the landscape carbon budget has become recognized. In the Canadian Taiga Shield Ecozone there are few estimates of the terrestrial or aquatic components of the landscape carbon budget. In this ecozone, aquatic component of the carbon balance is particularly important since lakes cover up to a quarter of the landscape. Here, we present dissolved CO2 and CH4 along with carbon burial rates from lakes along a climate gradient in the Western Canadian Taiga (Northwest Territories, Canada). We use relationships developed here, published models and remotely sensed data to integrate aquatic and terrestrial components of the landscape carbon balance. We find that the land carbon sink was 20 to 90 C g m-2 yr-1 and lake carbon burial was 1 to 30 C g m-2 yr-1 per area of landscape. Carbon emissions from streams and rivers were small (0.5 C g m-2 yr-1) while emissions from lakes were up to an order of magnitude higher. Together stream and lake emissions can offset a significant proportion of the terrestrial carbon sink. Aquatic emissions were highly sensitive to climate suggesting that warming climates might cause lake emissions to further offset the landscape carbon sink.

Soils are connected to the aquatic network through the continuous water film that begins in upland soils and ends in the ocean. Despite its importance in the global C budget, there continues to be widely diverging views on the “nature” of soil and soil-derived dissolved organic matter (DOM) entering streams. Given new high resolution molecular data, revisions to the concepts surrounding the soil-aquatic continuum are emerging. The idea of two levels of molecular diversity: beta and alpha (similar to the ecological concept) were introduced. When this concept is applied to the stream-ocean continuum, it is expected that headwater streams will have high beta-diversity that reflects the location-specific source material and microbial community, whereas oceans will have higher α diversity as they integrate molecules (and isomers of the same structural molecules) from a large source area along the degradation cascade. It is unclear if this concept has a terrestrial counterpart or if similar processes apply to the soil-aquatic degradation cascade. Here we demonstrate that the molecular composition and properties of dissolved organic matter continuously change during soil passage and towards aquatic ecosystems. We present patterns in dissolved organic matter diversity from four hardwood Canadian experimental catchments at several positions and depths (soil surface to 60cm) along flow paths feeding streams.