Dynamics of dissolved organic matter composition in Scottish rivers and headwater streams – resolving environmental and biogeochemical process interactions

Dissolved organic matter (DOM) has a wide range of chemical structures that give it a multifunctional role in the natural environment. Although the role of DOM in aquatic ecosystems has been the focus of previous work, a comprehensive understanding of the compositional behaviour of DOM under different environmental processes is still incomplete. New field-based geochemistry data is presented from a two-year study (03/2017- 03/2019) in Scottish headwaters and a 9-month study in large Scottish rivers. This research shows that the DOM mobilisation follows seasonality with enhanced exports of DOM during winter months compared to the summer. At a larger spatial scale, the seasonal trend is overprinted by the catchments soil type. Size-Exclusion Chromatography combined with high-resolution time series of DOM variables reveal that precipitation events preferentially mobilise humics from the surrounding soils, while humics concentration decline during low flow conditions. Furthermore, the data show that non-UV absorbing (“invisible”) low molecular weight (LMW) neutrals (iDOM) contribute up to 50 % to the total DOM pool in headwaters, especially during low flow conditions, and on average 13 % to the DOM in larger river systems. The source of iDOM was found to be the topsoil of peatland and peaty podzols. Consequently, more labile OM can be leached from soils into the aquatic environment in the future through disturbed soils promoting instream microbial growth and act as a nutrient source for aquatic plants

The riparian reactive interface: a climate-sensitive gatekeeper of global nutrient cycles

Riparian zones are critical interfaces to freshwater systems, acting as gateways for the conveyance and modification of macronutrient fluxes from land to rivers and oceans. In this paper, we propose that certain riparian conditions and processes (conceptually 'Riparian Reactive Interfaces') may be susceptible to environmental change with consequences of accelerating local nutrient cycling cascading to global impacts on the cycles of carbon (C), nitrogen (N), and phosphorus (P). However, we argue that this concept is insufficiently understood and that research has not yet established robust baseline data to predict and measure change at the key riparian ecosystem interface. We suggest one contributing factor as lack of interdisciplinary study of abiotic and biotic processes linking C, N, and P dynamics and another being emphasis on riparian ecology and restoration that limits frameworks for handling and scaling topography-soil-water-climate physical and biogeochemical observations from plot to large catchment scales. Scientific effort is required now to evaluate riparian current and future controls on global nutrient cycles through multi-nutrient (and controlling element) studies, grounded in landscape frameworks for dynamic riparian behaviour variation, facilitating scaling to catchment predictions

The riparian reactive interface: a climate-sensitive gatekeeper of global nutrient cycles

Riparian zones are critical interfaces to freshwater systems, acting as gateways for the conveyance and modification of macronutrient fluxes from land to rivers and oceans. In this paper, we propose that certain riparian conditions and processes (conceptually ‘Riparian Reactive Interfaces’) may be susceptible to environmental change with consequences of accelerating local nutrient cycling cascading to global impacts on the cycles of carbon (C), nitrogen (N), and phosphorus (P). However, we argue that this concept is insufficiently understood and that research has not yet established robust baseline data to predict and measure change at the key riparian ecosystem interface. We suggest one contributing factor as lack of interdisciplinary study of abiotic and biotic processes linking C, N, and P dynamics and another being emphasis on riparian ecology and restoration that limits frameworks for handling and scaling topography–soil–water–climate physical and biogeochemical observations from plot to large catchment scales. Scientific effort is required now to evaluate riparian current and future controls on global nutrient cycles through multi-nutrient (and controlling element) studies, grounded in landscape frameworks for dynamic riparian behaviour variation, facilitating scaling to catchment predictions.</jats:p

Assessment of the provenance of organic matter discharged from a permafrost watershed (Lena River) using lignin phenols

The Lena River (central Siberia) is one of the substantial pathways shifting terrestrial organic matter from its catchment area to the coastal zone of the Laptev Sea and the Arctic Ocean. The permafrost soils, which store huge amounts of OM, will most likely respond differently to climate warming and remobilize previously frozen OM with distinct properties specific for the source vegetation and soil. Particulate organic matter (POM) discharged by rivers and deposited close to their mouth is commonly assumed to record an integrated signal from the watershed. Furthermore, the POM likely undergoes degradation during its transport from source to sink. Therefore, investigating the different organic matter (OM) sources within a watershed will improve our understanding of OM sources and transport in large river systems. The present study investigated the composition of organic matter (OM) along the land–ocean continuum by characterizing lignin phenol composition in different grain size fraction in soils from the different vegetation zones (the boreal and northern Taiga and the Tundra) and marine surface sediments collected in the south-east Laptev Sea. Lignin is the rigidifying component of terrestrial higher plants, and it consists of different phenolic units, which allow to distinguish different vegetation sources, such as woody and non-woody tissues as well as gymnosperm and angiosperm tissues. The end-member calibration with plant tissues show that the taiga soils are dominated by gymnosperms, what reflect the predominant vegetation in the southern watershed of the Lena river, and that the actual tundra soils show a maximum angiosperm percentage of 50 %. In this case, the high S/V values in the marine surface sediments provide a further angiosperm source beside the watershed. The analyzes of the grain size fractions show that the finer fraction is generally more degraded than the coarser fraction in the soils, as well as in the marine surface sediments, which could be assume that the finer grain size fraction is transported more likely, e.g. during the spring freshet. All in all, the study indicated that in the marine sediments angiosperm tissues are more important than assumed in previous studies, in particular in the coarse grain size fraction, and that the gymnosperm signal in the marine sediments could be a real gymnosperm vegetation signal and not a result of high degradation as presumed before

Sources of particulate organic matter discharged by the Lena River using lignin phenols

Particulate organic matter (POM) discharged by rivers and deposited offshore their mouths is generally assumed to record an integrated signal from the watershed and therefore provides an archive of past environmental changes. Yet, in large river systems the riverine POM might be trapped in flood plains and the lower reaches resulting in an inefficient transport of POM particularly from the distal parts of the watershed. Further, the POM likely undergoes degradation during transport from source to sink. The Lena River is one of these large river systems stretching from 53°N to 71°N in central Siberia. The watershed can be broadly divided into two different biomes, taiga in the south and tundra in the northernmost part. The relative contribution of these biomes to the POM load of the river and its discharge to the ocean as well as the changes it is undergoing during transport are not well understood. Here we present the lignin phenol composition of different grain size fractions (bulk, 2mm-63µm, <63µm) of soil samples taken along a latitudinal transect (63°N to 72°N) as well as in marine surface sediments and two short sediment cores covering the last 120 years offshore the main Lena discharge channels. The lignin phenol composition of the soil samples (bulk, 2mm-63µm, <63µm) reflects the change in vegetation from south to north with increasing contribution of tundra vegetation. The degree of degradation between the soil sample locations as well as grain size fractions was very heterogeneous and did not show a clear trend. However, the POM seems to be slightly more degraded in the tundra, which is unexpected as the summer period when degradation in the upper thawed part of the soil can take place is shorter in the tundra compared to the southern taiga region. The marine surface sediments were dominated by gymnosperm-derived POM, particularly close to the river mouth and in the <63µm fraction. Because of the large heterogeneity of organic matter degradation in the soil samples and their grain size fractions, it is not quite clear to which degree the POM gets mineralized within the soils and during transport in the river compared to degradation occurring during cross shelf transport

Stabilization of carbon through co-addition of water treatment residuals with anaerobic digested sludge in a coarse textured soil

Coarse textured soils have low potential to store carbon (C) due to lack of mineral oxides and have low clay content to protect C from biodegradation and leaching. This study evaluated the potential of stabilizing C by adding metal oxyhydroxide-rich water treatment residuals (WTRs) to an aeolian pure sand (&lt;5% clay) topsoil amended with anaerobic digestate (AD) sludge. The AD sludge was applied at 5% (w/w) with aluminum based WTR (Al-WTR) and iron based WTR (Fe-WTR) co-applied at 1:1 and 2:1 WTR:AD (w/w) ratios and incubated at room temperature for 132 days. The cumulative mineralized C was normalized to the total organic C of the treatments. Co-addition with Al-WTR showed to be more effective in stabilizing C through decreased cumulative mineralized C by 48% and 57% in 1Al-WTR:1AD and 2Al-WTR:1AD, respectively, compared to AD sludge sole amendment. Co-application with Al-WTR also decreased permanganate oxidizable C by 37% and dissolved organic C by 51%. Co-application with Fe-WTR did not decrease the concentration of these labile C pools to the same extent, possibly due to the selective use of Fe-WTRs to treat organic-rich raw water. This makes it less effective in stabilizing C in a pure sand relative to Al-WTR due to chemical instability of the Fe-organic complexes. The Al-WTR provides a promising co-amendment to increase C sequestration in pure sands when co-applied with biosolids. The co-amendment approach will not only facilitate C sequestration but also contributes to waste management, aligning to the objectives of a circular economy

Unravelling Light and Microbial Activity as Drivers of Organic Matter Transformations in Tropical Headwater Rivers

Connecting tropical rainforests to larger rivers, tropical headwaters export large quantities of carbon and nutrients as dissolved organic matter (DOM), and are thus a key component of the global carbon cycle. This DOM transport is not passive, however; sunlight and microbial activity alter DOM concentrations and compositions, affecting riverine greenhouse gas emissions and downstream ecosystems. The effects of sunlight and microbial turnover/activity on DOM concentrations and compositions in tropical headwaters are currently poorly understood, but novel Size Exclusion Chromatography (SEC) techniques coupled to suitable detectors can for the first time quantify their influences. Here, we present in-situ incubation experiments from from headwaters of the Essequibo River, in the Iwokrama Rainforest, Guyana, where sunlight oxidised up to 9% of dissolved organic carbon (DOC) over 12 hours, at higher rates than in larger tropical rivers. DOM transformations occurred in both photo-sensitive and supposedly photo-resistant pools. Microbial activity had varying, less clear influences on DOC concentrations over the same time span; compositionally, this appeared to extend beyond known bio-labile components. Biopolymers were particularly reactive to both processes. We show sunlight has the greater potential to mineralise headwater DOM and thus potentially influence degassing. Our approach provides a future template to constrain DOM transformations along river networks, identify biogeochemical activity hotspots, and improve greenhouse gas emissions estimations under changing environmental conditions.</p