title: Exploring how land use may influence the export, composition, and reprocessing of dissolved organic carbon in peat-rich catchment drainage
Creators: name: Zheng, Ying
Publisher: name: University of Glasgow
Year: 2018

description: Organic soils play an important role in the fluvial carbon cycle through production and export of dissolved organic carbon (DOC) from the breakdown of plant material. However little is known about the changes in organic soil-derived fluvial DOC composition and its potential impacts on in-stream DOC breakdown dynamics. This research explores how aquatic DOC composition is influenced by land use, and how the biological utilisation of DOC may be influenced by its composition. Specifically, this research focusses on peat- rich catchments, subject to disturbance for wind farm construction to:
1. explore for how long the disturbance from wind farm development affects water chemistry in peaty catchments through analysing the long-term trends in fluvial DOC and nutrient concentrations in the draining streams;
2. assess if different wind farm-associated land uses, (construction and deforestation), exert different influences on fluvial DOC quantity and composition in peatland catchments;
3. investigate relationships between total dissolved iron (Fe) concentration [Fe] and concentrations of DOC and nutrients (phosphorus and nitrate), as this interaction may ultimately shape the DOC biodegradability during fluvial transport.
4. quantify how much DOC is biodegraded in peatland streams and what controls this.
To examine the wind farm disturbance on fluvial DOC concentration, [DOC], and spectrophotometric composition, streamwater samples from five peatland catchments draining the south of the Whitelee wind farm in Scotland were analysed monthly from 2014 to 2016. To further understand the impacts of wind farm-associated land uses, spatial and temporal variation of DOC concentration and composition were assessed between sub-catchments D-WF and D-FF within a small catchment, Drumtee. D-WF was draining the wind farm construction areas and D-FF draining the felled forestry. The concentrations of soluble reactive phosphorus, [SRP], and total oxidised nitrogen, [TON], were also measured.
Using existing and data new to this study, a long-term [DOC] increase (2006 - 2016) was observed in the Whitelee catchments since the start of wind farm development (October 2006, with the associated felling activities starting in November 2006). The increase may be a result of wind farm long-term disturbance, by generating more DOC which could be exported when discharge increases. Immediate impacts from wind farm were observed, with [DOC] and [SRP] increasing quickly after the construction began at the catchments most affected by the original wind farm development and its extension. Within Drumtee
catchment (WL13), greater mean [DOC] was observed in D-FF than D-WF. This may suggest
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that wind farm-related deforestation led to more peat decomposition and greater DOC release than the construction activities.
DOC is dominated by humic substances (HS), which generally can be further divided into humic acids (HAs) and fulvic acids (FAs). In WL13, DOC was less aromatic and humic than the other Whitelee catchments, with smaller SUVA410 (an indicator for more-complex aromatic C) and larger E2/E4 (an indicator for proportion of humic substances) (p &lt; 0.05 for both parameters). This could be a result of shorter water retention time and smaller [TON] in WL13 leading to less DOC biodegradation. Moreover, within-catchment difference of DOC composition was observed in WL13, with smaller SUVA410 and larger E2/E4 indicating less DOC humification in the felled catchment (p &lt; 0.05 for both parameters). This is interpreted related to different land use effects, as ‘younger’ DOC was produced from the decomposition of residual forest branch and leaves in the felled area, or ‘older’ DOC was released from the deeper peat in the wind farm areas. In addition, DOC in the two sub-catchments showed inverse changes during the dry periods, and one possible reason could be the different DOC sources from soil with low flow due to land use differences.
Total dissolved [Fe] measured in the fluvial samples allowed analysis of the interaction between dissolved Fe and DOC, and nutrients in fluvial systems. A positive significant relationship was found between dissolved [Fe] and [DOC], supporting interpretation that humic substances (HS) maintain Fe3+ in soluble state during transport. Fe-DOC complexation may be particularly facilitated by fulvic acids (FAs) than humic acids (HAs), supported by the relationships between Fe/DOC (Fe and C-DOC molar ratio) and E2/E4, and E4/E6 (FAs:HAs).
However, high [Fe] and occurrence of Fe oxides may limit DOC export by co-precipitation. This interaction is important as it may limit DOC export, impact terrestrial DOC characteristics and thus aquatic bioavailability. In addition, Fe-DOC complexation may increase P export and the Fe2+-Fe3+ oxidation mediate TON reduction.
The in-stream breakdown dynamics of compositionally-different DOC were considered through incubations with water samples from Drumtee tributaries in autumn and early winter of 2015, and late winter and summer of 2016. Although [DOC] was higher in Drumtee than many other aquatic systems, only 2.9 – 12.1 % of the total DOC was biodegradable during 21 days. However, when normalized by the incubation length breakdown rates were comparable to other studies. A general seasonal variation of biodegradable DOC (BDOC) was observed across the sites, with more lost in late winter
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and summer and less in autumn and early winter. Spatially, in autumn and early winter similar BDOC % loss was observed across sites, while in late winter and summer larger BDOC % removal was found in D-WF1 and WL13.
Weekly measurement of DOC composition and nutrient concentrations during the incubation revealed the dynamic process of DOC biodegradation. Protein-like fluorescence component C6 and TON changed actively with BDOC removal, suggesting C6 may be an important BDOC pool, and TON may provide substrate for the labile DOM to form. Initial DOC composition (SUVAs and C6) and nutrient availability seem important controls on BDOC loss. SUVAs showed negative correlations with BDOC which may because SUVAs represented the humic and refractory DOC pools. C6 and nutrients were all positively related to BDOC loss, and is likely due to their support for DOC consumption in different ways. The relationships help to understand the observed seasonal and spatial variation of Drumtee BDOC loss. The inter-stream difference of DOC composition may reflect the influence of wind farm-associated land use differences on in-stream DOC reprocessing and fate.
The understanding generated by this research advances knowledge of connecting the terrestrial and fluvial C cycle subject to land use changes. It reveals that wind farm development may have caused [DOC] increase. Particularly, deforestation can lead to larger fluvial [DOC] but with less refractory fractions than construction activities. DOC concentrations and composition can vary over space and time and in response to land use, and in turn this ‘inherited’ composition can influence the fate of DOC in aquatic systems.