Reactive dissolved organic carbon dynamics in a changing environment : Experimental evidence from soil and water

Sammanfattning: Dissolved organic carbon (DOC) is the major form of organic carbon in aquatic ecosystems. Biological and photochemical degradation of DOC are major causes of greenhouse gas emissions from aquatic ecosystems. In response to current changes in climate, studies at different northern-hemisphere locations have shown both increases and decreases in total DOC export from land to water. However, there is lack of knowledge on the effect of changing environmental conditions on bio- and photo-degradation potentials. Therefore, this thesis aimed to combine empirical and experimental methods to determine the dynamics of DOC reactivity in response to key environmental parameters related to soil frost conditions and hydrology. I collected samples from the boreal region to carry out laboratory degradation experiments and to analyze the DOC degradation potentials in relation to environmental variables and intrinsic properties of the DOC.In a boreal forest region that has seasonal soil frost, the DOC bio-reactivity decreased with experimental reductions in the extent and duration of soil frost, while photo-reactivity remained constant. However, DOC from experimentally thawed permafrost peat in the subarctic north showed higher bio- and photo-degradation potentials than the DOC from active peat layers above the permafrost. The nature of the soil and intrinsic DOC properties explained these patterns in soil DOC reactivity. In the surface water network, bio-degradation was the dominant DOC degradation mechanism in brow-water lakes, while photo-degradation played a relatively larger role in clearer waters. The surface water photo-reactivity per unit of light absorption was high at both acidic and at alkaline conditions, while lower at intermediate pH, probably due to effects of the protonation state of DOC on photo-reactivity. Therefore, pH change along the aquatic network caused extrinsic control on the photo-reactivity at the landscape scale.In conclusion the results show that climate warming decreases the export of bio-reactive DOC in a boreal forest, but it likely increases the export of bio- and photo-reactive DOC in a permafrost peatland. In a wetter climate with shorter residence times, clear waters may experience browning and relative shift from photo-degradation to bio-degradation, while already brown lakes will remain brown and dominated by bio-degradation. Furthermore, the loss of photo reactive DOC during transport in aquatic network may be counter balanced by the changes in extrinsic variables such as pH.