Dissolved organic matter (DOM) is exported from terrestrial to freshwater ecosystems where, not only is it being degraded and eventually lost as CO2, but such degradation occurs faster than in soils or marine systems. Across freshwaters, variations in DOM degradation and reactivity have been related to compositional changes in DOM. The flux from terrestrial to aquatic systems seems to be increasing associated to anthropogenic perturbations. However, despite the relevance of these fluxes for the global C cycle, Earth System Models (ESMs) are just starting to consider them. In that sense, a particularly crucial region deserving urgent attention is the Arctic, as is suffering exacerbated effects of climate change, holds a massive C stock that is vulnerable to being mobilized towards freshwaters and is th region with the highest density of lakes in the world. Such transfer could turn that vulnerable C stock from a sink into a CO2 source. Therefore, determining the reactivity of that OC flux and incorporating it in surface models is key at the moment. The foundation of CHROME is the idea that the chemical diversity of DOM explains its reactivity and, as such, should be considered in biogeochemical models. CHROME is based on the good match between the previous experience of the applicant and the excellence of the host institutions, leaders in the field of biogeochemical research in Arctic ecosystems (USGS- Boulder, US) and in global modelling (LSCE-CNRS, France). CHROME will constitute a key advance in C biogeochemical understanding and modelling at the forefront of geosciences research.The action responds to three identified needs (1) to advance our biogeochemical understanding of DOM degradation processes, (2) to improve its representation in regional surface system models and (3) to contribute to understand the fate of DOM matter being thawed from Arctic soils.
The main conclusions reached by the project have been:
i) Using molecular-level data of DOM composition obtained through ultra-high resolution mass spectrometry, we identified that chemical diversity indices based on the presence/absence were better predictors of DOM degradation and bacterial respiration parameters than indices based on the expected chemical function of the compounds
ii and iii) On the links between landscape predictors and DOM degradation with the aim of finding key relationships to be implemented in ESM, we can conclude:
a. Rock-weathering impacts DOM and nutrients content and processing in high- Arctic streams, due to the petrogenic origin of those elements. That was specially important in specific geologies, such those dominated by Jurassic sedimentary materials
b. For lakes and ponds in the high-Arctic, isolation (in terms of hydrological connectivity) and distance to the sea are good predictors of DOC concentration and DOM composition at the molecular level and ultimately, of the CO2 and CH4 emissions of those sites.
