Assessing the fate of polycondensed aromatic natural organic matter (NOM) in a high-alpine aquatic system

With the Anthropocene, thermogenic organic carbon, also known as black carbon (BC) is increasingly introduced to ecosystems worldwide. BC is formed by incomplete combustion or pyrolysis of fossil fuels, at wildfires or by the intentional burning of biomass, emitted to the atmosphere and hydrosphere or remaining in the bio-/pedosphere. With its high molecular proportion of elemental carbon, BC is regraded to be protected from fast microbial degradation by its inherent molecular properties. In aquatic systems, BC is partly sedimented or buried when reaching endorheic lakes or the ocean, where it is withdrawn from the carbon cycle. The biogeochemical implications of BC or its degradation products in aquatic systems have therefore not received much attention. Especially in sensitive oligotrophic alpine systems, such as the treeless Tibetan Nam Co catchment, part of the biggest connected alpine pasture system in the world, which is exposed to increasing anthropogenic pressure, any nutrient and carbon surplus can threaten the ecological status.

This was studied by an ultra-high resolution mass spectrometry approach that identifies several thousands of molecular formulae in dissolved organic matter (DOM). Including polycondensed aromatics (pcAro) that are considered to include thermogenic DOM.

Incubation experiments of water samples indicated that pcAro DOM was transformed by microbes to a comparable degree such as other natural organic matter (NOM) in the samples, removing pcAro DOM by defunctionalization or metabolization. In the environmental samples we found that pcAro DOM discharged to the endorheic Nam Co Lake was transformed by photodegradation along with other aromatic compounds. For DOM of streams and the lake of the high-alpine Nam Co watershed, most pcAro is likely of local origin, i.e. derived from burning of yak faeces by pastoralist households. This pcAro formed from cellulose-rich and lignin-poor Cyperaceae fodder is chemically distinct from low-land natural biomass, usually including the burning of wood.

Thus pcAro DOM appeared to be a more viable part of the carbon cycle than previously assumed. Our data support the hypothesis that the fate of polycondensed aromatic DOM depends on several environmental factors, such as catchment characteristics, water opacity, solar irradiation and actual light penetration into the water column, as well as on the carbon source, driving the molecular composition of thermogenic NOM.