Impact of Dissolved Organic Matter Photodegradation on Carbon and Nitrogen Cycling in Freshwaters

Abstract

Freshwaters emit comparable amounts of carbon dioxide (CO2) to the atmosphere as the net amount taken up by all land on Earth. The degradation of dissolved organic matter (DOM) to CO2 impacts these emissions from freshwaters, making the fate of DOM a critical component of the global carbon (C) cycle. Yet, controls on DOM degradation remain too poorly understood to quantify how much CO2 will be emitted from freshwaters as our planet warms. The sunlight-driven or photochemical degradation of DOM can impact freshwater CO2 emissions by producing CO2 or altering DOM to forms that are more or less labile to bacterial respiration to CO2. Photodegradation of DOM can also produce ammonium (NH4+), providing a source of nitrogen (N) to the primary producers carrying out photosynthetic uptake of CO2 in freshwaters. Here, chemical controls on these three photodegradation pathways were investigated and their impacts on C and N cycles were assessed in arctic and temperate freshwaters.

First, the photochemical production of NH4+ from DOM was investigated as a source of inorganic N to oligotrophic, arctic lakes. NH4+ was produced during the photodegradation of protein-like compounds within DOM, but rates of NH4+ photo-production were limited by the availability of protein-like compounds because they were also degraded by bacteria in the water column. The NH4+ photo-produced could account for ~5% of the N taken up by primary producers in arctic lakes, suppling N at rates comparable to the export of inorganic N from land to streams and streams to lakes. These findings demonstrate how the photodegradation of DOM creates a strong linkage between land and freshwater N cycling.

Second, the impact of thawing permafrost soils on freshwater CO2 emissions was determined by showing that millennia-aged DOM from arctic permafrost is rapidly photodegraded to CO2. Dissolved iron was identified as a major control on this photo-production of CO2 from permafrost DOM because it catalyzed the photo-decarboxylation of terrestrially-derived compounds to CO2. Rates of CO2 photo-production from permafrost DOM were two-fold higher than rates from DOM currently draining from thawed surface soils to arctic freshwaters. These findings demonstrate that more DOM will be photodegraded to CO2 as deeper permafrost soils thaw and deliver DOM to freshwaters in proportion to the amount of dissolved iron present.

Lastly, the effect of photodegradation on the amount of DOM respired by bacteria in the streambed to CO2 was studied in a temperate stream. This effect of sunlight exposure depended strongly on the photodegradation rate and which compounds were photo-produced. For instance, photodegradation rates were fast enough to impact the respiration of terrestrially-derived compounds that take longer for bacteria to respire in the streambed. Photodegradation increased the respiration of terrestrially-derived compounds by breaking down higher molecular weight aromatic compounds and decreased their respiration by altering them to less labile forms. The balance of these DOM compounds photo-produced resulted in positive effects of sunlight on respiration after short exposures and negative effects after longer exposures. Given that only minutes to hours of sunlight exposure in the stream were needed to change bacterial respiration by ~50%, these findings indicate that photodegradation greatly impacts the amount of DOM respired to CO2 even in periodically-shaded streams. Together, this dissertation demonstrates that DOM photodegradation substantially impacts freshwater CO2 emissions now and will impact them in the future, improving our predicting understanding of this process.