The Effect of Organic Matter Lability on Nitrous Oxide Production Dynamics in Coastal Sediments

• Anna Tevald, Marine Science, University of Delaware

• Nicholas Ray, School of Marine Science and Policy, University of Delaware
• Andrew Wozniak, School of Marine Science and Policy, University of Delaware

Nitrous Oxide (N2O) is a potent greenhouse gas and ozone depleter. N2O can be generated in coastal ecosystems as a by-product of different microbial metabolisms. There are two microbial processes that produce N2O: nitrification, which requires oxygen, and denitrification, which needs to take place when no oxygen is present. Denitrification competes with another nitrogen cycling process called anammox for nitrate. Since anammox does not produce N2O, understanding when denitrification and anammox dominate competition for nitrate might help to understand when and where N2O is produced in coastal ecosystems when little oxygen is present. A variable that may be useful for understanding the winner of this competition is organic matter lability/quality, because organic matter with higher lability is more easily broken down by denitrifying microbes in the soil, allowing more denitrification to occur. Anoxic bottle incubations were used to determine whether organic matter labilities affect N2O production. Treatments were: sediment + Spartina alterniflora (low lability treatment), sediment + Ulva lactuca (high lability treatment), sediment + 50% mix, and sediment alone (control treatment). Each bottle had seawater and nutrients needed for denitrification and anammox to proceed. Gas samples were collected from each bottle over an 11 day period and analyzed for N2O concentration with a gas chromatograph. Bottles with 50% Spartina alterniflora and 50% Ulva lactuca produced the highest concentration of N2O, followed by bottles with just Ulva lactuca. Bottles with just Spartina alterniflora and sediment produced little N2O, as did the control with sediment alone. These results show that organic matter is a clear control on N2O production in sediments. Further experiments could consist of measuring other nitrogen end products such as dinitrogen gas and testing other organic material liabilities to further understand the relationship between organic matter lability and N2O production.