Seasonal Variation in Rates of Nitrification Associated with Patterns of Carbon and Nitrogen Supply in a Southern Appalachian Headwater Stream


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Virginia Tech


Nitrification, the chemoautotrophic process via which ammonium-nitrogen (NH₄-N) is converted to nitrate-nitrogen (NO₃-N), is an important nitrogen (N) transformation in stream ecosystems. Experimental addition of dissolved organic carbon (DOC) has been shown to inhibit rates of nitrification, and rates have been stimulated by NH4-N addition. Insights regarding the role of particulate organic matter (POM) in this scenario could further enhance our understanding of linkages between ecosystem carbon (C) and N cycles. Hugh White Creek, a headwater stream located in the southern Appalachian mountains of North Carolina, USA, receives large amounts of allochthonous POM inputs each fall. To address the effects of these inputs on nitrification, I conducted a seasonal survey of organic matter standing stocks and nitrification rates along with experimental manipulation of dissolved C and N supplies in stream sediment microcosms to determine: 1) how rates of nitrification compare across seasons, and 2) to what extent nitrification rates are influenced by seasonal changes in standing stocks and relative abundances of both sedimentary and dissolved forms of C and N. Rates of nitrification were most closely and positively related to rates of ammonification, which, in turn were negatively related to C:N of fine benthic organic matter (FBOM). Uniform additions of C and N throughout the year had different effects on rates of nitrification and ammonification due to their changing relative importance as sediment organic matter stocks were depleted and underwent changes in quality. Slow rates of nitrification for much of the year could be attributed to large quantities of C relative to N in stream sediments. To the extent that changes in OM stocks dictate change in C and N availability, seasonal patterns in OM dynamics represent changes in ecosystem structure relevant to rates of nitrification, emphasizing the importance of terrestrial/aquatic linkages for predicting rates of N transformation in aquatic ecosystems.



carbon, nitrification, Nitrogen