Nitrification rates in a headwater stream: Influences of seasonal variation in C and N supply

Nitrification, the chemoautotrophic process by which NH₄-N is converted to NO₃-N, is an integral biogeochemical transformation in stream ecosystems. Previous research has shown that experimental addition of dissolved organic C inhibits rates of nitrification, and that NH₄-N addition stimulates rates of nitrification. In many streams, large amounts of C and N may be present in particulate and sorbed forms. Hugh White Creek, a headwater stream located in the southern Appalachian mountains of North Carolina, USA, has very low concentrations of dissolved N and receives large inputs of allochthonous particulate organic matter (POM) each autumn. We conducted a seasonal survey of organic matter (OM) standing stocks and nitrification rates, and we experimentally manipulated 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 particulate, sorbed, and dissolved forms of C and N. Rates of nitrification were closely and positively related to rates of ammonification which, in turn, were negatively related to C:N ratios of fine benthic organic matter (FBOM). Uniform additions of dissolved C and N had varying effects on sediment N-transformation rates during different seasons. Variable responses to experimental additions probably reflected the changing relative importance of C and N as sediment OM stocks were depleted and underwent changes in quality. Slow rates of nitrification for much of the year may be attributed to colder temperatures and large quantities of particulate C relative to N. To the extent that changes in POM stocks dictate changes in C and N availability, seasonal OM dynamics are closely linked to rates of nitrification.