The influence of organic carbon on nitrogen transformations in five wetland soils

Today we see an increased use of wetlands for N removal in agricultural catchments. Since the most important process for nitrate (NO3-) removal, denitrification, requires organic C, different soils could be expected to be differently suited for wetland construction. In this study, we evaluate the importance of soil organic C and the effects of added dissolved organic C on N transformations in existing and proposed wetlands. We used N-15-labeled NO3- to study N transformations in soil columns from five locations (a forest peaty soil, a field peaty soil, a silt loam, a loam, and a sandy loam). All five soils removed NO3- at substantial rates (13-73% of the load). The held peaty soil had highest denitrification rate (11 mmol m(-2) d(-1)), while sandy loam soil had the lowest rate (2 mmol m(-2) d(-1)). Dissolved organic C did not seem to limit N removal in the soils, as glucose additions affected N turnover only slightly. The forest peat soil differed from the others by exhibiting low nitrification, and relatively high production of nitrite (NO2-), probably a result of low pH, Nitrate removal in the field peat soil and the sandy loam soil was counteracted by production of ammonium (NH4+) and dissolved organic N, causing net N release. Although there was a positive relationship between soil organic matter and NO3- consumption, we conclude that all soils were suited for N removal. The lack of response to glucose additions indicate that there was no short-term lack of electron donor in any of the soils, including the sandy loam soil.