Nitrification and denitrification estimates in a Louisiana swamp forest soil as assessed BY 15N isotope dilution and direct gaseous measurements

The transformations of applied (100 kg N ha(-1)) N-15 labelled NO3 and NH4 in Mississippi River deltaic plain swamp forest soil which receives agriculture run-off from adjacent sugarcane fields were determined. Using an isotopic dilution technique, the rates of NO3 production (nitrification) and reduction in the (NO3)-N-15 treated soil-water-columns were approximately 240 and 2,320 g N ha(-1) d(-1) whereas NH4 production (mineralization) and removal rates in the (NH4)-N-15 treated soil-water-columns were 270 and 2160 g N ha(-1) d(-1), respectively. It was shown that if nitrification and NH4 assimilation were the primary processes responsible for NH4 removal, average NH4 assimilation would be 145 g N ha(-1) d(-1). Based on labelled N-2-emission, denitrification was 3 fold greater in the NO3 treatment compared to the NH4 treated soil water-columns with rates of 818 and 266 g N ha(-1) d(-1) respectively. Even though the rate was lower in the NH4 treatment, results show that nitrification-denitrification of NH4 is a significant process. Nitrogen losses determined by N-15(2) emissions were 20.4 and 6.4% and N2O emissions were 0.10 and 0.03% of the applied NO3-N and NH4-N, respectively, over 32 days of incubation. Fertilizer loss through N2O emission was only of minor significance compared to the fertilizer loss through N-2 evolution. Nitrous oxide fluxes from the control soil-water-columns averaged 9.4 g N ha(-1) d(-1). Addition of NO3-N to the columns increased N2O production 56% as compared to a 15% increase from the NH4-N addition. Results show that this wetland soil has a large capacity to process inorganic nitrogen entering the system as a result of agriculture run-off.