Nitrification and Denitrification Estimates in a Louisiana Swamp Forest Soil as Assessed by ¹⁵N Isotope Dilution and Direct Gaseous Measurements

Abstract

The transformations of applied (100 kg N ha^-1)¹⁵ N 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 ¹⁵NO₃ 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 ¹⁵NH₄ 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 N₂-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 by¹⁵ N₂ 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 N₂ 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.