Nitric and nitrous oxide emission from an upland agricultural grassland soil

The aim of this study was to determine how land management affected nitrogen cycling by nirtifiers and denitrifiers in an upland agricultural grassland soil and to determine the effects of changing environmental conditions on nitric and nitrous oxide production and emission as a result of land management. Soil microcosm experiments used changing soil moisture and temperature as an environmental factor to quantify production of nitric and nitrous oxide in soil sampled from an un-improved upland grassland field in the Scottish borders (Sourhope). Microcosms were amended with NH₄NO₃ at the field application rate of 126 kg N ha^-1 yr^-1, nitric oxide flux increased significantly at soil moisture contents below 60% WFPS (Water Filled Pore Space), the maximum flux was 0.134 μg NP – N g^-1 h^-1 at 40% WFPS. Nitrous oxide flux however, only increased at 80% WFPS with a flux of 9.942 x 10^-3 μg N₂O = N g^-1 h^-1 and decreased over time to 2.09 x 10^-6 μg N²O – N g^-1 h^-1, similar to that of the other soil WFPS microcosms. The effects of changing soil temperature was determined at 40 and 80% WFPS at 1,5,10,15,20 and 25°C. At low soil moisture contents the maximum flux of nitric and nitrous oxide, 0.17 μg NO-N g soil^-1 h^-1 and 4.19 x 10-5 μg N₂O-N g soil^-1 h^‑1 respectively, was at 15°C. The flux of both decreased with increasing temperature at 40% WFPS. A collaborative long term enrichment study using soil microcosms with soil sampled from the Sourhope plots found that liming increased the gross rates of nitrification relative to controls (1.6 μg N d^-1) and the ratio of nitric to nitrous oxide flux. Addition of nitrogen in all combinations increased both nitric acid nitrous oxide flux however gross nitrification rates at all N treatments decreased relative to control and lime and nitrite treatments.