Nitrogen transformations and greenhouse gas emissions from a riparian wetland soil: an undisturbed soil column study

Riparian wetlands bordering intensively managed agricultural fields can act as biological filters that retain and transform agrochemicals such as nitrate and pesticides. Nitrate removal in wetlands has usually been attributed to denitrification processes which in turn imply the production of greenhouse gases (CO(2) and N(2)O). Denitrification processes were studied in the Salburua wetland (northern Spain) by using undisturbed soil columns which were subsequently divided into three sections corresponding to A-, Bg- and B2g-soil horizons. Soil horizons were subjected to leaching with a 200 mg NO₃⁻L⁻¹ solution (rate: 90 mL day⁻¹) for 125 days at two different temperatures (10 and 20°C), using a new experimental design for leaching assays which enabled not only to evaluate leachate composition but also to measure gas emissions during the leaching process. Column leachate samples were analyzed for NO₃⁻concentration, NH(4)(+) concentration, and dissolved organic carbon. Emissions of greenhouse gases (CO₂ and N₂O) were determined in the undisturbed soil columns. The A horizon at 20°C showed the highest rates of NO₃⁻ removal (1.56 mg N-NO₃⁻kg⁻¹ DW soil day⁻¹) and CO₂ and N₂O production (5.89 mg CO₂ kg⁻¹ DW soil day⁻¹ and 55.71 μg N-N₂O kg⁻¹ DW soil day⁻¹). For the Salburua wetland riparian soil, we estimated a potential nitrate removal capacity of 1012 kg N-NO₃⁻ha⁻¹ year⁻¹, and potential greenhouse gas emissions of 5620 kg CO₂ ha⁻¹ year⁻¹ and 240 kg N-N₂O ha⁻¹ year⁻¹.