Denitrification Capacity of Hill Country Wet and Dry Area Soils as Influenced by Dissolved Organic Carbon Concentration and Chemistry

The quantification of the nitrate attenuation capacity of pastoral hill country wet areas, especially seepage wetlands, would aid in the proper management of hill country farms for improved water quality outcomes. This study investigated the denitrification capacity, dissolved organic carbon (DOC) concentration and chemistry of wet areas and adjacent dry areas soils in a hill country landscape in New Zealand. Soil samples were collected during spring (November 2017) from different soil depths down to 100 cm. The results showed that the mean DOC concentration in the surface 30 cm soil depth was in the order: seepage wetland (498 mg kg(-1)) > hillside seep (172 mg kg(-1)) > dry area (109 mg kg(-1)). The denitrification capacity of the seepage wetland within the 0-30 and 30-60 cm soil depths was 7 and 69 times greater (p <= 0.05), respectively, than that of the dry area. The high concentration of readily-decomposable (e.g. lower molecular weight) DOC in the seepage wetland soil could have contributed to its higher denitrification capacity. The contrasting nitrate attenuation capacities of the seepage wetland soil versus that of the dry area soil highlight the important contribution of seepage wetlands to water quality improvement in pastoral hill country landscapes.