In-Situ Denitrification and N2O Emission from Natural and Semi-natural Land Use Types in two UK Catchments

Abstract:

Whilst data and understanding of the controls of denitrification process and the subsequent emission of N₂O at microbial and plot scale exist, quantification of in situ annual denitrification rates at catchment scales is scarce due to methodological constraints in measuring in situ denitrification in large temporal and spatial scales. In situ denitrification (DNT) was measured monthly (April 2013 - October 2014) in organic (peat bog, heathland, acid grassland), forest (mixed and deciduous), and grassland (improved and semi-improved) land use types in the Ribble-Wyre and Conwy River catchments in the UK. A static chamber technique according to the ¹⁵N-Gas Flux method¹ was employed for quantifying the fluxes of ¹⁵N-N₂ and ¹⁵N-N₂O gases after labelling the soil with 98 at% K¹⁵NO₃^- at tracer level amounts (10% of the ambient nitrate concentration) and sampling the chamber headspace at 0, 1, 2 and 20 hour intervals. The DNT rates ranged between 0 and 2.3 mg N m^-2 h^-1 and were significantly influenced by land use type (p<0.05). The annual denitrification rate of organic and forest soils (4 kg N ha^-1 y^-1) was 3 and 6 times less than that of semi-improved (12 kg N ha^-1 y^-1) and improved (23 kg N ha^-1 y^-1) grassland soils, respectively. The N₂O emission, due to denitrification, followed a similar trend with lower fluxes from organic and higher from improved grassland soils (range: 0 – 0.04 mg N m^-2 h^-1), whilst the N₂O:N₂ ratio ranged between 0.2 and 4%. The relative contribution of denitrification to net N₂O flux varied temporally and across the different land use types and ranged from 0.2 to 75%. The ¹⁵N-Gas Flux method can be successfully applied in a variety of land use types for relatively high temporal and spatial resolution measurement of in situ denitrification and the simultaneous quantification of N₂ and N₂O fluxes due to denitrification. Therefore the ratio of N₂O:N₂ and also the source apportionment for N₂O can be estimated more accurately. The results suggested a difference in DNT rates between unmanaged/ low nitrate content versus managed/ high nitrate content land use types, which was further supported by the significant positive correlation between DNT and soil nitrate content.

¹R. Stevens and R. Laughlin, Nutr. Cycling Agroecosyst., 1998, 52, 131-139.