Quantifying denitritication losses in subsoils: A methodology

Abstract

The impact of nitrate (N0₃⁻) leached from agricultural soils is of local, regional and global concern. Groundwater concentrations of N0₃⁻ have increased as a consequence of intensification of agriculture. In many cases measured concentrations of N0₃⁻ in leachate exceed drinking water standards (11.3 mg L- However, N0₃⁻ concentrations of shallow groundwater are often lower than predicted and in New Zealand rarely exceed drinking water standards. The importance of denitrification in subsoils as a process for reducing No₃- contamination to the groundwater is unclear, but may be significant. Nitrate leachate amounts for arable systems are affected by the type of cropping, and the timing and type of cultivation (Francis et al. 1994). In Canterbury, measured leachate concentrations of N0₃⁻ for cropping rotations range between 3 to 53 mg L⁻¹ , equivalent to 5 to 80 kg N ha⁻¹ (Thomas et al. 1999). There have been few direct measurements of the extent of denitrification in subsoils. Availability of a carbon substrate in the subsoil is considered to limit subsoil denitrification and not the availability of denitrifiers (McCarty and Bremner 1992). However, activity and distribution of denitrifiers may also be affected by management history (Sotomayor and Rice 1996). Our hypothesis is that management history may affect subsoil denitrification by regulating supply of both carbon and nitrate.