Simultaneous examination of nitrous oxide emissions in grazed pastures using paddock-scale measurements and process-based models

Abstract

{ }In-situ{ } farm-scale measurements are a prerequisite to improve the accuracy of the greenhouse gas inventory and to assess the viability of mitigation options. These measurements are also important in developing and verifying the empirical, as well as process-based, modelling approaches to emission estimates at farm-scales and beyond. Here, we assess the practicalities of implementing process-based modelling methods of quantifying nitrous oxide (N{ }2{ }O) emissions through validation and testing at the farm-scale, using environmental inputs and N{ }2{ }O emissions data from an emissions measurement campaign on dairy-grazed pastures in New Zealand. The median emissions measured with an atmospheric technique gave an emission factor (EF) of about 0.01, similar to the EF currently used by New Zealand inventory reporting for animal excreta (0.01) or for N fertiliser (0.0125). Emissions were simulated at the farm scale using two process-based models: NZ-DNDC and DayCent. The NZ-DNDC and DayCent models predicted EFs of 0.022 and 0.015, respectively for non-irrigated conditions and 0.041 and 0.032, respectively with irrigation. Lack of information on the grazing (excretal N inputs) and irrigation schedules before the campaign, and on soil ammonium and nitrate levels, was the main limitation in these validations. Identification of these shortcomings has been useful for the design of further integrated work on model development with farm-scale validation.