Quantifying N₂O emissions from intensive grassland production: the role of synthetic fertilizer type, application rate, timing and nitrification inhibitors

Abstract

SUMMARYIncreasing recognition of the extent to which nitrous oxide (N2O) contributes to climate change has resulted in greater demand to improve quantification of N2O emissions, identify emission sources and suggest mitigation options. Agriculture is by far the largest source and grasslands, occupying c. 0·22 of European agricultural land, are a major land-use within this sector. The application of mineral fertilizers to optimize pasture yields is a major source of N2O and with increasing pressure to increase agricultural productivity, options to quantify and reduce emissions whilst maintaining sufficient grassland for a given intensity of production are required. Identification of the source and extent of emissions will help to improve reporting in national inventories, with the most common approach using the IPCC emission factor (EF) default, where 0·01 of added nitrogen fertilizer is assumed to be emitted directly as N2O. The current experiment aimed to establish the suitability of applying this EF to fertilized Scottish grasslands and to identify variation in the EF depending on the application rate of ammonium nitrate (AN). Mitigation options to reduce N2O emissions were also investigated, including the use of urea fertilizer in place of AN, addition of a nitrification inhibitor dicyandiamide (DCD) and application of AN in smaller, more frequent doses. Nitrous oxide emissions were measured from a cut grassland in south-west Scotland from March 2011 to March 2012. Grass yield was also measured to establish the impact of mitigation options on grass production, along with soil and environmental variables to improve understanding of the controls on N2O emissions. A monotonic increase in annual cumulative N2O emissions was observed with increasing AN application rate. Emission factors ranging from 1·06–1·34% were measured for AN application rates between 80 and 320 kg N/ha, with a mean of 1·19%. A lack of any significant difference between these EFs indicates that use of a uniform EF is suitable over these application rates. The mean EF of 1·19% exceeds the IPCC default 1%, suggesting that use of the default value may underestimate emissions of AN-fertilizer-induced N2O loss from Scottish grasslands. The increase in emissions beyond an application rate of 320 kg N/ha produced an EF of 1·74%, significantly different to that from lower application rates and much greater than the 1% default. An EF of 0·89% for urea fertilizer and 0·59% for urea with DCD suggests that N2O quantification using the IPCC default EF will overestimate emissions for grasslands where these fertilizers are applied. Large rainfall shortly after fertilizer application appears to be the main trigger for N2O emissions, thus applicability of the 1% EF could vary and depend on the weather conditions at the time of fertilizer application.

The use of biogeochemical models to evaluate mitigation of greenhouse gas emissions from managed grasslands

“…The reduction of N fertilizer logically decreases the N 2 O-N emissions, as reported here ( Fig. J in Supplementary material) and by experimental studies (Cardenas et al, 2010;Bell et al, 2016;Hörtnagl et al, 2018). Our results at the G3 site showed the same trend (Table 5), when the N 2 O-N emissions are compared to the applied N fertilizer amounts, the estimated (simplified) N 2 O-N emission factors (percent ratios of the total yearly N 2 O-N emissions over the amount of annually applied N fertilizer, both in kg N ha −1 ).…”

Changing views of nitrous oxide emissions from agricultural soil: key controlling processes and assessment at different spatial scales

“…Rain may be unpredictable, but sampling frequencies can be adjusted to record the outcome of fertilizer applications and also irrigations where they occur. In one recently reported example of this event‐related approach (Bell et al ., ), ‘daily gas samples were taken on ten occasions over the first 2 weeks after fertilizer application. Sampling frequency was reduced to 2 days per week for the following 3 weeks.…”

Isolating the effect of soil properties on agricultural soil greenhouse gas emissions under controlled conditions

“…Cumulative fluxes were calculated using the trapezoidal rule (area under the curve) to interpolate fluxes between sampling days (Bell, Hinton, et al., ; Bell, Rees, et al., ; Bell et al., ; Hinton et al., ) as follows:…”