Lowering foliage nitrogen contents reduces N2O emissions in grazed pastures: An emission factor-based modelling assessment constrained by eddy covariance measurements

Abstract

Context

Modern agriculture faces the challenge of maintaining production while reducing environmental impacts, particularly nitrogen (N) pollution from grazed pastures. Recent research has suggested introducing low foliage N content species, such as plantain, into traditional ryegrass/white clover pastures as a potential solution.

Objective

This study aimed to evaluate the effect of lowering foliage N contents in pasture-based grazing systems on excretal N deposition from grazing animals and subsequent nitrous oxide (N₂O) emissions by using a combination of field measurements and process-based modelling. We further explored potential management options that could be beneficial for reducing N₂O emissions by changing the N content in feed for grazing animals.

Methods

We used eddy covariance measurements to parameterise carbon and water fluxes in the process-based model CenW. We estimated site-specific excretal emission factors, which were derived from continuous N₂O emission measurements using eddy covariance and modelled excretal N deposition from CenW, to simulate the effects of lowering foliage N content on N₂O emissions.

Results and conclusions

The model accurately captured carbon and water cycling at the site, with Nash-Sutcliffe model efficiencies ranging from 0.50 to 0.87. Decreasing foliage N content from 41 to 31 g N kg DM⁻¹ in pasture-based grazing systems reduced annual N₂O emissions by approximately 36 %. Modifying excretal N partitioning showed that increasing N excretion through dung rather than urine by 1 g N per kg dry matter intake reduced N₂O emissions by only 3.1 %.

Significance

Our findings demonstrate that management practices focusing on reducing urine N contents through the inclusion of feeds with lower N contents can be highly effective for mitigating N₂O emissions in grazed pastures, as urine N is the primary pathway for these emissions.