Nitrous oxide emission factors from an intensively grazed temperate grassland: A comparison of cumulative emissions determined by eddy covariance and static chamber methods

Nicholas Cowan,James Rambaud,Matthew Saunders,Karl G Richards,Dominika J Krol,Amanuel W Gebremichael,Rachael M Murphy,Gary J Lanigan

doi:10.1016/j.agee.2021.107725

Abstract

Quantifying nitrous oxide (N2O) emissions from grazed pastures can be problematic due to the presence of hotspots and hot moments of N2O from animal excreta and synthetic fertilisers. In this study, we quantified field scale N2O emissions from a temperate grassland under a rotational grazing management using eddy covariance (EC) and static chamber techniques. Measurements of N2O by static chambers were made for four out of nine grazing events for a control, calcium ammonium nitrate (CAN), synthetic urine (SU) + CAN and dung + CAN treatments. Static chamber N2O flux measurements were upscaled to the field scale (FCH FIELD) using site specific emission factors (EF) for CAN, SU+CAN and dung + CAN. Mean N2O EFs were greatest from the CAN treatment while dung + CAN and SU + CAN emitted similar N2O-N emissions. Cumulative N2O-N emissions over the study period measured by FCH FIELD measurements were lower than gap-filled EC measurements. Emission factors of N2O from grazing calculated by FCH FIELD and gap-filled were 0.72% and 0.96%, respectively. N2O-N emissions were derived mainly from animal excreta (dung and urine) contributing 50% while N2O-N losses from CAN and background accounted for 36% and 14%, respectively. The study highlights the advantage of using both the EC and static chamber techniques in tandem to better quantify both total N2O-N losses from grazed pastures while also constraining the contribution of individual N sources. The EC technique was most accurate in quantifying N2O emissions, showing a range of uncertainty that was seven times lower relative to that attributed to static chamber measurements, due to the small chamber sample size per treatment and highly variable N2O flux measurements over space and time.

Highlights

Nitrous oxide (N2O) is a potent greenhouse gas (GHG), with a global warming potential (GWP) 265 times higher than carbon dioxide (CO2), over a 100 year lifespan (Pachauri et al, 2014)
Soil temperature at 6 cm depth was greatest in June and lowest in January with values of 20.3 ◦C and 2.1 ◦C (Fig. 2a), respectively, which represented a warmer June and colder January compared to the 10 year mean for these months (Table 1A)
The N content of dung and urine is often unknown or is simulated using a constant N content to evaluate the effect of deposition timing on emis sions

Summary

Introduction

Nitrous oxide (N2O) is a potent greenhouse gas (GHG), with a global warming potential (GWP) 265 times higher than carbon dioxide (CO2), over a 100 year lifespan (Pachauri et al, 2014). The IPCCs default (Tier 1) EFs for mineral fertilisers (EF1) is 1% with an uncertainty range of 0.3–3%, and for urine and dung N deposition on pasture, range and paddocks by grazing animals (EF3PRP) is 2% with an uncertainty range of 0.7–6% (Eggleston et al, 2006). The IPCC has revised the default EF3PRP from 2% down to 0.6% (0–2.6%) and has disaggregated grazing EFs for dung at 0.13% (0–0.53%) and urine 0.77% (0.03–3.82%), as well as revising the EF1 at 1.6% (1.3–1.9%) in wet temperate climates (Buendia et al, 2019). Improvements in modelling N2O EFs would in turn avoid the burden of conducting dedicated measurement campaigns for estimating local EFs (Lopez-Aizpún et al, 2020)

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