Meta‐analysis of global livestock urine‐derived nitrous oxide emissions from agricultural soils

Abstract

Nitrous oxide (N2O) is an air pollutant of major environmental concern, with agriculture representing 60% of anthropogenic global N2O emissions. Much of the N2O emissions from livestock production systems result from transformation of N deposited to soil within animal excreta. There exists a substantial body of literature on urine patch N2O dynamics, we aimed to identify key controlling factors influencing N2O emissions and to aid understanding of knowledge gaps to improve GHG reporting and prioritize future research. We conducted an extensive literature review and random effect meta‐analysis (using REML) of results to identify key relationships between multiple potential independent factors and global N2O emissions factors (EFs) from urine patches. Mean air temperature, soil pH and ruminant animal species (sheep or cow) were significant factors influencing the EFs reviewed. However, several factors that are known to influence N2O emissions, such as animal diet and urine composition, could not be considered due to the lack of reported data. The review highlighted a widespread tendency for inadequate metadata and uncertainty reporting in the published studies, as well as the limited geographical extent of investigations, which are more often conducted in temperate regions thus far. Therefore, here we give recommendations for factors that are likely to affect the EFs and should be included in all future studies, these include the following: soil pH and texture; experimental set‐up; direct measurement of soil moisture and temperature during the study period; amount and composition of urine applied; animal type and diet; N2O emissions with a measure of uncertainty; data from a control with zero‐N application and meteorological data.

Global Research Alliance N2O chamber methodology guidelines: Recommendations for deployment and accounting for sources of variability

“…Different approaches also exist in calculating treatment EFs from static chambers with partial treatment coverage, with some researchers using only the wetted area and others the whole chamber area in calculations (López‐Aizpún et al., 2020; Mori & Hojito, 2015). Care must therefore be taken when comparing EFs between studies.…”

“…(2020). If the aim is to generate new N 2 O EFs, soil mineral N contents are usually recorded but may not be necessary (López‐Aizpún et al., 2020). A recommended minimum set of ancillary measurements for N 2 O EF studies would improve the potential for subsequent meta‐analyses (de Klein, Alfaro, et al., 2020; López‐Aizpún et al., 2020).…”

“…If the aim is to generate new N 2 O EFs, soil mineral N contents are usually recorded but may not be necessary (López‐Aizpún et al., 2020). A recommended minimum set of ancillary measurements for N 2 O EF studies would improve the potential for subsequent meta‐analyses (de Klein, Alfaro, et al., 2020; López‐Aizpún et al., 2020). If the goal is to understand temporal patterns in N 2 O emissions, or for model development or verification, then a wider range of (frequent) ancillary measurements are necessary (de Klein, Alfaro, et al., 2020; Dorich et al., 2020; Giltrap et al., 2020).…”

Optimising storage conditions and processing of sheep urine for nitrogen cycle and gaseous emission measurements from urine patches

“… 15 . Differing concentrations of minor constituents was posited to be the reason for lower sheep urine N 2 O emission factors than those from cattle urine, independently of the volume and N concentration of the urine 16 , but other factors (e.g. potassium ion [K + ] concentration or urine pH), for which insufficient data for meta-analysis were provided, could also have an effect.…”

Elucidating three-way interactions between soil, pasture and animals that regulate nitrous oxide emissions from temperate grazing systems

“…Arguably the most archetypical example in the present context is the application of synthetic N fertilisers to non-leguminous swards, a routine task performed across a large area of temperate grasslands ( Buckingham et al, 2014 ). Nevertheless, a recent review of 81 studies encompassing 254 unique urine treatments from around the world ( López-Aizpún et al, 2020 ) was unable to identify any single case where the dosage of synthetic fertiliser was determined to mirror local farmers’ practices (e.g. incorporating split applications) or, even more crucially, adjusted according to the presence or absence of N-fixing legumes in the sward ( Hoogendoorn et al, 2016 ).…”