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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.

Most nitrogen (N) lost to the environment from grazed grassland is produced as a result of N excreted by livestock, released in the form of nitrous oxide (N2O) emissions, nitrate leaching and ammonia volatilisation. In addition to the N fertiliser applied, excreta deposited by grazing livestock constitute a heterogeneous excess of N, creating spatial hotspots of N losses. This study presents a yearlong N2O emissions map from a typical intensively managed temperate grassland, grazed periodically by a dairy herd. The excreta deposition mapping was undertaken using high-resolution RGB images captured with a remotely piloted aircraft system combined with N2O emissions measurements using closed statics chambers. The annual N2O emissions were estimated to be 3.36 ± 0.30 kg N2O–N ha−1 after a total N applied from fertiliser and excreta of 608 ± 40 kg N ha−1 yr−1. Emissions of N2O were 1.9, 3.6 and 4.4 times lower than that estimated using the default IPCC 2019, 2006 or country-specific emission factors, respectively. The spatial distribution and size of excreta deposits was non-uniform, and in each grazing period, an average of 15.1% of the field was covered by urine patches and 1.0% by dung deposits. Some areas of the field repeatedly received urine deposits, accounting for an estimated total of 2410 kg N ha−1. The method reported in this study can provide better estimates of how management practices can mitigate N2O emissions, to develop more efficient selective approaches to fertiliser application, targeted nitrification inhibitor application and improvements in the current N2O inventory estimation.

The deposition of livestock urine and feces in grazed fields results in a sizable input of available nitrogen (N) in these soils; therefore significantly increasing potential nitrogen pollution from agricultural areas in the form of nitrous oxide (N2O), ammonia (NH3), and nitrate (NO3−). Livestock deposition events contributes to high spatial variability within the field and generate uncertainties when assessing the contribution that animal waste has on nitrogen pollution pathways. This study investigated an innovative technique for identifying the spatial coverage of urine deposition in grasslands without the need for manual soil measurements. A Remotely Piloted Aircraft System (RPAS) using a twin camera system was used to identify urine patches in a 5 ha field, which had been grazed by sheep 3 weeks previous to measurements. The imagery was processed using Agisoft Photoscan (Agisoft LLC) to produce true and false color orthomosaic imagery of the entire field. Imagery of five areas (225 m2) within the field were analyzed using a custom R script. For a total of 1,125 m2 of grassland, 12.2% of the area consisted of what was classified as urine patch. A simple up-scaling method was applied to these data to calculate N2O emissions for the entire field providing an estimate of 1.3–2.0 kg N2O-N ha−1 emissions from urine and fertilizer inputs.

peer-reviewed, Soil plays a key role in the global carbon (C) cycle. Most current assessments of SOC stocks and the guidelines given by Intergovernmental Panel on Climate Change (IPCC) focus on the top 30 cm of soil. Our research shows that, when considering only total quantities, most of the SOC stocks are found in this top layer. However, not all forms of SOC are equally valuable as long-term stable stores of carbon: the majority of SOC is available for mineralisation and can potentially be re-emitted to the atmosphere. SOC associated with micro-aggregates and silt plus clay fractions is more stable and therefore represents a long-term carbon store. Our research shows that most of this stable carbon is located at depths below 30 cm (42% of subsoil SOC is located in micro-aggregates and silt and clay, compared to 16% in the topsoil), specifically in soils that are subject to clay illuviation. This has implications for land management decisions in temperate grassland regions, defining the trade-offs between primary productivity and C emissions in clay-illuviated soils, as a result of drainage. Therefore, climate smart land management should consider the balance between SOC stabilisation in topsoils for productivity versus sequestration in subsoils for climate mitigation., PUBLISHED, peer-reviewed

Soil plays a key role in the global carbon (C) cycle. Most current assessments of SOC stocks and the guidelines given by Intergovernmental Panel on Climate Change (IPCC) focus on the top 30 cm of soil. Our research shows that, when considering only total quantities, most of the SOC stocks are found in this top layer. However, not all forms of SOC are equally valuable as long-term stable stores of carbon: the majority of SOC is available for mineralisation and can potentially be re-emitted to the atmosphere. SOC associated with micro-aggregates and silt plus clay fractions is more stable and therefore represents a long-term carbon store. Our research shows that most of this stable carbon is located at depths below 30 cm (42% of subsoil SOC is located in microaggregates and silt and clay, compared to 16% in the topsoil), specifically in soils that are subject to clay illuviation. This has implications for land management decisions in temperate grassland regions, defining the trade-offs between primary productivity and C emissions in clay-illuviated soils, as a result of drainage. Therefore, climate smart land management should consider the balance between SOC stabilisation in topsoils for productivity versus sequestration in subsoils for climate mitigation.

peer-reviewed, Soil plays a key role in the global carbon (C) cycle, since it holds 3.5 times more C than the atmosphere, and is the largest C pool after the oceans. Therefore, changes in soil organic carbon (SOC) can contribute to important emissions or sequestration of CO2 from the atmosphere. Grassland soils have are one of the world biomes with a greater C sequestration potential. More than 46% of SOC can be found below 30 cm, and the dynamics and factors affecting subsoil SOC are different than the topsoil. The aim of this thesis is to physically and biochemically characterise SOC in topsoil and subsoil of Irish grasslands, up to 1 m depth, and to understand if there is a soil type effect on SOC sequestration potential. A subset of soils sampled for National Soil Survey project (the Irish Soil Information System) in 2012/2013 were selected, with the aim of being representative of the main soil types occurring under grassland systems in Ireland. For each horizon, a 1 kg sample was taken, and several analysis were carried out: 1) separation of four aggregate sizes (Large and small macroaggregates, microaggregates and silt and clay), 2) isolation of fractions within small macroaggregates (particulate organic matter, microaggregates and silt and clay within macroaggregates), and 3) biochemical characterisation: non-hydrolysable C and N, as a measure of the biochemically recalcitrant fraction, and hot water extractable C and N, as a measure of the most labile fraction. Topsoil only showed differences between soil types in the case of percentage of microaggregates within macroaggregates, which was higher in Typical Sufrace-water gley than in Typical Brown Earth, while all the other analysed characteristics were equal between soil types. Subsoil horizons showed significant differences in many of the fractions. Differences in SOC characteristics were mainly influenced by clay content and stagnation properties.

peer-reviewed, Soil plays a key role in the global carbon (C) cycle, since it holds 3.5 times more C than the atmosphere, and is the largest C pool after the oceans. Therefore, changes in soil organic carbon (SOC) can contribute to important emissions or sequestration of CO2 from the atmosphere. Grassland soils have are one of the world biomes with a greater C sequestration potential. More than 46% of SOC can be found below 30 cm, and the dynamics and factors affecting subsoil SOC are different than the topsoil. The aim of this thesis is to physically and biochemically characterise SOC in topsoil and subsoil of Irish grasslands, up to 1 m depth, and to understand if there is a soil type effect on SOC sequestration potential. A subset of soils sampled for National Soil Survey project (the Irish Soil Information System) in 2012/2013 were selected, with the aim of being representative of the main soil types occurring under grassland systems in Ireland. For each horizon, a 1 kg sample was taken, and several analysis were carried out: 1) separation of four aggregate sizes (Large and small macroaggregates, microaggregates and silt and clay), 2) isolation of fractions within small macroaggregates (particulate organic matter, microaggregates and silt and clay within macroaggregates), and 3) biochemical characterisation: non-hydrolysable C and N, as a measure of the biochemically recalcitrant fraction, and hot water extractable C and N, as a measure of the most labile fraction. Topsoil only showed differences between soil types in the case of percentage of microaggregates within macroaggregates, which was higher in Typical Sufrace-water gley than in Typical Brown Earth, while all the other analysed characteristics were equal between soil types. Subsoil horizons showed significant differences in many of the fractions. Differences in SOC characteristics were mainly influenced by clay content and stagnation properties.

peer-reviewed, Soil plays a key role in the global carbon (C) cycle, since it holds 3.5 times more C than the atmosphere, and is the largest C pool after the oceans. Therefore, changes in soil organic carbon (SOC) can contribute to important emissions or sequestration of CO2 from the atmosphere. Grassland soils have are one of the world biomes with a greater C sequestration potential. More than 46% of SOC can be found below 30 cm, and the dynamics and factors affecting subsoil SOC are different than the topsoil. The aim of this thesis is to physically and biochemically characterise SOC in topsoil and subsoil of Irish grasslands, up to 1 m depth, and to understand if there is a soil type effect on SOC sequestration potential. A subset of soils sampled for National Soil Survey project (the Irish Soil Information System) in 2012/2013 were selected, with the aim of being representative of the main soil types occurring under grassland systems in Ireland. For each horizon, a 1 kg sample was taken, and several analysis were carried out: 1) separation of four aggregate sizes (Large and small macroaggregates, microaggregates and silt and clay), 2) isolation of fractions within small macroaggregates (particulate organic matter, microaggregates and silt and clay within macroaggregates), and 3) biochemical characterisation: non-hydrolysable C and N, as a measure of the biochemically recalcitrant fraction, and hot water extractable C and N, as a measure of the most labile fraction. Topsoil only showed differences between soil types in the case of percentage of microaggregates within macroaggregates, which was higher in Typical Sufrace-water gley than in Typical Brown Earth, while all the other analysed characteristics were equal between soil types. Subsoil horizons showed significant differences in many of the fractions. Differences in SOC characteristics were mainly influenced by clay content and stagnation properties.