Paddock scale nitrous oxide emissions from intensively grazed pasture: Quantification and mitigation

Abstract

Agricultural soils are the main contributor to global emissions of the potent greenhouse gas nitrous oxide (N₂O) to the atmosphere. Nitrous oxide can absorb and transform radiative energy emitted from the sun to earth into heat. The ability of N₂O to absorb energy is part of the greenhouse effect and naturally contributes to habitable temperature conditions for life on earth. Atmospheric concentrations of N₂O have never before increased as rapidly as observed since the onset of industrialisation. High N₂O concentrations enhance the greenhouse effect and, at present, add to a warming atmosphere and a changing climate. A major pathway for N₂O production is from soils that receive large surpluses of reactive nitrogen, e.g., in the form of animal excreta or fertiliser. Once available in the soil, microbes can access and transform this nitrogen into different compounds: N₂O, for example. The current contribution of agricultural soils to global N₂O emissions is about 60% but higher in New Zealand, where soils are responsible for 94% of national N₂O emissions. However, robust quantifications are not straightforward and challenging for science to overcome. This thesis is sited at the interface of this challenge and aimed at measuring the exchange of N₂O over an intensively grazed dairy pasture in New Zealand. The core of the work was based on eddy covariance (EC), a technique that allows measuring the N₂O exchange between the soil and the atmosphere at ten times per second. Since EC measurements of N₂O over dairy grazed land are rare, main objectives of this work were: to 1) compare EC to traditional measurement approaches, i.e. static chambers, 2) quantify annual N₂O emission budgets and 3) determine the effect of farm management including pasture renewal on N₂O emissions. The findings of this thesis show that EC provided a realistic picture of the N₂O exchange at paddock scales. This means EC data identified cattle grazing, land history, farm management, soil moisture and temperature as the main factors controlling N₂O exchange throughout the year. Interestingly, using one EC system for measurements across adjacent but differently managed paddocks also allowed to compare the effect of different pasture management scenarios on N₂O emissions. This will help researchers to test different mitigation options more easily in the future, ideally, to reduce N₂O emissions and avoid further atmospheric warming. This thesis advances our understanding of the N₂O exchange process from intensively grazed land as well as it shows how limited this knowledge still is. Micrometeorological N₂O flux measurements will have to be used more frequently in the future to provide researchers with answers in the face of a rapidly changing climate on earth.