Clays could underpin a viable agricultural greenhouse gas (GHG) abatement technology given their affinity for nitrogen and carbon compounds. We provide the first investigation into the efficacy of clays to decrease agricultural nitrogen GHG emissions (i.e., NO and NH). Via laboratory experiments using an automated closed-vessel analysis system, we tested the capacity of two clays (vermiculite and bentonite) to decrease NO and NH emissions and organic carbon losses from livestock manures (beef, pig, poultry, and egg layer) incorporated into an agricultural soil. Clay addition levels varied, with a maximum of 1:1 to manure (dry weight). Cumulative gas emissions were modeled using the biological logistic function, with 15 of 16 treatments successfully fitted ( < 0.05) by this model. When assessing all of the manures together, NH emissions were lower (×2) at the highest clay addition level compared with no clay addition, but this difference was not significant ( = 0.17). Nitrous oxide emissions were significantly lower (×3; < 0.05) at the highest clay addition level compared with no clay addition. When assessing manures individually, we observed generally decreasing trends in NH and NO emissions with increasing clay addition, albeit with widely varying statistical significance between manure types. Most of the treatments also showed strong evidence of increased C retention with increasing clay additions, with up to 10 times more carbon retained in treatments containing clay compared with treatments containing no clay. This preliminary assessment of the efficacy of clays to mitigate agricultural GHG emissions indicates strong promise.

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The concentration of the potent greenhouse gas nitrous oxide (N2O) in the Earth's atmosphere is increasing. The main reason is agricultural activity, especially the application of nitrogenous fertilisers and animal manures to soils. In tropical and subtropical climates, N2O emissions from fertilised soils can be particularly high, however, there is considerable uncertainty in N2O estimates as data coverage is poor. The research presented here aimed to close this knowledge gap and investigate strategies for abating N2O emissions from agricultural soils. This thesis focused on animal manures because (i) intensive livestock production is expanding in tropical and subtropical regions, and (ii) manures from intensive animal production are increasingly considered as alternative nutrient sources for crops. N2O mitigation strategies were evaluated by amending soils with geological or plant-derived materials that have shown potential to decrease N2O emissions from agricultural soil in previous research but have not been studied extensively. Experiments across different spatial scales (microcosm to field) have been conducted to investigate the transferability of observed effects.In the first study (Chapter 3), manures were applied to the soil surface as a simulation of no-till farming practice using microcosm systems. It was hypothesised that adding bentonite, a clay with ion adsorption capacity, would decrease N2O emissions from manures. Blends of bentonite with beef, pig or poultry layer manure were applied to three different soils from South East Queensland and N2O fluxes were quantified over three weeks. Contrary to expectations, blending bentonite with manures tended to increase N2O emissions. This observation was interpreted as the combined effect of increased moisture content at the soil surface caused by the strong water binding capacity of bentonite and decreased oxygen concentrations in the soil due to restricted gas exchange. In the following studies, bentonite was therefore incorporated into the soil rather than applied to the surface.The second study (Chapter 4) examined the effect of bentonite, biochar or green waste compost additions on N2O emissions from poultry litter at a commercial sugarcane farm. Sugarcane was chosen because it is a major crop in Queensland and globally and N2O emissions from sugarcane soils can be high. Poultry litter and blends of poultry litter were applied subsurface along sugarcane rows. Over ten months, fluxes of N2O, and also CO2 and CH4 were quantified with static chambers.The early phase of the field experiment was simulated in a parallel laboratory experiment. Biochar addition to poultry litter decreased N2O emission in the field (-36%) and in the laboratory (-18%) compared to poultry litter only application. Bentonite addition decreased N2O emissions from poultry litter in the field (-16%) but increased N2O emissions in the laboratory (+8%). Differences in soil aeration caused by bentonite's swelling-shrinking characteristics are the likely cause for 3 these opposite...