Mitigating N2O emissions from pasture soils by optimising irrigation scheduling based on relative gas diffusivity : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

It is projected that the World’s average temperature could rise by 0.2 oC over each of the next two decades, and by 1.1 up to 6.4 oC this century. Grazed pasture irrigation practices play a vital role in meeting the global food demand of a growing population, are keys for the efficient use of water, and conjointly, could be used to meet the climate change targets by decreasing greenhouse gas emissions levels. Grazed pasture systems are a significant source of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting-substance. In pastures, N2O emissions are driven by changes in soil inorganic-N substrate supply following urine and/or fertiliser inputs. Increasing soil moisture as a result of irrigation and rainfall events potentially increases N2O production via nitrifier-denitrification and denitrification, since these are hypoxic and anaerobic processes, respectively, and increasing soil moisture reduces oxygen (O2) supply. Net surface emissions rely on subsurface gas transfer which is controlled mainly by diffusion. Soil relative gas diffusivity (Dp/Do) is the predominant parameter that describes O2 transport in soils and has been shown to be a promising integrator of soil physical conditions to better understand and mitigate N2O emissions. The value of Dp/Do is a function of the soil air content and gaseous phase tortuosity, both of which vary with soil physical properties including soil texture, soil structure and water content. The principal outcomes of this project were: • The confirmation that Dp/Do can be used as a useful measure to predict soil physical conditions where N2O emissions occur and that recent observations of a critical diffusivity window under 0.02, and especially close to 0.006, need to be avoided when managing pastures if excessive N2O emissions are to be prevented. • Soil texture and structure were shown to have an effect on drainage rate which in turn directly affected Dp/Do, and thus N2O emissions following irrigation events. • Optimised irrigation minimised soil anaerobic conditions when compared to standard irrigation, identified through higher soil Dp/Do, which, in turn, was shown to increase plant growth and N uptake in a lysimeter study. • Lower N2O emissions were also measured under optimised irrigation in field trials, but conversely, higher emissions were observed in the lysimeter study with optimal irrigation. It was hypothesised that this may have resulted from the “Birch effect” being enhanced due to rewetting after extended drying periods (reduced in the standard irrigation) that in turn could promote hypoxia and N2O generating processes. The first laboratory experiment (Chapter III) examined the consistency of Dp/Do across different soil types to predict peaks of N2O emissions. It was hypothesised that regardless of soil type, the N2O emissions would peak at the previously reported Dp/Do value of 0.006. Increasing soil bulk density and soil matric potential caused Dp/Do to decline. As Dp/Do declined to a value of 0.006 N2O fluxes increased, peaking at a Dp/Do value of ≤ 0.006. This experiment shows that the elevation of N2O fluxes as a Dp/Do threshold of 0.006 is approached, holds across soil types. However, the variability in the magnitude of the N2O flux as Dp/Do declines is not explained by Dp/Do and is likely to be dependent on factors affecting the N2O:(N2O+N2) ratio. In Chapter IV, the effect of successive wetting-drainage cycles on both Dp/Do dynamics and associated N2O and N2 emissions in two soils; a pallic silt loam and an allophanic sandy-silt, with the later also having a higher organic matter content was determined. For both soils each wetting-drainage cycle induced N2O fluxes but with 5-fold lower fluxes in the allophanic soil. Greater aggregation and sand content in the allophanic soil generated higher porosity and Dp/Do values that were almost always greater than recognised anaerobic limits (Dp/Do < 0.02). While wetting-drainage events induce N2O emissions by altering Dp/Do and the soil aeration status, the drainage of soils, especially soils high in organic matter, may enhance O2 demand generating anaerobic zones conducive to denitrification. Low N2 emissions were potentially due to pH and high nitrate concentration effects in the pallic soil. Chapter V evaluated the potential for managing soil N2O emissions by altering irrigation timing based upon modelled Dp/Do using repacked lysimeters, sown with perennial ryegrass (Lolium perenne), that received either a standard irrigation treatment (15 mm every three days), or an optimised irrigation treatment where irrigation was applied when soil Dp/Do was 0.085 (equivalent to 50% of plant available water) and top up below Dp/Do = 0.02 (based on the previous results). Emission factors over the first 39 days of the experiment for optimised and standard irrigation treatments with urine were 1.26 and 0.12%, respectively, with an increase of N2O emissions in the optimised irrigation possibly due to the “Birch effect”. Cumulative pasture dry matter production and N uptake were higher in the optimised irrigation treatment. Macropores flow of urine derived N was lower under optimised irrigation. Finally, Chapter VI re-evaluated the effect of irrigation scheduling (standard and optimised), using soil gas diffusivity as a decision tool to mitigate N2O emissions, in the field using automatic chambers. The optimised irrigation treatment was favoured as there was a significant decrease in cumulative N2O emissions compared to the standard irrigation. Despite growth rates lower than the summer average for fertilised irrigated pastures, dry matter production for both irrigation treatments with urine were not significantly different. It was concluded that Dp/Do appears to be a relevant parameter to predict N2O emissions and future implementation into models is conceivable. Moreover, this study demonstrates the need to adjust irrigation cycles to prevent excessive drying of the soil profile between water events while anticipating the rain events and keeping Dp/Do > 0.02. Further detailed studies are needed to: (i) examine the interaction between soil structure and soil organic matter content and their effect on N2O emissions under wetting-drainage events, with measures of soil O2 (ii) consider avoiding intra-aggregate disruption through irrigation management which might minimise the Birch effect while also helping to minimise N2O emissions through reducing the increase of C supply for denitrifiers.