|dc.description.abstract||Nitrous oxide (N₂O) is problematic as it is a potent greenhouse gas with a global warming potential about 298 times that of carbon dioxide, and it also contributes to the depletion of stratospheric ozone. The use of nitrification inhibitors to reduce nitrification, and consequently N₂O emissions, has been extensively researched. Much of this work was carried out using synthetic inhibitors and these inhibitors have proved to be very effective. Some plants also have the ability to inhibit nitrification through release of secondary metabolites produced in plant tissues. This is termed biological nitrification inhibition (BNI). Brassica crops are considered plants with potential biological nitrification inhibitors, as they contain glucosinolates (GLS) whose hydrolysis products (e.g. isothiocyanates and nitriles) have been shown to reduce soil nitrification. Brassicas may, therefore, provide a practical forage based tool for mitigating nitrification and N₂O emissions. The aim of this study was to determine whether brassicas and GLS hydrolysis products inhibited nitrification, and whether N₂O emissions were reduced as a result of that inhibition. This aim was achieved through a series of laboratory incubations and a field trial that sequentially tested whether GLS hydrolysis products, brassicas, or urine derived from cows fed brassicas, reduced nitrification and N₂O emissions.
The first study was a laboratory incubation where urea (600 µg N g⁻¹ soil) along with a selection of GLS hydrolysis products (at 2 rates: 30 and 60 µg N g⁻¹ soil) were applied to soil. Ammonia oxidising bacteria populations, soil mineral N concentrations, N₂O emissions and soil respiration were monitored throughout the 40 day incubation. The results showed that some GLS hydrolysis products inhibited soil nitrification and reduced N₂O emissions by up to 51%. The effective products identified in the laboratory study were then tested in a field using a small plot trial. Artificial urine (600 kg N ha⁻¹; 10 L m²) and GLS hydrolysis products (60 kg ha⁻¹) were applied to the plots, and N mineralisation and N₂O emissions were measured. No inhibition of nitrification or reduction in N₂O emissions were observed in the field study. In the laboratory study, there was evidence that the reduction in N₂O emissions was a result of inhibition of nitrification, however, the results suggested that the inhibition by GLS hydrolysis products was not strong and was short-lived. Multiple applications may therefore be required to achieve a meaningful reduction in N₂O emissions from urine affected soil.
The second study examined whether brassica tissues incorporated into soil inhibited nitrification and reduced N₂O emissions. A laboratory incubation was conducted where three types of brassica tissue and ryegrass (as a control) were incorporated into soil with urea added (600 µg N g⁻¹ soil). Ammonia oxidising bacteria populations, soil mineral N concentrations, N₂O emissions and soil respiration were monitored throughout the 52 day incubation. The results showed that incorporation of brassica tissues reduced urea-derived N₂O emissions, relative to ryegrass tissues, by up to 62%. However, there was no evidence that this reduction was a result of inhibition of nitrification, rather it was likely due to the difference in labile C provided by the different plants. While there was a reduction in urea-derived N₂O emissions, total N₂O emissions increased as a result of tissue incorporation into soil and this trade-off must be investigated if brassica tissues are considered as an option for N₂O reduction.
The final study examined whether the BNI capacity of GLS hydrolysis products was transferred into the urine of cows grazing brassica crops and subsequently reduced N₂O emissions from the deposited urine patch. A secondary objective was to determine if soil growing brassica crops contained BNI compounds that decreased N₂O emissions following addition of urine. These were addressed in a laboratory incubation where urine (600 kg N ha⁻¹) derived from animals fed on pasture or a kale crop, was applied to soils growing either pasture or a kale crop. Ammonia oxidising bacteria populations, soil mineral N concentrations, N₂O emissions and soil respiration were monitored throughout the 60 day incubation. Urine from cows fed kale did not show decreased N₂O emissions compared to urine from cows fed on pasture when applied to soil. N₂O emissions were higher from the kale-cropped soil than the pasture soil, which was attributed to the higher fertility status of the cropped soil. Overall, these results provided no evidence that feeding kale to grazing animals reduced nitrification rates or N₂O production following urine inputs to soil.
In summary, this study showed that GLS hydrolysis products exhibit BNI capacity when applied directly to soil which is in agreement with other published studies. Glucosinolate hydrolysis products also reduced N₂O emissions when applied to soil with urea. There was no evidence that BNI capacity remained following decomposition of brassica tissues incorporated into soil or when brassica derived cow urine was applied to soil. Other literature investigating the incorporation of brassica tissues into soil generally supports BNI activity, however, those studies have all been under low N conditions. Although urea-derived N₂O emissions were reduced following incorporation of brassica tissues into soil, this reduction in emissions could not be attributed to inhibition of nitrification. The impact of brassica fed urine on soil nitrification and N₂O emissions remains inconclusive. Further work examining the mechanism by which GLS hydrolysis products inhibit nitrification, and how that may be exploited, is required to determine whether GLS hydrolysis products or brassicas may be a useful N₂O mitigation tool.||