Effect of soil aggregate size and pH on nitrous oxide emissions, ammonia oxidising communities and DCD effectiveness in a grazed pasture.

Agricultural soils have been identified as the main anthropogenic source of nitrous oxide (N₂O) emissions. N₂O is produced microbially through the processes of denitrification and nitrification as part of the nitrogen cycle. Soil properties can amplify N₂O emissions by creating a favourable environment for N₂O production or by altering microbial pathways. However, the impacts of soil properties such as aggregate size and soil pH on N₂O emissions have yet to be fully understood. Thus the objectives of this research were to: 1) quantify N₂O emissions from a grazed pasture soil with different soil aggregate sizes and soil pH; 2) determine the effectiveness of the nitrification inhibitor DCD in reducing N₂O emissions from a soil over a range of soil aggregate sizes and pH values; and 3) determine changes in ammonia oxidising bacteria (AOB) and ammonia oxidising archaea (AOA) abundance as affected by soil aggregate size and pH. An incubation trial and field trial was carried out to assess the effects of soil aggregate size and soil pH, respectively, on N₂O emissions, ammonia oxidising communities and DCD effectiveness. For the incubation trial a Temuka clay loam soil was sieved to produce three aggregate sizes: large (4-5.6 mm), medium (2-4 mm) and small (1-2 mm). These aggregate sized soils were iii incubated at 10˚C for 397 days in gas sampling jars and soil sampling tubes. Temporally, N₂O emissions were different, with higher peak N₂O emissions seen in the large and medium aggregates. However, high N₂O emissions after day 66 from the small aggregates meant that total emissions were not significantly different between aggregate sizes. Increased N₂O emissions after day 66 from the small aggregates are thought to be caused by greater aggregate instability causing aggregate disruption and a release of previously unavailable carbon. Ammonia oxidising communities were not affected by aggregate size, and DCD was effective in all aggregate size treatments, reducing N₂O emissions by an average of 79%. The field trial was established at Lincoln University in a Temuka clay loam soil. The soil pH was altered using HCl for the ‘acidic’ pH plots (pH 6). Water was used for the control pH plot and refered to as the ‘native’ pH soil. Total N₂O emissons were significanly higher in the acidic pH soil compared to the native and basic pH soils. This is hypothesised to have been caused by inhibition of the N₂O-reductase enzyme in the denitrification pathway. Ammonia oxidising microbes were affected by soil pH with AOB amoA gene copy numbers increasing in the basic pH soil and AOA amoA gene abundance increasing in the acidic pH soil. The addition of urine enhanced AOB growth and inhibited AOA growth. This supports the previous reseach that AOA prefer low nutrient, low pH environments whilst AOB prefer high N concentrated soil. DCD was most effective in the acidic pH soil reducing total N₂O emissions by 64%