Ammonia oxidising microbial communities and relationships with nitrous oxide emissions as affected by different land uses

Abstract

Nitrous oxide (N₂O) emissions in New Zealand largely come from agriculture through the processes of nitrification and denitrification as a part of the nitrogen cycle. Ammonia oxidising bacteria (AOB) and ammonia oxidising archaea (AOA) are the main drivers of ammonia oxidation (the first step of nitrification) in soil thus they affect N₂O emissions. It is thought that different land uses and land use change can impact on ammonia oxidising (AO)-populations and N₂O emissions, however, the influence of different land uses on AO-populations and N₂O emissions are not fully understood. Therefore the objectives of this thesis were to: 1) Determine the impact of three different land uses (pine tree plantation, dairy farming and sheep farming) in the same geographic location with the same soil type on the abundance of AOB and AOA; 2) Determine the effect of urine application on AO community abundance in the three soils; and 3) Determine the effect of urine application on N₂O emissions from soils from the three different land uses. Two projects were conducted to assess the effects of different land uses and the effect of urine addition on AO communities and N₂O emissions. In Project 1, AO-populations in the Templeton silt loam soil under three different land uses were analysed. The soils were collected from three adjacent sites: pine tree plantation, dairy farm and sheep farm. The results showed that AOB abundance was higher in the dairy (P < 0.05) and sheep (P < 0.05) farming soils than in the pine tree soil. When the AOB and AOA abundance was compared at each site, AOB abundance was higher than AOA abundance in the dairy farming soil (P < 0.05), however, AOA abundance was higher than AOB abundance in the sheep farming soil (P < 0.05). In addition, the AOA abundance in the pine tree soil was lower than that in the dairy (P < 0.001) and sheep (P < 0.05) pasture soils. These results support the the hypothesis that AOB prefer higher N environments whilst AOA prefer lower N environments. However, the reason for the lower AOA abundance in the pine tree soil compared to the dairy and sheep farm soils requires further research. Project 2 was an incubation study, where cow urine was applied to the three different soils and incubated at 20°C for 126 days in jars for gas sampling and in pottles for soil sampling. The N₂O emission trends in the urine-treated dairy and sheep farm soils were similar. They reached a peak shortly after urine application and then decreased rapidly to almost background levels afterwards. However, N₂O emissions in the urine-treated pine tree soil increased gradually and reached a peak at a much delayed time at day 91. The total amount of N₂O emitted from the urine-treated pine tree soil (P < 0.05) was the highest of the three soils investigated. A large amount of available carbon in the urine-treated pine tree soil was probably the main reason for the high N₂O emission. The results from the analysis of microbial populations also supported the hypothesis that urine application will increase AOB abundance, but not that of AOA. AOB abundance in the urine-treated dairy and sheep farm soil increased and reached peak abundance at day 60. The rapid increase in AOB abundance in these two soils following urine application probably reflected the influence of past land use history on the AO communities where the AOB populations had adapted to the higher nitrogen environment in these two land uses where nitrogen is applied as fertilisers and animal excreta returns. AOB abundance in the urine-treated pine tree soil increased more slowly but continued for an extended period until the end of the incubation study. These results suggest that land use history can have a major effect on AO microbial population abundance and these effects have implications on N₂O emissions.