N cycling and microbial dynamics in pasture soils

Abstract

Pasture soils are a significant source of the greenhouse gas, nitrous oxide (N2O) and as such they contribute to global warming. It has been reported that N2O is approx. 300 times more potent than carbon dioxide (CO2) as a greenhouse gas. Thus, understanding the mechanisms for controlling N2O emissions from soil is key to developing new soil management strategies to counter or prevent climate change throughout the world. Despite this, very little is known about the key regulators of production and consumption of N2O in pasture soils, especially under urine patch conditions. To address this, we used pasture soils representing both Northern (Ireland) and Southern (New Zealand) Hemispheres in experiments designed to understand both phenotypic and genotypic characteristics associated with N2O emissions. We used a combination of gas kinetics, soil physicochemical characterization, metagenomics, 16S amplicon sequencing and quantitative PCR (of denitrifier: nirS, nirK, nosZI and nosZII; and nitrifier: bacterial and archaeal amoA genes) to link physical, chemical and biological parameters associated with emissions. This thesis work was able to show how in nitrate-amended pasture soils the rate of carbon mineralization under oxic and anoxic conditions is positively linked to the rate of denitrification. In addition, the emission ratio of N2O is negatively linked to pH. Both pH and N2O emission ratio were significantly associated with 16S microbial community composition as well as microbial richness. This result confirms that pH imposes a general selective pressure on the entire community and that this is associated with changes in emission potentials. This supports the general ecological hypothesis that with increased microbial diversity, efficiency of N2 production increases (i.e. more efficient conversation of N2O to N2). Worked performed in a simulated urine patch (oxic conditions) suggested other pathway (e.g., nitrifier-denitrification) as a source of N2O emissions. No clear trend was observed between emission ratio of N2O under urine patch condition and emission ratio under true denitrification conditions (i.e. under anoxic environment). The urine patch accelerated the rate of C mineralization about 10 times, concurrent with a decrease in prokaryotic richness and a shift in community composition. Community response identified two major groups of responders: negatively affected prokaryotes we hypothesized utilized energy from N-linked redox reaction for maintenance and positively responding populations that use this energy for growth. Overall, this study provides new insights into the N2O emissions and microbial dynamics for reduction of N2O in pasture soils.