Chemical and microbial determinants of N2O emission from denitrification in pasture soils

Abstract

Nitrous oxide (N2O) is a potent greenhouse gas and ozone depleter accounting for 6.2% of globally emitted CO2 equivalents. Denitrification, the microbial reduction of NO3->NO2->NO->N2O->N2, is a significant source of N2O in agricultural soils. The end product of the process (N2O or N2) is determined by the net balance of N2O production and consumption steps which are in turn sensitive to soil conditions. Therefore, mitigation of N2O emission from denitrification is dependent on our basic understanding of factors regulating the production and consumption of this gas. Here, I present results contrasting the potential biological and chemical causes of varied N2O emission potential in 20 New Zealand pastures soils. Soil N2O/N2O+N2 was determined by gas chromatography of anoxic soil incubations amended with NH4NO3 and compared correlatively to potential chemical or biological controls or directly through chemical amendments to soil incubations. Investigations revealed soil N2O/N2O+N2 was largely determined by the timing of N2O reduction: some soils concurrently produced and reduced N2O from the start of incubations, while others delayed N2O reduction until almost all added N was accumulated as N2O in incubation vial headspace. The later pattern is hypothesized to result in high N2O emissions in situ. Differential nitrite (NO2-) accumulation between alternative phenotype soils and responses to exogenous NO2- suggest high NO2- accumulation could account for delayed N2O reduction activity. However, the impact of carbon additions (Concurrent -> Sequential) and successive nitrate additions (Sequential -> Concurrent) suggest carbon availability and delayed N2O reductase synthesis could also be important driving factors. NO accumulation and inhibition is proposed as a potential proximal mechanism linking alternate drivers of soil denitrification phenotypes. In addition, correlations between changes in microbial community composition as measured by 16S rRNA gene sequencing and denitrification phenotypes suggest a potential distal cause for delayed N2O reduction activity but also highlight an ongoing issue with denitrification research: our inability to satisfactorily identify whether the correlations between soil microbial community composition and N2O emission potential reflect a true causative relationship. To address this, I utilized a soil extracted cell based experimental design to answer a fundamental question about denitrification in complex soil communities: Does microbial community composition really matter for a N2O emission outcome and if so, how much compared to chemical controls? Incubations of various combinations of soil extracted cells and water extractable chemical compoenents (e.g. carbon, pH) from contrasting high N2O/N2O+N2 and low N2O/N2O+N2 ratio soils suggested microbial community origin did indeed impact N2O/N2O+N2 ratio but impacts were smaller than that of extractable chemical components. In the absence of pH effects, predictable impacts of cell origin imply some generalizations can be made about what is a low N2O emitting community; low N2O/N2O+N2 soil cells or chemistry typically lowered incubation N2O/N2O+N2 and high N2O/N2O+N2 soil cells or chemistry vice versa. Serendipitously, these cell based incubations also provided evidence that low carbon availability can induce increased N2O emissions. Together, these investigations build evidence for a combined role of soil chemistry and microbial community composition as determinants of soil N2O emissions, support the increasing focus on microbial community composition in denitrification research and imply both soil chemistry and microbial community composition must be considered in prediction and management of soil N2O emissions in the future.