Influence of vegetation spp. on nitrous oxide production and reduction.

The overarching aim of this thesis was to explore how vegetation spp. influence potent nitrous oxide (N2O) production and reduction to harmless dinitrogen (N2) in the soil. In the rhizosphere, organic carbon (C) exuded from plant roots (e.g. rhizodeposits), and recalcitrant soil organic matter (OM) is transformed during catabolic processes to fuel most heterotrophic organisms including in soil denitrification. C is known to drive soil denitrification, but knowledge on whether the more active microbial community would select for C derived from plant roots against soil OM pool is limited. Experiments were conducted targeting vegetation with distinct traits from two land use scenarios. In a glasshouse experiment, two rice cultivars were grown in two agricultural soils of contrasting soil OM contents with C and N stable isotopes applied at three growth stages to quantify plant-derived C influence on N2O production and reduction processes. The relationship between plant-derived C and nitrate reducers’ N2O and N2 was generally positive but were stronger during flowering and maturity growth stages especially in low OM soil. The 15N-N2O/(N2O+N2) product ratio was low (<0.10), indicating a high nitrate reducing condition and was associated with high plant-derived C input when N availability was low due to N demand during flowering and maturity stage. Also, N2O production was lowered in low OM soil, but the N2O reduction to N2 was enhanced in high OM soil when planted with the late-flowering and high yielding cultivar. In the girdling experiment, two tropical trees spp. from different genera were girdled, and the soil was treated with N stable isotope. Girdling of the two trees were conducted to stop the C transfer from photosynthesis to root by removing a ring-section of the phloem. The 15N-N2O/(N2O+N2) product ratio was also low (<0.10) for the girdling experiment indicating a high nitrate reducing condition. A positive relationship was observed between CO2 emissions and nitrate reducers’ N2O production when trees were not girdled. However, the N2O production was lowered when the tree C flow for fast-growing pioneer spp. was cut off while girdling did not impact N2O production for the old-growth tree genera. Overall, it was demonstrated that plant direct and indirect influence on N2O production and reduction to N2 varied between crop/trees, growth strategies/stages and the level of soil OM-C and –N. The information obtained from this study is crucial for the development of strategies to mitigate N2O emissions through soil OM management and selection of plant species or cultivars in rice cultivation and understanding the impact of tropical forest disturbance (i.e. girdling) on N cycle.