"Nitrous oxide (N2O) is a potent greenhouse gas with a global warming potential 3003030 times higher than CO2. As the primary source of reactive nitrogen oxides (NOx) in the stratosphere, N2O also depletes stratospheric ozone. N2O concentrations in the atmosphere are increasing rapidly, primarily due to agricultural activity. Nitrification, an autotrophic process that converts ammonia (NH3) into nitrite (NO2-) and nitrate (NO3-), and denitrification, a heterotrophic process that reduces NO3- into NO, N2O and N2, are the two major processes leading to N2O emissions. Nitrification has been reported to dominate N2O emissions from agricultural soils under aerobic conditions.Ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) are the two main taxa involved in nitrification. Both AOA and AOB are capable of producing N2O, but their relative importance in nitrification is still largely unknown. In this dissertation I address three nitrification knowledge gaps: 1) Importance: what is the contribution of nitrification versus other microbial processes for producing N2O in systems under different management intensities (Chapter 2)? 2) Ecology: can high NH4+ inputs induce niche differentiation between AOA and AOB (Chapter 3)? 3) Complexity: how do plants mediate N2O emissions from AOA and AOB in situ in annual and perennial bioenergy cropping systems (Chapter 4)?In Chapters 2 and 3, I sampled soils from ecosystems under a management intensity gradient ranging from heavily-managed row crop agriculture to unmanaged deciduous forest. Results in chapter 2 show that soil nitrification is unlikely to be the dominant source of N2O in annual row crop systems, as the 25th-75th percentile of the maximum potential contribution ranged only between 13-42% of total N2O. In contrast, a maximum potential contribution of 52-63% of total N2O emissions could be attributed to nitrification in perennial or successional systems. In Chapter 3, I found high NH4+ inputs could inhibit nitrification of AOB but not AOA, especially in perennial and successional systems. Moreover, long-term N fertilization significantly promoted nitrification potentials of both AOA and AOB in the early succession but not in the deciduous forest systems. In summary, results from these two chapters suggest 1) nitrification is a minor source of N2O, especially in row crop systems, and 2) NH4+ inhibition of AOB could be another mechanism leading to niche differentiation between AOA and AOB in terrestrial environments.In Chapter 4, I examined nitrifier N2O emissions from annual (corn) and perennial (switchgrass) bioenergy cropping systems during different seasons that differ in plant nutrient demands. Both AOA and AOB responded to N fertilizer applications in situ but N fertilizer-induced N2O emissions were mainly observed in corn but not in switchgrass system. Because plants can compete with soil nitrifiers for NH4+ during the growing season, competition for NH4+ appeared to reduce N2O emissions from nitrification. Thus, synchronizing fertilizer application with plant nutrient uptake can be an important strategy for mitigating nitrification-derived N2O. Overall, results from this dissertation suggest that nitrifier-derived N2O in terrestrial ecosystems is significant but not a dominant source of N2O, and although AOB are more responsive to added N than are AOA, AOB can also be inhibited by high NH4+ concentrations in soil."--Pages ii-iii.
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