Quantifying Nitrogen Fixation and Nitrous Oxide Emissions in Global Natural Terrestrial Ecosystems Using Trait-Based Biogeochemistry Models
Nitrous oxide (N2O) contributes to the global climate system and is the third most important greenhouse gas. It is also one of the largest ozone-depleting substances emitted from the biosphere, with great influences on the atmosphere. Therefore, it is important to quantify its emissions from soils in natural terrestrial ecosystems. This dissertation research quantifies N2O emissions from the global natural ecosystems by explicitly considering the influence of soil microbial activities. This dissertation consists of three studies. I first develop a N2O emission model based on an extant process-based biogeochemistry model, the Terrestrial Ecosystem Model, to estimate soil N2O emissions in Northern Eurasia for the latter half of the 20th century and the 21st century. Results show a slight decreasing trend for the regional N2O emissions in the latter half of the 20th century, and contrasting trends for the 21st century under two climate scenarios. In the second study, I improve the model by incorporating the detailed microbial physiological processes of nitrification. The global total soil emissions were estimated to be 8.7 Tg N yr-1, which generally agreed with other previous estimates. Tropical forests are a major emitter, accounting for 42% of the global emissions. Global sensitivity analysis indicated that the model was more sensitive to temperature and precipitation, and less sensitive to soil organic carbon and nitrogen contents. Compared to our precious model without considering the detailed microbial activities, the new model shows more variations in response to seasonal changes of climate. Following this study, biological nitrogen fixation (BNF) is incorporated into N cycling and coupled with TEM. The model is then extrapolated to global terrestrial ecosystems to estimate BNF and its influence on soil N2O emissions. Our regional simulations indicated that tropical forests have the highest N2 fixation rate among all vegetation types, and decreased in northern high latitudes. A sensitivity analysis indicated that air temperature was the most important factor dominating BNF. Soil nitrogen content was also a key factor, directly limiting N fixation. The total nitrogen fixation from the global terrestrial ecosystems was estimated to be 61.5 Tg N yr-1 with a range of 19.8 - 107.9 Tg N yr-1. Our estimation was relatively lower compared to earlier published studies, but comparable to more recent findings using various approaches.
Zhuang, Purdue University.
Environmental science|Atmospheric sciences|Biogeochemistry
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