Closing the Gaps in the Budgets of Methane and Nitrous Oxide

Together methane and nitrous oxide contribute almost 40% of the estimated increase in radiative forcing caused by the buildup of greenhouse gases during the last 250 years (IPCC, 2007). These increases are attributed to human activities. Since the emissions of these gases are from biogenic sources and closely associated with living things in the major terrestrial ecosystems of the world, climate change is expected to cause feedbacks that may further increase emissions even from systems normally classified as natural. Our results support the idea that while past increases of methane were driven by direct emissions from human activities, some of these have reached their limits and that the future of methane changes may be determined by feedbacks from warming temperatures. The greatly increased current focus on the arctic and the fate of the carbon frozen in its permafrost is an example of such a feedback that could exceed the direct increases caused by future human activities (Zimov et al. 2006). Our research was aimed at three broad areas to address open questions about the global budgets of methane and nitrous oxide. These areas of inquiry were: The processes by which methane and nitrous oxide are emitted, new sources such as trees and plants, and integration of results to refine the global budgets both at present and of the past decades. For the process studies the main research was to quantify the effect of changes in the ambient temperature on the emissions of methane and nitrous oxide from rice agriculture. Additionally, the emissions of methane and nitrous oxide under present conditions were estimated using the experimental data on how fertilizer applications and water management affect emissions. Rice was chosen for detailed study because it is a prototype system of the wider terrestrial source, its role in methane emissions is well established, it is easy to cultivate and it represents a major anthropogenic source. Here we will discuss the highlights of the results that were obtained.