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Nitrous oxide (N20) is often the largest single component of the greenhouse-gas budget of individual cropping systems, as well as for the US agricultural sector as a whole. Here, we highlight the factors that make mitigating N20 emissions from fertilized agroecosystems such a difficult challenge, and discuss how these factors limit the effectiveness of existing practices and therefore require new technologies and fresh ideas. Modification of the rate, source, placement, and/or timing of nitrogen fertilizer application has in some cases been an effective way to reduce N20 emissions. However, the efficacy of existing approaches to reducing N20 emissions while maintaining crop yields across locations and growing seasons is uncertain because of the interaction of multiple factors that regulate several different N20-producing processes in soil. Although these processes have been well studied, our understanding of key aspects and our ability to manage them to mitigate NzO emissions remain limited. INSETS: In a nutshell:;Panel 1. N20-producing processes in soil. [ABSTRACT FROM AUTHOR]

The potential for mitigating increasing atmospheric carbon dioxide concentrations through the use of terres-trial biological carbon (C) sequestration is substantial. Here, we estimate the amount of C being sequestered by natural processes at global, North American, and national US scales. We present and quantify, where possible, the potential for deliberate human actions - through forestry, agriculture, and use of biomass-based fuels - to augment these natural sinks. Carbon sequestration may potentially be achieved through some of these activities but at the expense of substantial changes in land-use management. Some practices (eg reduced tillage, improved silviculture, woody bioenergy crops) are already being implemented because of their economic benefits and associated ecosystem services. Given their cumulative greenhouse-gas impacts, other strategies (eg the use of biochar and cellulosic bioenergy crops) require further evaluation to determine whether widespread implemen-tation is warranted. INSET: In a nutshell:. [ABSTRACT FROM AUTHOR]

The use of commercial nitrogen (N) fertilizers has led to enormous increases in US agricultural productivity. However, N losses from agricultural systems have resulted in numerous deleterious environmental impacts, including a continuing increase in atmospheric nitrous oxide (N20), a greenhouse gas (GHG) and an important catalyst of stratospheric ozone depletion. Although associated with about 7% of total US GHG emissions, agri-cultural systems account for 75% of total US N20 emissions. Increased productivity in the crop and livestock sectors during the past 30 to 70 years has resulted in decreased N20 emissions per unit of production, but N20 emissions from US agriculture continue to increase at a rate of approximately 0.46 teragrams of carbon dioxide equivalents per year (2002-2009). This rate is lower than that during the late 20th century. Improvements in agricultural productivity alone may be insufficient to lead to reduced emissions; implementing strategies specifically targeted at reducing N20 emissions may therefore be necessary. INSET: In a nutshell:. [ABSTRACT FROM AUTHOR]