Nitrous oxide (N 2 O) is a potent greenhouse gas (GHG) that also depletes stratospheric ozone. Nitrogen (N) fertilizer rate is the best single predictor of N 2 O emissions from agricultural soils, which are responsible for ∼50% of the total global anthropogenic flux, but it is a relatively imprecise estimator. Accumulating evidence suggests that the emission response to increasing N input is exponential rather than linear, as assumed by Intergovernmental Panel on Climate Change methodologies. We performed a metaanalysis to test the generalizability of this pattern. From 78 published studies (233 site-years) with at least three N-input levels, we calculated N 2 O emission factors (EFs) for each nonzero input level as a percentage of N input converted to N 2 O emissions. We found that the N 2 O response to N inputs grew significantly faster than linear for synthetic fertilizers and for most crop types. N-fixing crops had a higher rate of change in EF (ΔEF) than others. A higher ΔEF was also evident in soils with carbon >1.5% and soils with pH <7, and where fertilizer was applied only once annually. Our results suggest a general trend of exponentially increasing N 2 O emissions as N inputs increase to exceed crop needs. Use of this knowledge in GHG inventories should improve assessments of fertilizer-derived N 2 O emissions, help address disparities in the global N 2 O budget, and refine the accuracy of N 2 O mitigation protocols. In low-input systems typical of sub-Saharan Africa, for example, modest N additions will have little impact on estimated N 2 O emissions, whereas equivalent additions (or reductions) in excessively fertilized systems will have a disproportionately major impact.fertilizer response | greenhouse gas emissions | agriculture | bioenergy | greenhouse gas mitigation

Row-crop agriculture is a major source of nitrous oxide (N 2 O) globally, and results from recent field experiments suggest that significant decreases in N 2 O emissions may be possible by decreasing nitrogen (N) fertilizer inputs without affecting economic return from grain yield. We tested this hypothesis on five commercially farmed fields in Michigan, USA planted with corn in 2007 and 2008. Six rates of N fertilizer (0-225 kg N ha À1 ) were broadcast and incorporated before planting, as per local practice. Across all sites and years, increases in N 2 O flux were best described by a nonlinear, exponentially increasing response to increasing N rate. N 2 O emission factors per unit of N applied ranged from 0.6% to 1.5% and increased with increasing N application across all sites and years, especially at N rates above those required for maximum crop yield. At the two N fertilizer rates above those recommended for maximum economic return (135 kg N ha À1 ), average N 2 O fluxes were 43% (18 g N 2 O-N ha À1 day À1 ) and 115% (26 g N 2 O-N ha À1 day À1 ) higher than were fluxes at the recommended rate, respectively. The maximum return to nitrogen rate of 154 kg N ha À1 yielded an average 8.3 Mg grain ha À1 . Our study shows the potential to lower agricultural N 2 O fluxes within a range of N fertilization that does not affect economic return from grain yield.

Nitrous oxide (N 2 O) is a major greenhouse gas (GHG) product of intensive agriculture. Fertilizer nitrogen (N) rate is the best single predictor of N 2 O emissions in rowcrop agriculture in the US Midwest. We use this relationship to propose a transparent, scientifically robust protocol that can be utilized by developers of agricultural offset projects for generating fungible GHG emission reduction credits for the emerging US carbon cap and trade market. By coupling predicted N 2 O flux with the recently developed maximum return to N (MRTN) approach for determining economically profitable N input rates for optimized crop yield, we provide the basis for incentivizing N 2 O reductions without affecting yields. The protocol, if widely adopted, could reduce N 2 O from fertilized row-crop agriculture by more than 50%. Although other management and environmental factors can influence N 2 O emissions, fertilizer N rate can be viewed as a single unambiguous proxy-a transparent, tangible, and readily manageable commodity. Our protocol addresses baseline establishment, additionality, permanence, variability, and leakage, and provides for producers and other stakeholders the economic and environmental incentives necessary for adoption of agricultural N 2 O reduction offset projects.

Management decisions both at the field and off-site have the potential to contribute to climate change mitigation and adaptation. Climate change threatens to increase the potential for soil erosion, reduce soil quality, lower agricultural productivity and negatively impact food security and global sustainability, making it one of the most severe challenges we will face in the 21st century. This paper looks at the potential of management to help us, not only mitigate climate change, but also to help us adapt to a changing climate. Different aspects of carbon management, nitrogen management, manure management, management in low-input systems (sustainable agriculture), and grazing land management are discussed as examples. Management decisions regarding conservation practices, such as no-till, conservation agriculture, and returning crop residue to the field to increase nutrient cycling, can contribute to carbon sequestration and help us mitigate and adapt to climate change. Additionally, management of grasslands, restoration of degraded/desertified lands, nitrogen management to reduce greenhouse gas emissions, precision conservation management at a field and/or watershed level, and other management alternatives can also help us mitigate and/or adapt to climate change. Management for climate change mitigation and adaptation is key for environmental conservation, sustainability of cropping systems, soil and water quality, and food security. This paper suggests, based on a review of the literature, that management decisions that reduce soil erosion, increase carbon sequestration to improve soil functions, soil quality, and soil health, and contribute to the resilience of soils and cropping systems will be needed to respond to climate change and related challenges such as food security. Our review suggests that without management decisions that increase soil and water conservation, food security for the world's growing population will be harder to achieve.