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.

Agriculture in the United States (US) cycles large quantities of nitrogen (N) to produce food, fuel, and fiber and is a major source of excess reactive nitrogen (Nr) in the environment. Nitrogen lost from cropping systems and animal operations moves to waterways, groundwater, and the atmosphere. Changes in climate and climate variability may further affect the ability of agricultural systems to conserve N. The N that escapes affects climate directly through the emissions of nitrous oxide (N 2 O), and indirectly through the loss of nitrate (NO 3 -), nitrogen oxides (NO x ) and ammonia to downstream and downwind ecosystems that then emit some of the N received as N 2 O and NO x . Emissions of NO x lead to the formation of tropospheric ozone, a greenhouse gas that can also harm crops directly. There are many opportunities to mitigate the impact of agricultural N on climate and the impact of climate on agricultural N. Some are available today; many need further research; and all await effective incentives to become adopted. Research needs can be grouped into four major categories: (1) an improved understanding of agricultural N cycle responses to changing climate; (2) a systems-level understanding of important crop and animal systems sufficient to identify key interactions and feedbacks; (3) the further development and testing of quantitative models capable of predicting N-climate interactions with confidence across a wide variety of crop-soil-climate combinations; and (4) socioecological research to better understand the incentives necessary to achieve meaningful deployment of realistic solutions.

Increasing fertilizer costs have resulted in more growers evaluating the use of alternative nutrient sources such as manure. Coincidentally, the questions about fertilizer eff ects on oil yield and oil fatty acid composition have been a concern. A 2-yr study was conducted to investigate nutrient source (fertilizer urea plus S and manure) and N level (0, 84, and 168 kg N ha -1 ) eff ects on canola seed yield, total oil content, and oil composition at East Lansing and Chatham, MI. Results indicated nutrient applications were not necessary to increase canola yield (865-1991 kg ha -1 ) in fertile fi elds. However, N fertilizer appeared to reduce total oil content (444−536 mL kg -1 ), and at similar N levels, total oil content in canola with fertilizer was sometimes lower than that with manure application. Compared with the no nutrient control treatment, fertilizer application sometimes decreased linolenic acid (LN) content, and increased palmitic acid (P) and arachidic acid (A) at Chatham, while it appeared to decrease oleic acid (O) and increase P, linoleic (L) and A at East Lansing. Fertilizer applications oft en increased canola total saturated fatty acid content (6.80-8.32%) and decreased ratio of O/(L+LN) (2.04-2.52). Manure application had milder eff ects on oil composition than fertilizer application. Compared with less N (84 kg N ha -1 ) applications, greater N level (168 kg N ha -1 ) tended to lower oil quality by increasing total saturated fatty acid content and decreasing the O/(L+LN) ratio.