Reducing Nitrous Oxide Emissions Through Improved Nitrogen Stewardship: Balancing Crop Production Management and Environmental Protection.

Global demands for fertilizer nitrogen (N) - which are driven by the human family’s increased consumption of food, fiber, and fuel - exceeded 100 million tons (Mt) in 2007 and are expected to exceed 112 Mt by 2015. Sustaining or increasing soil organic carbon (SOC), for multiple crop-soil benefits,  is usually possible only if crop productivity is maintained or increased. Appropriate fertilizer N management can help stimulate crop biomass production, provided that crop residue is maintained on the soil surface through conservation or reduced tillage systems and soil disturbance is minimized. The full N cycle budget in many agricultural systems is not fully understood or has only been published for a few long-term cropping studies. It is however, fairly well recognized that growing season crop recovery of applied N by cereal grains often ranges below 50%, with the  remainder at risk of loss via  leaching, runoff,  volatilization, and denitrification, or retained in the soil. Risks of N loss from farms and fields varies among fertilizer N sources, and the rate, time, and place of application in different cropping systems, as well as with site- and weather-specific conditions. Direct gaseous loss of N from soils as nitrous oxide is important to society because it is one of the more potent greenhouse gases (GHGs), although it usually represents only a small fraction of the total potential N losses from cropping systems. Improved fertilizer N management, based on the ‘4R’ best management practice (BMP) principles - the right N source, at the right rate, right time, and right place – is likely to help enhance crop N use efficiency and effectiveness.  Through appropriate ‘4R’ fertilizer N stewardship, apparent in-season crop N recovery may be increased by as much as 25% above current levels, residual soil nitrate-N levels and risks of groundwater contamination may be minimized,  and direct and indirect nitrous oxide emissions and may be reduced. Nitrous oxide emission reductions from 20 to 50% may be attainable in many field environments.  The International Plant Nutrition Institute has developed a preliminary GIS-based county and watershed-scale assessment of N and other major nutrient balances in the U.S. The potential for international adaptation of the GIS-based approach will be discussed as a means to help address crop production needs, improved N use efficiency and effectiveness,  and nitrous oxide emission reduction goals.