Climate-Nitrogen Interactions In Agriculture: Implications For Nitrogen Use Efficiency.

Vast amounts of nitrogen (N) are needed for the manufacture of food, fuel and fiber from agriculture. Cropping systems and animal production represent a major source of excess reactive nitrogen (Nr) being released into the environment as it moves to aquatic systems and the atmosphere. The N that escapes affects the climate system directly through radiative forcing from emissions of nitrous oxide (N₂O) to the atmosphere, and indirectly through the loss of nitrate (NO₃^-), nitrogen oxides (NO_x) and ammonia (NH₃) to ecosystems downwind of the source. Changes to the climate system (e.g., increased temperatures and shifts in precipitation events) may affect the ability of agricultural systems to utilize N effectively. Major impacts include:

• Higher rates of temperature-dependent ozone formation from precursor NO_x emissions will damage crops and reduce yield. 
• Temperature changes will likely increase NH₃ emissions from manure handling in the livestock industry, as well as raising the risk of heat stress to the animals and reducing production rates. 
• Extreme precipitation events may cause flooding and increased run off of excess Nr to nearby waterways and ground water.
• Loss of productivity due to increasingly unpredictable climate variations and extreme events will likely increase challenges for farmers to match crop N needs with fertilizer inputs. This will likely end up magnifying the flow of Nr to the environment.

An improved understanding of the responses between climate, nitrogen and agriculture is needed for future mitigation. Here we detail the findings of the recent National Climate Assessment report on climate-nitrogen interactions and discuss the implications and future options for nitrogen use efficiency in light of a changing climate.