Nitrogen fertilization — if one includes CO2 released during the manufacture, distribution, and application of the fertilizer and N2O released during microbial transformation of fertilizer in soils (denitrification)—is responsible for the majority of greenhouse gas emissions from agriculture. Some of the nitrate (NO3-) fertilizer applied to crops leaches from fields and contaminates groundwater; groundwater NO3– concentrations above 4 milligrams of nitrate nitrogen per liter (4 mg NO3– -N L–1) are associated with increased risk of cancer and other human health problems.  Moreover, U.S. maize farmers spend more than $150 per hectare on nitrogen fertilizer each year. Therefore, higher nitrogen-use efficiencies—higher yields per nitrogen fertilizer applied—would not only decrease greenhouse gas emissions but would also decrease crop production costs and water pollution.
Breeding crops for better yields has inadvertently selected for cultivars with nearly double the nitrogen-use efficiency of their predecessors. Newer cultivars attain higher yields per nitrogen absorbed from the soil (nitrogen-utilization efficiency) as well as acquire more of the nitrogen applied to the soil (nitrogen-uptake efficiency). Consequently, grain yields in the United States have increased about 3% per year without requiring additional nitrogen fertilization or emitting additional greenhouse gas emissions. Crop breeding programs are beginning to target high nitrogen-use efficiency as a primary objective. Optimizing nitrogen levels in various plant parts and enhancing nitrogen translocation within plants should enhance nitrogen utilization efficiency. Higher nitrogen-uptake efficiency may entail breeding crops that allocate a larger share of their resources to roots, a characteristic that may sacrifice some yield but would also benefit soil carbon sequestration and water acquisition from soils.
Complementing genetic improvement of crops is more sophisticated fertilizer management. Previously, farmers took a “one size fits all” approach, whereby most fields received a similar amount of fertilizer in a single application just before planting. To compensate for possible variation among locations or over years, farmers would err on the “safe” side and apply an excessive amount of nitrogen. Now, farmers optimize fertilizer rates among fields and sometimes within fields, and they add small amounts of nitrogen to the soil several times through the growing season. Fertigation, the practice in which farmers dissolve fertilizer in the irrigation water that they apply regularly through drip irrigation systems, has become commonplace in California. This practice both conserves water and minimizes fertilizer losses from denitrification or leaching.
The majority of agricultural N2O emissions derive from the process of denitrification. This process occurs when patches in a soil become anaerobic (oxygen deprived) during flooding or compaction. Certain soil microorganisms can sustain themselves in such anaerobic patches on the energy released during the stripping of oxygen from nitrogen compounds.
Transfers between the steps of this pathway are not perfectly efficient, and some of the immediate compounds escape to the external environment. In particular, leaks before the last reaction step release substantial amounts of N2O to the atmosphere. Indeed, the standard agricultural practice of heavy fertilization and irrigation at the beginning of the growing season to establish a crop creates temporary anaerobic conditions that generate a burst of N2O (Matson et al. 1998). Management practices that diminish N2O emissions from agricultural soils include distributing fertilizer and water more evenly across a field and throughout the growing season and improving field drainage through accumulation of soil carbon and avoidance of soil compaction. Elevated CO2 concentrations in the atmosphere inhibit the assimilation of NO3– in the shoots of most plants. To compensate for this phenomenon without exacerbating greenhouse gas emissions, farmers will need to manage soil nitrogen more carefully. In particular, multiple small applications of ammonium-based fertilizers will avoid the deleterious effects of excessive ammonium and promote slow but steady nitrification (microbial conversion of soil ammonium to nitrate) and root NO3– assimilation.
 Hitt, K. J. and B. T. Nolan (2005) Nitrate in ground water: Using a model to simulate the probability of nitrate contamination of shallow ground water in the conterminous United States. U.S. Geological Survey, http://pubs.usgs.gov/sim/2005/2881/, accessed Sept. 20, 2007.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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