Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2004
Publication Date: 12/1/2004
Citation: Russelle, M.P., Birr, A.S. 2004. Large-scale assessment of symbiotic dinitrogen fixation by crops: soybean and alfalfa in the Mississippi River Basin. Agronomy Journal. 96:1754-1760. Interpretive Summary: Innovative ways of preventing water contamination are needed to achieve public water quality goals, such as reducing the occurrence of the so-called 'Dead Zone' in the Gulf of Mexico. This is a large area of water that contains too little oxygen for most organisms to live. It is caused by 'blooms' (population explosions) of microscopic organisms that feed on nutrients carried to the Gulf by the Mississippi River. One of these nutrients, nitrogen, appears to come largely from farm field runoff because too much nitrogen is available on some farms. This research is based on the idea that farmers can reduce the amount of excess nitrogen on their farms by applying livestock manure to legumes like alfalfa. Legumes can obtain nitrogen from the air and also remove it from soil. Although legumes are usually prized as a source of 'free' nitrogen, their contribution can be reduced where it is not needed and they can help achieve a better balance of nitrogen when they are grown where nitrogen supply is high. This idea was applied to the entire Mississippi River Basin to generate maps showing how much nitrogen alfalfa obtains from the air, which indicates how much manure nitrogen it could utilize. This new approach could help farmers reduce nitrogen supplies on their farms with little expense. If broadly adopted, it should help limit the size of the 'Dead Zone.'
Technical Abstract: Hypoxia in the Gulf of Mexico is an outcome of phytoplankton growth supported by nitrate-nitrogen and other nutrients delivered from the Mississippi River Basin. Because symbiotic N2 fixation is a facultative process that is reduced by other plant-available nitrogen, we propose that strategic application of livestock manure to alfalfa could reduce excess nitrogen supply in the Basin. We estimate symbiotic N2 fixation by alfalfa based on harvested nitrogen and inorganic nitrogen supply. Our analysis suggests that up to one million metric tons of fixed nitrogen could be replaced by manure nitrogen annually, an amount equal to the annual delivery of nitrate-nitrogen to the Gulf by the Mississippi River.