Submitted to: US Environment
Publication Type: Proceedings
Publication Acceptance Date: 7/28/2003
Publication Date: 7/28/2003
Citation: Olness, A.E. 2003. Intensification of nitrogen cycling from agricultural lands: An unintended consequence of national policy?. US Environment. Available: www.usenvironment-2003.com.
Interpretive Summary: Fertilizer nitrogen has been implicated as an important factor in the development of hypoxia in the Gulf of Mexico due to the fact that nitrogen limits algal production. Several solutions have been advanced to limit the perceived loss of nitrogen from the heart of the Corn Belt to the drainage system including a 70% tax on fertilizer nitrogen in the hope that this will effect a reduced rate of use and increase the care with which fertilizer nitrogen is applied. However, analysis of the data in the Corn Belt shows that the rate of nitrogen fertilizer applications reached a near maximal rate in the early 1980s at the expected optimal rate of about 160 pounds per acre. Also, the expansion of the area of the hypoxic zone in the Gulf of Mexico occurred well after the period in which nitrogen fertilizer rates reached their maximum. This fails to support any relationship between nitrogen fertilizer and the area of the hypoxic zone. Several changes have occurred in the cropping systems within the Corn Belt. Hay, barley, rye, and oat production have declined precipitously and soybean production has expanded at the rate of nearly a million acres a year for the past 50 years within the Corn Belt. The rate of soybean expansion shows a more stable relationship with the expansion of the hypoxic zone than does fertilizer nitrogen. When drought years are eliminated from consideration, no relationship is observed between the area of the hypoxic zone and nitrogen fertilization rates. It is speculated that, if nitrogen is a major driving force in the expansion of the hypoxic zone in the Gulf of Mexico, the main source of this nitrogen is probably coming from erosion of soybean fields and accelerated mineralization within fields previously planted to soybean. Because soils previously planted to soybean tend to be warmer than soils devoted to corn production, nitrogen mineralization occurs more rapidly within these soils and more nitrates are produced. Soybean has become a major crop within the US Corn Belt apparently as a consequence of national food policy and it is suggested that this policy is no longer useful. This work clearly shows that the current target, fertilizer nitrogen, is not a major factor in development of the hypoxic zone of the Gulf of Mexico.
Technical Abstract: Changes in agricultural production in the US Corn Belt over the last 50 years have been quite dramatic. Increased nitrogen (N) fertilization has been implicated as a cause of greater N in stream water concentrations. Increased N concentrations in the Mississippi watershed, in turn, have been implicated as factors in the development of larger hypoxic zones in the Gulf of Mexico. However, an inexpensive national food policy has clearly contributed to exacerbation of N movement to the Gulf of Mexico from the Corn Belt. This policy has resulted in conversion of > 6 million ha from hay production and > 12 million ha from small grain production to maize (Zea mays L.) and soybean (Glycine max L.) production. Shifting crop production has had the subtle effect of accelerating mineralization of soil organic matter and the production of nitrate-N during a time period in which no plants are available to capture the N; hence, greater leaching loss of N. Further, removal of soil cover during this critical period has contributed to soil warming which further accelerated mineralization of organic matter and production of inorganic N. Converting land, once dedicated to hay and small grain to corn and soybean production, has accelerated soil erosion and this has also contributed organic N to streams and rivers during the fallow period. This paper discusses the factors affecting microbial mineralization of organic matter and explains how they interact to intensify N movement in the environment.