From 1998 to 2008, the researchers evaluated and compared potential management strategies for reducing nitrogen and nitrate nitrogen levels in soil and groundwater.
The research, led by ARS soil scientist Ardell Halvorson, focused on irrigated cropping systems in the Arkansas River Valley, an agricultural region of Colorado that has high levels of nitrate nitrogen in the fields and groundwater--due, in part, to heavy application of nitrogen fertilizer and the prevalence of shallow-rooted crops such as onions.
The first study showed that onions used only about 12 to 15 percent of the fertilizer nitrogen applied to the crop. Much of the remainder stayed in the top six feet of soil. The next year, Halvorson and his colleagues planted corn on the same land and found that it recovered about 24 percent of the fertilizer nitrogen that had been applied to the onion crop.
Following that study, the scientists grew alfalfa on the land for five years, then followed it with a watermelon crop, followed by a corn crop. In the first year that the corn was grown, an unfertilized control plot yielded about 250 bushels of corn.
By comparison, a plot fertilized with 250 pounds of nitrogen per acre yielded about 260 bushels, a small increase that required a significantly higher investment of time and money. Additional corn studies following onion in rotation showed corn was a good residual nitrogen scavenger crop.
The results suggest that when managing fields with relatively high nitrogen levels, farmers could benefit economically from reduced nitrogen fertilization rates. Recommendations based on this research could have important economic and environmental benefits for the Arkansas River Valley and similar regions.