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Fluid Journal : Fluid Journal 2005-2007
Conclusions For this five-year study, total fertilizer N applied and forage N removed was similar in plots with and without added N, and no increase in soil profile inorganic N accumulation was observed. The soil-plant system was apparently able to compensate for the surplus N. If increased subsoil inorganic N accumulation was an indicator of increased risk for NO3-N leaching, applied N in this non-irrigated alfalfa experiment cannot be considered to increase leaching risk. Because no increase in soil profile inorganic accumulation was observed in this study, biological nitrogen fixation probably was lower in plots receiving additional N, especially since total N removed was similar for fertilized and unfertilized plots. Increased total N in the surface (0-6 inch) horizon was evident at the 40 lbs/A N rate; however no differences in total N were noted at depths greater than 6 inches. Decreased biological N fixation as a result of adding N is yet another buffering mechanism in a legume production system, and this helps explain why no observed increase in soil profile inorganic accumulation was found. This work suggests that low N rates can be applied to alfalfa following each cutting without increasing the risk of NO3-N accumulation. In our work, increased yields due to applied N were found in either the last or second to last harvest. We speculate that the potential benefits of applying low N rates in alfalfa will take place in later harvests and in arid irrigated systems with high yield potential, where fertigation and good water management are possible. Dr. Raun is regents professor, Oklahoma State University; Dr. Phillips is associate professor and Dr. Thomason is assistant professor at Virginia Polytechnic Institute; Mr. Dennis is a soil scientist with NRCS; and Mr. Cossey is a supervisor with ServiTech Labs in Dodge City, KS.
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