Nitrogen Budget and Soil N Dynamics after Multiple Applications of Unlabeled or 15 Nitrogen-Enriched Dairy Manure

cially if manure is surface-applied (Thompson et al., 1987). Conversely, denitrification losses of manure N Repeated N applications to field crops, either as inorganic fertilizers are increased by incorporation or injection (Comfort et or animal manures, can lead to N buildup in soils with potential longterm environmental hazards. The objective of this 3-yr field study al., 1988), and because organic matter in manure serves was to monitor totaland mineral-N levels in soil after repeated as a substrate for denitrifier microorganisms, tend to be fertilizer or single or repeated dairy manure applications, and to comhigher than denitrification losses in fertilized soils pute an N balance for the soil-crop system. Unlabeled and 15N-enriched (Cates and Keeney, 1987). Immobilization of inorganic dairy manure were used. The experiment was conducted on a Plano N in manure, plus the greater volatilization and denitrificorn silt loam continuously cropped to corn (Zea mays L.) Manure cation losses, cause inorganic N in manure to be less increased totaland NO3–N levels in soil, especially in the 0to 30-cm plant-available than equal rates of inorganic fertilizer depth and in plots receiving frequent and recent manure applications. Manure increased NO3–N in the 0to 30-cm soil layer more than (Paul and Beauchamp, 1993). Because of its lower N fertilizer N, whereas the opposite was true in the 30to 60and 60availability, greater amounts of manure than fertilizer to 90-cm layers. There was a clear NO3–N buildup with repeated N are applied to crops. This can result in a steady accumanure treatments. Unlabeled N measurements were not accurate mulation of soil N. Long-term soil N accumulation on enough to track trends in soil total N levels, hampering the calculation dairy farms poses a serious environmental risk (Bouldin of an N balance. 15Nitrogen-labeled manure allowed for direct meaet al., 1984). Heavy and/or repeated manure applications surement and provided more accurate estimates of N recovery in soils and crops. During the 3-yr study period, an average of 18% of applied can lead to NO3–N buildup in soil and losses through manure 15N was recovered in corn silage and 46% remained in the leaching (Adriano et al., 1971; Mathers and Stewart, soil. Unaccounted-for 15N (36%) was assumed to be lost mainly by NH3 1974; Smith et al., 1980; Cooper et al., 1984). Dairy farms volatilization and denitrification. Most (82%) of the 15N remaining in in the Midwest are considered significant contributors of soil was present in the top 30 cm, irrespective of frequency of manure N to the hypoxic zone in the Gulf of Mexico (Burkart application. Although costly and time-consuming, the use of 15Nand James, 1999). labeled manure provided a much better approach to study the fate Excessive soil nutrient accumulation and losses to of manure N within the soil-crop system, compared with unlabeled manure. surface and ground water are pressing environmental challenges facing the dairy and other animal industries. As dairy herds expand to remain economically viable, I most agricultural soils, N is the most limiting a larger percentage of the available cropland is devoted nutrient, and it has to be supplied to cereal crops, to corn silage. The noted expansion of corn silage proparticularly in high productivity systems (Meisinger, duction (Battaglia, 1999; Shaver, 2000) is due to this 1984). Fertilizers, manure, and in some cases, legumes, crop’s ability to feed more cows (Bos taurus) than other are the principal N sources for crop production in mixed, forages per unit of cultivated area (Seglar, 1998), as well dairy-crop production systems. Whereas fertilizer N is as favorable economics (Klemme, 1998) to the farmer. readily soluble in soils and becomes immediately availHowever, the effects of shifting more land to corn silage able for crop uptake, it can also be highly susceptible on other system components, such as N use, buildup to leaching losses. Comfort et al. (1987) found that soil and loss remains to be determined. Since only a relainorganic N and downward movement were increased tively small amount of applied N is ultimately taken up to a greater extent by fertilizer than by manure N. On by the crop, we wanted to track the fate of the unused the other hand, only about half of manure N is inorganic, portion to see whether it was lost or remained in the soil. with the rest being present in organic forms. Organic N The objective of this study was to determine total and must be mineralized before it can be used by plants or inorganic soil N and the N balance of a continuous corn it becomes susceptible to losses. However, when fresh silage cropping system receiving two fertilizer or dairy manure or slurry contain appreciable amounts of urea manure N rates of different application frequency across or NH4, N can be easily lost via NH3 volatilization, espe3 yr. Unlabeled and 15N-enriched dairy manure were used, and the ability of each manure type to detect G.R. Muñoz and K.A. Kelling, Dep. of Soil Science, Univ. of Wiscontrends in soil N levels and account for applied N was sin, 1525 Observatory Dr., Madison, WI 53706; and J. Mark Powell, USDA-ARS Dairy Forage Research Ctr. , 1925 Linden Dr. West, compared. The use of 15N-labeled manure was an essenMadison, WI 53706. Received 22 Feb. 2002. *Corresponding author tial part of the study because it allowed direct N tracking (grmunoz@uwalumni.com). in the cropping system, and provided more accurate measurements than unlabeled manure. Published in Soil Sci. Soc. Am. J. 67:817–825 (2003).