Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2003
Publication Date: 4/15/2004
Citation: Munoz, G.R., Kelling, K.A., Powell, J.M., Speth, P.E. 2004. Comparison of estimates of first-year dairy manure n availability or recovery using 15n and other techniques. Journal of Environmental Quality. 33:719-727. Interpretive Summary: The difficulty of accurately predicting the nutrient release from land-applied manure makes manure a somewhat uncertain source of crop nutrients. Although many farmers acknowledge the beneficial effects of manure on soil quality and soil nutrient levels, many do not credit manure-applied nutrients when calculating the amount of fertilizer to apply. Accurate credit of manure nutrients would reduce fertilizer costs, thus yielding economic benefits. Optimal manure N use that ensures adequate crop nutrition while avoiding pollution requires accurate and reliable estimates of manure N availability and crop recovery. Two commonly used methods are the fertilizer equivalent (FE) approach to determine manure N availability to crops, and measurement of apparent N recovery by the difference method (Diff Meth). These indirect methods are highly variable. The objective of this research was to compare estimates of dairy manure N availability and recovery by corn using indirect (Diff Meth and FE) and direct (15N labeled manure) techniques in a 3-year field study. Estimates of manure N availability by the FE and Diff Meth were highly variable, yet similar (32 and 41%, respectively). Although average apparent N recovery by the Diff Meth (14%) and 15N recovery (16%) were similar, variability tended to be lower for the 15N method. Although more costly and time-consuming, 15N-labeled manure provides the most consistent, and therefore, reliable result of manure N uptake by corn. The 15N labeling technique is a valuable research tool that may greatly improve the precision by which we can determine dairy manure N cycling in soils and availability to crops. Field trial results using 15N-labeled manure may reduce the risk associated with N crediting and improve manure management.
Technical Abstract: Measurements of dairy manure nutrient availability to crops typically show great variability. Approaches that are more accurate are needed to improve manure management. In this study we compared the capability of direct (15N recovery) and indirect [difference method (Diff Meth) and fertilizer equivalence (FE) approach] methods of determining first-year dairy manure N availability and recovery during three cropping seasons. A field experiment was conducted on a Plano silt loam planted to corn. Plots received either manure, inorganic N, or no N. Microplots receiving 15N-labeled manure were also established each study year. Manure was applied to a new plot each cropping season. Whole-plant N uptake was the best crop response to use for FE estimates. Estimates of N availability by relative effectiveness derived from the Diff Meth and the FE method were similar (32 and 41%, respectively) and higher than N recovery measurements because these indexes factor in N use efficiency. Measures of the relative effectiveness (Rel Eff) of manure N use were highly affected by crop responses in control plots. The FE approach is less influenced by control plots, but it requires the inclusion of several more treatments and use of mathematical functions to describe crop response to N. These limitations are reflected in the wide ranges obtained for N availability estimates (-60 to 140%). Although apparent N recovery by the Diff Meth (14%) or direct measurements of 15N recovery (16%) were close on average, variability tended to be lower for the 15N method. In addition, the Diff Meth was highly dependent on initial soil conditions. Use of 15N-labeled manure, although more costly and time-consuming, provided more consistent and reliable results.