ARS | Publication request: USE OF 15N TO EVALUATE DAIRY MANURE NITROGEN AVAILABILITY TO CORN AND CYCLING IN SOILS

Submitted to: Sustainable Land Application Conference
Publication Type: Proceedings
Publication Acceptance Date: January 2, 2004
Publication Date: N/A

Technical Abstract: Estimates of dairy manure N availability to crops are based on indirect measurements, such as the "difference method" and the "fertilizer equivalent approach" and can vary greatly. More accurate estimates of manure N availability to crops are needed if we are to expect farmers to credit the N contained in manure. This paper describes two methods that differentially label dairy manure N components (urine and feces) in 15N and the use of 15N-labeled and -unlabeled manure and fertilizer N to compare direct and indirect determinations of manure N availability to corn and the fate of residual manure N in soils. Manure has been applied every year, every second, and every third year to a Plano silt loam in central Wisconsin since 1998. Of the total manure 15N applied, approximately 15 to 30% has been taken up by corn the first year, 4 to 8% the second year, and 2 to 4% the third year after application. Most of the 15NO3 and total soil 15N in plots that received manure continuously for three years was found in the upper 30 cm of soil indicating little downward movement of applied manure 15N. In these plots, an average of 70% of applied manure 15N was accounted for, either in crop uptake (22%) or in the soil (48%). Most of the 15N unaccounted for was probably lost through NH3 volatilization and denitrification. It is expected that this information will increase our confidence in manure N credits.