Seasonal nitrogen availability from current and past applications of manure

Proper management of manure nitrogen (N) requires the ability to match the rate and extent of manure N availability with crop needs. This includes recognizing the potential importance of N contributions from residual manure N that accumulates with repeated applications. Nitrogen availability relative to barley needs was assessed in plots with 13-16 years continuous histories of contrasting manure-based (solid-bedded beef) and fertilizer-based soil treatments in the Maine Potato Ecosystem Project. Soil and barley samples were collected every 7-14 days during 2003-2005, and once in 2006. Barley dry matter and N content were equivalent between the two systems. In the manure-based system, temporal patterns of N availability were more synchronous with early season crop needs than in the fertilizer-based system, but continued mineralization after harvest was also observed. In 2004-2006, samples were collected from subplots where manure/fertilizer was withheld to estimate the proportion of available N originating from current versus previous manure applications. Apparent N recovery of current years' applications of manure organic N was 8-11% and less than predicted by a standard decay series model for beef manure (25%), highlighting the need to adjust manure N credits for crops with shorter growing seasons and lower N uptake capacities than corn. The relative contribution of residual manure N to total manure N uptake was greater than predicted from the decay series model, providing support for a residual N effect from repeated manure applications that is not accounted for in standard manure recommendations.