Long-Term Manure Application and Forages Reduce Nitrogen Fertilizer Requirements of Silage Corn-Cereal Cropping Systems

Assessment of the soil N supply capacity is essential to optimize fertilizer N use. We investigated soil N supply capacity and fertilizer N recovery for three cropping systems established in 1977: silage corn (Zen mays L.)-cereal without (CC) and with 20 Mg ha(-1) yr(-1) manure (CCM), and silage corn-forage (3-yr) with manure (CFM). During the present study (2005-2008), manure applications were suspended and a silage corn-silage corn-barley (Hordeum vulgaris L.)-wheat (Triticum aestivum L.) sequence was imposed to all systems. Fertilizer ((NH4NO3)-N-15-N-15, 3.1 atom % N-15) was applied in 2005 to silage corn (160 kg N ha(-1)) and in 2007 to barley (80 kg N ha(-1)). The N-15 recovery in silage corn and barley ranged from 40 to 59%, with the lowest values measured in CFM. Compared to the CC systems (47 kg N ha(-1)) in 2005, soil-derived N in silage corn was two times higher under CCM (98 kg N ha(-1)), and four times higher under CFM (208 kg N ha(-1)). These differences decreased over years, but were still noticeable at the end of the experiment. Twenty-two to 58% of applied N-15 was recovered in the soil at harvest. More than 50% of this residual N was present in macroaggregates (>0.25 mm), whereas <20% was present in particulate organic matter (POM). Up to 75% of residual N-15 was lost during the winter period, and little residual N was transferred to the following crop. These findings indicate that soil macroaggregates may be a preferential sink for residual fertilizer N, but this N is not present in stable forms and is vulnerable to environmental loss.