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 ( Zea 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 ( 15 NH 4 15 NO 3 , 3.1 atom % 15 N) was applied in 2005 to silage corn (160 kg N ha −1 ) and in 2007 to barley (80 kg N ha −1 ). The 15 N 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 15 N 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 15 N 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.