Plant available nitrogen varies with crop rotation due to legacy effects of legume and poultry litter inputs

Poultry litter (PL) is used by grain farmers in poultry producing regions to supply nitrogen (N). Knowledge of the effects of long-term applications on plant available N (PAN) is limited, particularly when combined with historical inputs from legume cover crops and forages. Previous research at the Farming Systems Project (FSP) in Maryland, USA found that historical inputs from legumes and PL increased PAN, likely reducing maize PL requirements. We quantified the effects of three reduced PL application rates on PAN and the effects of PAN and weeds on maize yields in three organically managed rotations at FSP. We established PL application rate microplots in two-year hairy vetch ( Vicia villosa Roth) plus rye ( Secale cereale L.) cover crop–maize (Zea mays L.)–rye cover crop–soybean ( Glycine max L.), three-year hairy vetch plus rye–maize-rye cover crop–soybean–wheat ( Triticum aestivum L.), and six-year maize–rye cover crop–soybean–wheat–alfalfa ( Medicago sativa L.) –alfalfa–alfalfa rotations. Rotational diversity increased microbially active carbon, N, and PAN concentrations, eliminating maize PL requirements in the six-year rotation. In the two- and three-year rotations, eliminating PL decreased yields by 1645 and 1009 kg ha −1 , respectively. However, application rates could be reduced by 43 % and 58 % without affecting maize yields. Weeds reduced yields in the two-year rotation by 1394 kg ha −1 . Results indicate that long-term PL and legume inputs increase N storage, N cycling, and soil PAN sufficient to support maize yields, but rates are rotation-dependent. Reducing PL recommendations in comparable rotations, whether organic or conventional, would reduce fertility costs and likely lower N loss risks.