Post‐harvest crop residue contribution to soil N availability or unavailability in North Dakota

North Dakota producers have adopted conservation tillage practices to conserve soil moisture and reduce wind and water erosion. As a result, an accumulation of crop residue has been observed but current recommendations encourage a fertilizer N credit in fields under no‐till for more than 6 yr. Producers are concerned that post‐harvest crop residues are not contributing to N needs of subsequent crops during the growing season. This study was established to assess N mineralization from common crop residue in conventional tillage systems using long‐term incubation studies in order to establish a baseline for future studies on no‐till systems. Three commonly cultivated North Dakota soil series were selected for study with seven residue treatments (varying C/N ratios) including corn ( Zea mays L.), soybean [ Glycine max (L.) Merr.], flax ( Linum usitatissimum L.), forage radish ( Raphanus sativus L.), winter pea ( Pisum sativum L.), spring and winter wheat ( Triticum aestivum L.), and a soil‐only control. Biweekly leachings were collected for nine incubation periods and analyzed for nitrate‐nitrogen (NO 3 –N). Soils with higher organic matter (OM) resulted in increased soil N mineralization (Fargo [1.63 mg kg –1 N] > Forman [0.65 mg kg –1 N] > Heimdal–Emrick [0.38 mg kg –1 N]). Radish and pea residues (narrow C/N ratios) were the only treatments showing N mineralization potential compared with the soil only controls. However, post‐harvest residues with wide C/N ratios (>25:1) promote N immobilization when compared with mineralization by the soil alone. These findings raise the question of whether the N credits for >6 yr of no‐till management are appropriate in the northern Great Plains.