We applied a mechanistic ecosystem model to investigate the production and environmental performances of (1) current agricultural practice on two fields of a stockless organic cereal farm in southeast Norway and (2) alternative cereal-ley rotations and plowing time scenarios. Scenarios were simulated using historic weather data and a climate change scenario. Measured and simulated soil mineral N concentrations were generally low (1-4 g N m(-2)) and in good agreement. Simulated nitrate leaching was similar for the two fields, except when an extended period of black fallow weeding was practiced on one of them. Scenario simulations indicated that continuous cereal cropping undersown with a clover-grass winter cover crop performed best when evaluated by whole-rotation grain yield, the N yield/input-, and N loss/yield-ratios, and greenhouse gas emissions. However, the rotation had the largest soil organic matter losses. The N use and loss efficiency indicators were especially poor when ley years occurred consecutively and under fall plowing. Total greenhouse gas emissions were, however, smaller for the fall-plowed scenarios. In conclusion, our results indicated a modest potential for improving stockless systems by management changes in plowing time or crop rotation, which was hardly different in the climate change scenarios, although nitrate leaching increased substantially in the winter. Alternative strategies seem necessary to substantially improve the N-use efficiency in stockless organic grain production systems, e.g., biogas production from green manure and subsequent recycling of the digestate. Abandoning the stockless system and reintegrating livestock should also be considered.
