Legacy effects of soil fertility management on cereal dry matter and nitrogen grain yield of organic arable cropping systems

•Legacy effects of soil fertility building last beyond individual crop rotation cycles•Legacy effects on crop yield span up to a decade, but decline with time.•Green manure crops, cover crops and manure vary in effect on cereal yields.•A holistic approach to soil and crop management should account for legacies. The yields of crops in organic farming are typically lower than in conventional farming, although this depend greatly on local climatic conditions and on crop and soil management. A long-term study over three consecutive 4-year cropping cycles at three locations in Denmark compared organic arable cropping systems varying in fertilization management (green manure crops, cover crops, livestock manure) to study the long-term effects of soil fertility management on crop yields. The analysis of the results showed that the nitrogen (N) derived from cover crops has a legacy effect on the dry matter and N grain yield of spring barley being traceable 5-8 years back in time. The net N input of crop residues and manure for the crop cycles 1-4 and 5-8 years prior to the winter wheat had a significant effect on both the dry matter and N grain yield of wheat. The management of organic cropping systems should focus on reducing the weed pressure, promoting soil fertility and increasing the soil N supply to optimize crop yields. For both winter wheat and spring barley, the weed pressure reduced both crop dry matter and the N grain yield. A cover crop as pre-crop had a significant positive effect on both dry matter and N yield of spring barley, while a cover crop in the rotation did not significantly affect grain yield of winter wheat. For both winter wheat and spring barley, manure application was most effective in increasing crop yields. However, N in applied crop residues also had considerable yield-enhancing effects. For winter wheat, the N inputs from crop residues from previous crop rotation cycles (1-4 and 5-8 years) significantly increased yields, while for spring barley, N input in crop residues from incorporated cover crop and the long-term crop rotation cycle (5-8 years) significantly enhanced both dry matter and N grain yield. These legacy effects of soil fertility management through enhancing and retaining organic N inputs of organic arable cropping suggest that a holistic approach to soil and crop management is necessary, and this needs to focus on increasing weed suppression, soil fertility and targeting manure inputs for enhanced N uptake.