More than 70% of fresh tomato is produced in plastic high tunnel or greenhouse and much fertilizer and agricultural chemicals have been applied for tomato production. Cover crop is one of the important tools for sustainable agro-production because of improving soil properties. Effect of cover cropping on soil properties and tomato production in plastic high tunnel was examined in Sapporo, Japan, snow cover region. Plastic high tunnel was set before snow-melting, late in March, 2007. Two cover crops, legume (hairy vetch; Vicia villosa R.) and non legume (wild oat; Avena sterigosa L.), were planted each alone or mix-planted in the plot (0.8 m in width and 3.5 m in length) early in April. Seeding density in mono-culture was 5 kg/10a in hairy vetch and 10 kg/10a in wild oat. For bi-culture, density was 3.5 kg/10a in hairy vetch and 5 kg/10a in wild oat. Above ground biomass (dry weight) was shown 445 kg/10a in hairy vetch and 982 kg/10a in wild oat late in May. However, in mix-culture, it was 190 kg/10a in hairy vetch and 602 kg/10a in wild oat. Cover crops were killed for residue mulch just before tomato planting. Seedlings of tomato 'Momotaro' were planted in the plots covered with cover crop residue and 12 kg/10a N fertilizer application (12 kg N/10a) and bare plot with 12 kg/10a or 24 kg/10a N fertilizer (24 kg N/10a) application. Nitrate concentration of the petiole sap in the leaf just below the first fruit cluster was higher in the tomatoes grown in bare plot with 24 kg N/10a and in hairy vetch mulch with 12 kg N/10a at 40 days after transplanting. Large yield was shown in bare plot with 24 kg N/10a (7.9 t/10a) and in hairy vetch plot with 12 kg N/10a (7.8 t/10a). The yield in bare plot with 12 kg N/10a and Mix plot with 12 kg N/10a was 6.9 t/10a. The yield in wild oat plot with 12 kg N/10a was the smallest (6.1 t/10a). Soil carbon in the plots with cover crop mulch became higher than that of bare plots after tomato production. It will be possible to obtain current yield and to increase soil carbon in cover cropping with reduction of N fertilizer. This technique will lead the organically grown system in plastic high tunnel production.
