Annual net production and utilization efficiency of solar energy conversion were studied in three double-cropping agro-ecosystems: upland rice-barley (R-B), peanut-wheat (P-W) and dentcorn-Italian ryegrass (C-I). The investigations were carried out in upland fields in Ibaraki Prefecture, central Japan, from June 1985 to May 1988. The average annual global solar radiation (GSR) recorded was 44-55 × 102 MJ m-2 during the 3 years. Annual net primary production was 24.4-25.3 MJ m-2 for the food crops (R-B, P-W) and 53.3 MJ m-2 for the forage crop (C-I), which was equivalent to 0.54 (R-B), 0.50 (P-W) and 1.20% (C-I) of GSR for the year including the fallow period. Of the total net energy fixed by the crops, ∼ 90% was harvested materials (H). The rest comprised litter (L), stubble and roots (SR), which may be utilized later after decomposition by soil microorganisms. Dry matter production for every 1.0 MJ of GSR absorbed is ∼ 1.0g dry matter for peanut, 1.3-1.5 g for cereal crops (R, B and W) and 2.0-2.3 g for forage crops (C and I). The annual net production and the efficiency of solar energy utilization were compared with various types of terrestrial ecosystems. Fertile, double-cropping fields in Japan were comparable to or higher in net production than a forest ecosystem.
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