Characteristics of decomposition of dissolved organic carbon by microbes in water of forest ecosystems

流式生物反应器法是测定森林生态系统水体中生物可降解溶解有机碳（BDOC）的有效方法。我们确定BDOC测量条件如下：空气温度≥20℃，溶液流速≥0.25 ml/min，样品DOC浓度≥10 mg/L。杉木（Cryptomeria japonica）、日本柏木（Chamecyparis obutsa）和红松（Pinus densiflora）林分有机层渗透水中BDOC对DOC的百分比（BDOC%）为20 ~ 30%，水原（Quercus mongolica var.grosseserrata）林分为50%。结果表明，大量施氮对水原林分BDOC%有影响。在一个小流域的老杉柏人工林中，DOC的浓度从高到低依次为有机层渗漏>到秋季降水>，在土壤水分中迅速降低。蒸汽水中DOC年排放量较小（6.4 kg/ha），说明土壤水中DOC被土壤微生物消耗并吸附在土壤胶体上。表层土壤水溶性有机碳耗量与碳矿化率呈显著正相关。因此，DOC和WSOC量是微生物活性的重要指标。然而，这些数量明显小于矿化碳，因此我们认为有效有机碳的测定是必要的。6种植物的腐殖土中木质素含量随分解等级的增加而增加。特别是不溶于碱性溶液的腐殖质类有机碳占木质素碳的70%。另一方面，6种落叶阔叶树凋落叶的分解导致木质素含量随质量重量的减少而减少。我们需要对有机物进行更详细的分类，比如易分解和难分解的木质素。

A flow-through bioreactor method is useful for measurement of biodegradable dissolved organic carbon(BDOC) in water of forest ecosystems. We determined the condition of BDOC measurement as follows ; over 20℃ for air temperature, 0.25 ml/min for solution flow rate and 10 mg/L for DOC concentration of samples. The percentages of BDOC on DOC(BDOC%) in percolated water of organic layer were 20-30% in Japanese cedar (Cryptomeria japonica), Japanese cypress (Chamecyparis obutsa) and red pine (Pinus densiflora) stand, while 50% in mizunara (Quercus mongolica var.grosseserrata) stand. This result suggested that the large amount of nitrogen influenced the BDOC% in mizunara stand.In a small watershed of aged Japanese cedar and cypress plantation, DOC in order of high concentrations were organic layer percolation > through fall > precipitation, and was rapidly low in soil water. The DOC annual discharge in steam water was small (6.4 kg/ha), suggesting the DOC in soil water was consumed by soil microbes and adsorbed on soil colloid. The amount of consumption of water soluble organic carbon (WSOC) had positive correlations with the rate of carbon mineralization in surface soil. Therefore, DOC and WSOC amounts were important indicators for microbe activities. However, these amounts were remarkably smaller than mineralized carbon, so we considered that determination of available organic carbon was necessary.The percentages of lignin on humus increased with increases of decomposition class in forest floor of six species. Especially, the organic carbon in the humus class insoluble by alkaline solution had 70% of carbon in lignin. On the other hand, the amounts of lignin decreased with decreases of mass weight due to decomposition of the leaf litter of six deciduous broad-leaved trees species. We need to make more detailed classification of organic matter, such as easily or hard decomposed lignin.