Nitrogen dynamics in the tropical savanna of Australia

白蚁是热带陆地生态系统中数量丰富的土壤动物，在分解含氮少的植物死亡物质中起着重要作用。大多数植物死亡材料含氮量仅为0.03-0.7%，C/N比值为70-500，而白蚁含氮量约为19%，C/N比值为4-12。因此，白蚁必须通过增加氮摄入量或选择性地消除碳来解决碳氮平衡问题。Tayasu et al .（1994）研究表明，低等白蚁Neotermes koshunensis体内约50%的氮是通过肠道中固氮细菌的活动从空气中获得的。本文主要从白蚁在生态系统中的作用出发，探讨了白蚁如何解决碳氮平衡问题。我们测量了白蚁的甲烷产量作为一个碳消除过程，表明土壤食用化白蚁的甲烷产量很高。白蚁在后肠中藏有丰富的细菌。因此，怀疑白蚁利用后肠细菌作为氮源。我们检测了白蚁肠道中分解细菌细胞壁的溶菌酶的活性。产自唾液腺的木食用虫网状白蚁（Reticulitermes speratus）前肠溶菌酶活性高，中肠蛋白酶活性高。从唾液腺分泌到前肠的溶菌酶很可能消化了中肠通过营养作用转移的共生细菌。我们怀疑土食性白蚁（占白蚁属的50%）的主要食物是溶菌酶，并在肠道中显示出溶菌酶的活性，但我们无法证明共生菌是否是它们的主要食物。利用稳定同位素分析了白蚁的食土倾向。我们将研究扩展到澳大利亚昆士兰热带稀树草原的种群水平氮经济。

Termites are super-abundant soil animals and play an important role on the decomposition of dead plant material containing little nitrogen in tropical terrestrial ecosystems. Most dead plant material contains only 0.03-0.7% nitrogen and C/N ratio is 70-500, while termites contain about 19% nitrogen and their C/N ratio is 4-12. Therefore, termites must solve this C/N balance problem by either increasing nitrogen intake or selectively eliminating carbon. Tayasu et al (1994) showed that about 50% of body nitrogen of a lower termite, Neotermes koshunensis comes from the air through the activity of nitrogen-fixing bacteria in the gut.In this study we examined mainly how termites solve C/N balance problem with reference to their role in ecosystems. We measured methane production by termites as a process of carbon elimination, showing that the methane production was high in soil feeding termites. Termites harbor abundant bacteria in the hindgut. Therefore, its was suspected that termites utilized the hindgut bacteria as a nitrogen source. We examined the activities of lysozyme which decomposes the cellwall of bacteria in termite guts. The activity of lysozyme of a wood-feeder, Reticulitermes speratus, which was produced in salivary gland, was high in the foregut, and the activity of protenase was high in the midgut. It was highly probable that lysozyme secreted from salivary gland to foregut digests the symbiont bacteria transferred by trophallaxis in the mid gut. We suspected that major food of soil feeding termites, which occupy about 50% of termites at genus level, and showed the presence of lysozyme activity in the gut, although we could not show if symbiont bacteria are their major food or not. We showed the tendency of soil-feeding in termites using stable isotope. We extended our studies into the nitrogen economy at colony level in the savanna of Queensland of Australia.