Nitrogen dynamics in the tropical savanna of Australia

白蚁是一种超丰富的土壤动物，在热带陆地生态系统中对含少量氮的枯死植物物质的分解起着重要作用。大多数枯死植物只含有0.03-0.7%的氮，C/N比为70-500，而白蚁的氮含量约为19%，C/N比为4-12。因此，白蚁必须通过增加氮的摄入量或选择性地清除碳来解决这一碳/氮平衡问题。Tayasu等人(1994)指出，新白蚁体内约50%的氮素是通过肠道内固氮细菌的活动从空气中获得的。我们测量了白蚁的甲烷产生量作为一个碳消除过程，表明白蚁饲养的土壤甲烷产生量很高。白蚁的后肠里有大量的细菌。因此，怀疑白蚁利用后肠道细菌作为氮源。我们检测了白蚁肠道中分解细菌细胞壁的溶菌酶的活性。在唾液腺中产生的精巢网虫的溶菌酶活性在前肠较高，在中肠中蛋白酶活性较高。很可能是由唾液腺分泌到前肠的溶菌酶消化了中肠营养转移的共生菌。我们怀疑土壤中的主要食物是白蚁，在属水平上约占白蚁总数的50%，并显示肠道中存在溶菌酶活性，尽管我们不能表明共生菌是否为它们的主要食物。我们用稳定同位素显示了白蚁取食土壤的趋势。我们将研究扩展到澳大利亚昆士兰稀树大草原群体水平的氮素经济。

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.