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This chapter presents an overview of the respiratory/carbon costs of symbiotic nitrogen fixation. The various theoretical costings for nitrogen fixation suggest that respiration directly associated with nitrogenase activity will require between 1.77 and 3.01 g C g-N (4.35–7.00 mol CO mol N), while respiration of the entire nitrogen-fixing nodules will require between 2.78 and 4.81 g C g-N (6.51–11.19 mol CO mol N). Early attempts to measure these costs were beset by methodological problems, but some reliable approaches were developed. Measured values based on root respiration during the period of active nitrogen fixation are in the range of 5–10 g C g-N (11.6–23.4 mol CO mol N), with an average value of 6.5 g C g-N (15.1 mol CO mol N). On a nodule basis, values in the range of 3–5 g C g-N (7–12 mol CO mol N) appear to represent the ‘normal’ for legume nodules, while values below about 2.5 g C g-N are likely to be erroneous. The implications of these costings are considered in terms of the need for legumes to carefully regulate nitrogen fixation and the requirement for such regulation systems to be operational in any novel nitrogen-fixing plants.

Mycorrhizal fungi form symbiotic and often mutually beneficial relationships with the roots of most terrestrial plants. In this chapter we review current literature concerned with plant respiratory requirements for supporting this important plant-fungal association, and its effect on the overall plant carbon economy. Controlled studies indicate that mycorrhizal respiratory costs are considerable, consuming between 2 to 17% of the photosynthate fixed daily, varying depending on the host and fungal species involved, the stage of colonization, and the environmental conditions. Respiratory energy is required by the mycobiont for construction of new intraradical and extraradical fungal tissue (including reproductive structures), for maintenance and repair of existing fungal tissue, and for cellular processes in the fungal tissue associated with the absorption, translocation and transfer of nutrients from the soil to the host. Additional respiration is also required by the host plant for stimulated root cellular processes, and potentially for increased production of root biomass. Field studies of these important processes will eventually lead us to better understand how significant mycorrhizal fungi are to the total carbon budgets of natural and managed plant communities.

Atmospheric CO concentrations have been increasing since the industrial revolution due to fossil fuel burning and deforestation. Elevated levels of atmospheric [CO] are likely to enhance photosynthesis and plant growth, which, in turn should result in increased specific and whole-plant respiration rates. However, a large body of literature has shown that specific respiration rates of plant tissues can be considerably reduced when plants are exposed to or grown at high [CO]. Reductions in respiration by [CO] have been explained by either direct inhibitory effects of [CO] on respiratory processes or by indirect effects associated with changes in the chemical composition of tissues of plants grown at high [CO]. The observed reductions in plant respiration rates by elevated [CO] can represent a large biospheric sink for atmospheric carbon. Although doubling current ambient levels of atmospheric [CO] could inhibit some mitochondrial enzymes directly in the short-term, the magnitude of the direct effect of [CO] on tissue respiration has now been shown to be largely explained by measurement artifacts, diminishing the impact that direct effects would have on the carbon cycle. A reduction in construction and maintenance costs of tissues of plants grown at high [CO] can explain an indirect reduction of respiration. Such indirect effects, however, may be offset by the larger biomass of plants exposed to elevated [CO]. A lack of clear understanding of the physiological control of plant respiration, of the role(s) of non-phosphorylating pathways, and effects associated with plant size, makes it difficult to predict how respiration and the processes it supports respond to elevated [CO]. Therefore, the role of plant respiration in augmenting or controlling the sink capacity of terrestrial ecosystems is still uncertain.