Clear patterns can be seen in the stable carbon and nitrogen isotope composition (expressed as δ13C and δ15N respectively) of natural vegetation throughout the world, closely reflecting climate and other environmental factors. Water availability appears to be the most important factor, usually showing a strong negative relationship with both δ13C (e.g. Stewart et al. 1995; Swap et al. 2004; Weiguo et al. 2005) and δ15N (e.g. Handley et al. 1999; Schuur & Matson 2001; Swap et al. 2004). In the case of δ13C however, the negative relationship with water availability is usually only present in plants using the C3 photosynthetic pathway. In C4 plants, δ13C is much less variable and relationships with climatic factors are usually absent (Van Der Water et al. 2002; Swap et al. 2004).
In C3 plants, δ13C is primarily influenced by the ratio of intercellular to ambient concentrations of CO2 (ci/ca) (Farquhar et al. 1989a). Plants regulate ci/ca by opening and closing their stomata, in response to a range of environmental factors, including light (Yakir & Israeli 1995), nutrients (Raven & Farquhar 1990) and water availability (Winter et al. 1982). A close relationship exists between ci/ca and plant water use efficiency (WUE), which means that δ13C can provide an estimate of the integrated long-term WUE of a plant (Ehleringer 1989; Farquhar et al. 1989b). The relationship between water availability and δ13C of natural vegetation arises in two ways. First, at an individual level, plants respond to decreasing water availability by increasing their WUE, and hence δ13C (Farquhar et al. 1989b). Secondly, plants adapted to more arid environments tend to have higher WUE, and hence δ13C, than plants adapted to more mesic environments, even when grown in the same environment (Anderson et al. 1996).
In C4 plants, the effect of ci/ca on δ13C is similar to that in C3 plants, but greatly diminished, and confounded by post-photosynthetic fractionation due to ‘leakiness’ of the bundle sheath cells to CO2 (Farquhar 1983). Depending on the degree of leakiness, the slope of the relationship between δ13C and water availability can range from positive (e.g. Weiguo et al. 2005) to negative (e.g. Wang et al. 2005), but in general, δ13C in C4 plants tends to vary much less in response to environmental factors than in C3 plants (Henderson et al. 1992). Perhaps the largest source of variation in δ13C in C4 plants is biochemical subtype, with C4 plants generally divided into three groups on the basis of biochemistry: NADP-ME, NAD-ME and PCK, named after their respective C4 acid decarboxylases. NADP-ME species tend to have the highest δ13C, followed by PCK and then NAD-ME species (Hattersley 1982).
The causes of variation in δ15N are much less clearly understood than the causes of variation in δ13C. Although extensive fractionation may occur within plants (Evans 2001), foliar δ15N tends to reflect soil δ15N (Austin & Vitousek 1998; Handley et al. 1999), and a number of authors have suggested that it is the ‘openness’ of the nitrogen cycle that primarily influences soil δ15N (Austin & Vitousek 1998, Handley et al. 1999, Schuur & Matson 2001). In an open nitrogen cycle, gains and losses of nitrogen are large relative to the total nitrogen pool. This is typical of arid areas, where water, rather than nitrogen, tends to be limiting. In more mesic areas, nitrogen, rather than water, tends to be limiting, such that it is efficiently recycled, with little leaving the nitrogen cycle.
Accurate prediction of foliar δ13C and δ15N has numerous applications in ecological studies. Values of δ13C of plant and animal remains can be used to estimate the proportion of plant biomass or diet that was C4, but typical values of δ13C for C3 and C4 plants must be estimated (Vogel 1978; Witt et al. 1998; Cerling et al. 2006; Codron et al. 2007). Accurate estimates of δ13C of C3 and C4 vegetation are also required for use in global carbon budgets that estimate the relative importance of terrestrial and marine carbon sinks from δ13C of atmospheric CO2 (Lloyd & Farquhar 1994; Fung et al. 1997; Suits et al. 2005). Values of δ15N of animal remains can be used to estimate trophic level, as animals tend to be enriched by 1–5‰ with each increasing trophic level (Hobson & Montevecchi 1991; Kwak & Zedler 1997). However, variation in the δ15N signature of plant material at the base of the trophic structure must be accounted for.
While most previous studies of variation in plant δ13C and δ15N have focused on woody plants, grasses provide an opportunity to directly compare the influence of the C3 and C4 photosynthetic pathways on the response of δ13C and δ15N to environmental factors, within a single functional group. This is not as readily achieved with woody plants due to the limited occurrence of C4 photosynthesis within this group. In this study, we examine the variation in foliar δ13C and δ15N of grasses throughout the Australian continent, in relation to a range of environmental factors. Given the existing evidence for a close relationship between water availability and both δ13C and δ15N in other plants, we focus primarily on this factor, and evaluate three hypotheses about isotopic variation in relation to water availability:
- 1 there is strong a negative relationship between δ13C and water availability in C3 grasses;
- 2 there is no relationship between δ13C and water availability in C4 grasses; and
- 3 there is a negative relationship between δ15N and water availability, similar in both C3 and C4 grasses.