The carbon and nitrogen isotope composition of Australian grasses in relation to climate
Murphy, BP and Bowman, DMJS, The carbon and nitrogen isotope composition of Australian grasses in relation to climate, Functional Ecology, 23, (6) pp. 1040-1049. ISSN 0269-8463 (2009) [Refereed Article]
1. The carbon and nitrogen isotope composition of plants are known to be affected by environmental
factors, especially water availability. While d13C of C4 plants is generally assumed to be
much less responsive to environmental variables than that of C3 plants, it is unclear whether the
response of d15N differs between the two photosynthetic pathways.
2. Focussing on differences in the response of members of the C3 and C4 photosynthetic pathways
to climate variables, we examined the foliar d13C and d15N of grasses collected from natural
vegetation in a wide range of climatic zones throughout Australia.
3. An index of water availability was clearly related to foliar d13C and d15N of both C3 and C4
grasses. There was a negative relationship between water availability and d13C of C3 grasses
(r2 = 0∆21), similar to that documented extensively in other C3 plants. An opposite, positive
relationship was found between water availability and d13C in C4 grasses (r2 = 0∆17), an effect
that has been reported only infrequently. Accounting for differences in d13C between the different
C4 types (NADP-ME, PCK, NAD-ME and Aristida) resulted in a substantial increase in the
fit of the model relating d13C to water availability (R2 = 0∆48).
4. There was a negative relationship between water availability and d15N (r2 = 0∆40), that was
similar in both C3 and C4 grasses, but temperature had no effect on d15N. This provides strong
support for the theory that water availability is the dominant factor determining soil and plant
d15N via its effect on the Ďopennessí of the nitrogen cycle.
5. We also found significant differences in d15N between the different C4 types, with the highest
values for PCK, followed by NADP-ME and NAD-ME, and the lowest values for Aristida. The
difference in d15N between PCK and Aristida was large, at 5∆1&.
6. The importance of water availability as a predictor of d13C in both C3 and C4 grasses suggests
that variation in water availability should be considered when estimating C4 grass abundance
based on d13C measurements (e.g. in herbivore diets or as a contributor to biospheric carbon
C3, C4, photosynthetic pathway, Poaceae, rainfall, stable isotope, water availability