Does season and grazing influence the δ13C and δ15N of C4 native grasses in semi-arid rangelands of the Pilbara region of NW Australia?

Much of the semi-arid, native rangelands Pilbara region of NW Australia is used for cattle production with the most productive grasslands for cattle grazing dominated by the C4 perennials grasses; Astrebla pectinata, Themeda triandra, and Eragrostis xerophila. In both grazed and ungrazed plots, we quantified the seasonal variation in the natural abundance of 13C and 15N of green foliage. We assigned the C4 pathway of individual species (NAD-ME vs. NADP-ME) on the basis of δ13C. Foliage δ13C varied with seasonal conditions that in turn dictated patterns of growth (and of photosynthate production), and possibly also with the 'leakiness' of the bundle sheath cells. However the significant differences present in grazed and ungrazed plots for Astrebla and Themeda, indicate that grazing may result in greater water use efficiency in these semi-arid rangelands. Seasonal patterns of δ15N were highly variable and exhibited little correlation with seasonal patterns of growth and periods of inorganic N uptake. While it is possible that N obtained from sources other than the simple uptake of inorganic N contributed to observed δ15N patterns, there is little additional evidence to support this hypothesis. There was no difference in δ15N between grazed and ungrazed treatments for any of these grasses.