Effects of frequent fires and grazing on stable nitrogen isotope ratios of vegetation in northern Australia

The ratios of stable nitrogen isotopes expressed as delta(15)N values can indicate the openness of nitrogen cycles in ecosystems. Southwards through the Northern Territory, values of foliar delta(15)N in savanna trees increase as mean annual rainfall decreases from approximately 1800 mm to approximately 750 mm, with foliar delta(15)N thereafter decreasing toward arid central Australia. Recent literature argues that this pattern is caused by higher grazing intensity in semi-arid savannas, but counter views have attributed the pattern more directly to variations in aridity. In this paper, grazed and ungrazed sites in a semi-arid savanna are compared, and it is shown that grazing has a relatively small effect on the positive foliar delta(15)N values of grasses, but no effect on delta(15)N values of trees. This gives little support to the argument that variations in grazing pressure at the scale of hundreds of kilometres could result in detectable differences in the foliar delta(15)N values of trees. I then compare the semi-arid savannas with mesic savannas, where fires are frequent, and with mesic rainforests, which are rarely burnt. Greater foliar delta(15)N values in rainforest and fire-excluded mesic savannas than in frequently burnt savannas suggests that fire regimes affect foliar delta(15)N. The previously observed pattern in delta(15)N values along the rainfall gradient in the Northern Territory is consistent with trends in fire frequency and possible direct effects of fire, but further work is required to determine the relative impacts of aridity and fire. Within a particular rainfall regime, foliar delta(15)N values may indicate historical fire frequencies.