The carbon‐isotope composition of both forest and grassland soils from Papua New Guinea exhibit predictable trends with increasing altitude. Soils under pure C3vegetation (forests and alpine grasslands above 4000 m) show an increase in δ13C value with altitude paralleling the increase in δ13C value observed in plant leaves byKörner et al.1988. Grassland soils show a decrease in δ13C value above about 1000 m, from maximum values which are close to pure C4values (−12 to −13‰ vs. PDB) to minimum values which are indistinguishable from pure C3values at 3500–4000 m (∼−26‰). Within this general framework, several factors can influence the soil δ13C value at individual locations. In local forest settings, soil δ13C values will be influenced by the degree to which respired CO2is re‐utilized during photosynthesis, the proportions of leaf and wood litter, and the degree of decomposition. In grassland settings the primary factor controlling the observed δ13C variability at any specific altitude is the amount of nongrass C3carbon present in the sample. It is also possible that other factors, such as moisture availability, may play some role in determining the proportions of C3and C4grasses at any given altitude, although further work would be required to substantiate such a link. The results provide a framework within which to more accurately constrain the carbon‐isotope composition of terrestrial carbon pools and to interpret variations in the isotopic composition of riverine
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