Stable Isotopes in Ecosystem Science: Structure, Function and Dynamics of a Subtropical Savanna†

Stable isotopes are often utilized as intrinsic tracers to study the effects of human land uses on the structural and functional characteristics of ecosystems. Here, we illustrate how stable isotopes of H, C, and O have been utilized to document changes in ecosystem structure and function using a case study from a subtropical savanna ecosystem. Specifically, we demonstrate that: (1) d 13C values of soil organic carbon record a vegetation change in this ecosystem from C4 grassland to C3 woodland during the past 40–120 years, and (2) d2H and d 18O of plant and soil water reveal changes in ecosystem hydrology that accompanied this grassland-to-woodland transition. In the Rio Grande Plains of North America, d13C values of plants and soils indicate that areas now dominated by C3 subtropical thorn woodland were once C4 grasslands. d13C values of current organic matter inputs from wooded landscape elements in this region are characteristic of C3 plants (ÿ28 toÿ25%), while those of the associated soil organic carbon are higher and range fromÿ20 to ÿ15%. Approximately 50–90 % of soil carbon beneath the present C3 woodlands is derived from C4 grasses. A strong memory of the C4 grasslands that once dominated this region is retained by d13C values of organic carbon associated with fine and coarse clay fractions. When d13C values are evaluated in conjunction with 14C measurements of that same soil carbon, it appears that grassland-to-woodland conversion occurred largely within the past 40–120 years, coincident with the intensification of livestock grazing and reduction