THE USE OF STABLE CARBON ISOTOPES FOR ESTIMATING SOIL ORGANIC-MATTER TURNOVER RATES

In natural ecosystems, soil organic carbon (C) is derived almost exclu sively from the residues of plants growing in situ. In agroecosystems, it has at least two origins: one is the remains from the previous nat ive vegetation, and the other is the remains of the crop and the decom position of its residues. Where vegetation has changed from plants wit h the C3 photosynthetic pathway to C4 pathway or vice versa, changes i n the natural abundance of C-13 in soil organic matter (SOM) over time can be used to identify sources of organic C in soil and to determine the turnover rate of SOM. For example, large areas of C3 tropical for est have been replaced with C4 pasture or cropland. Changes in the del ta(13)C values of soil organic C in these areas reflect soil organic m atter turnover rate, and provide insight regarding the functional role of tropical ecosystems in the global C cycle. This paper illustrates how the stable isotope C-13 can be used to estimate SOM turnover rates and the sensitivity of different models and different model parameter s, using a chronosequence of forest and pastures of different ages fro m the Brazilian Amazon. A single-compartment exponential decay model a nd a two-compartment model in which SOM was divided into stable and la bile components yielded similar estimates of soil C turnover time at t he surface but divergent estimates at depth. The one-compartment model gave the least variable estimates of model parameters and turnover ti mes and was also relatively insensitive to individual C stocks in sing le pastures of a particular age. Estimates of soil stable and labile C pools obtained using changes in forest soil delta(13)C With depth dif fered from estimates obtained using the chronosequence. This suggests that upon burning and pasture creation, a portion of the previously st able soil C pool is rendered less stable. Model r(2) was a poor criter ion for selecting an appropriate soil C turnover model to apply to chr onosequence data. In the absence of substantial justification for segr egating SOM into different compartments based on lability, modeling sh ould be done with the simplest models possible. (C) 1998 Elsevier Scie nce B.V.