Temporal variations in plant delta C-13 values and implications for using the C-13 technique in long-term soil organic matter studies

The C-13 natural labeling technique in combination with modeling could pote ntially be used to analyze SOM changes in long-term experiments involving c omplex cropping systems such as aeroforestry systems. It requires informati on on quantity and C-13 abundance of all plant materials entering the soil throughout the experiment. The occurrence of significant year-to-year chang es of 6 13C values of the plant inputs may complicate the technique, as it would require a systematic isotopic analysis of all plant inputs over the e ntire life span of the experiment. By analyzing crop and tree samples from different sampling times in a long-term agroforestry experiment under a sub -humid tropical climate we established that year-to-year variations in plan t delta C-13 values were of the order of only 0.4-0.6%, i.e. only 3-4% of t he maximum tracer signal difference between C-3 and C-4 plants (similar to 15 parts per thousand,) Hence plant sampling and isotopic analysis can be l imited to 2-3 years or even a single representative year in terms of weathe r. Important and consistent within-year variations were, however, observed for legume tree prunings (up to 2.4 parts per thousand) and weeds (up to 7 parts per thousand), pointing to the need for frequent sampling within a si ngle year if such seasonal variation is suspected. Observed delta C-13 valu es for the weed vegetation revealed a clear shift in weed composition in te rms of C-3 and C-4 weeds within the season and across treatments, as was co nfirmed by a visual weed species identification in the field. (C) 2001 Else vier Science Ltd. All rights reserved.