Temporal variations in plant δ13C values and implications for using the 13C technique in long-term soil organic matter studies

The C-13 natural labeling technique in combination with modeling could potentially be used to analyze SOM changes in long-term experiments involving complex cropping systems such as aeroforestry systems. It requires information on quantity and C-13 abundance of all plant materials entering the soil throughout the experiment. The occurrence of significant year-to-year changes 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 entire 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 plant delta C-13 values were of the order of only 0.4-0.6%, i.e. only 3-4% of the 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 limited to 2-3 years or even a single representative year in terms of weather. 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 single year if such seasonal variation is suspected. Observed delta C-13 values for the weed vegetation revealed a clear shift in weed composition in terms of C-3 and C-4 weeds within the season and across treatments, as was confirmed by a visual weed species identification in the field. (C) 2001 Elsevier Science Ltd. All rights reserved.