Respiration from C3 plant green manure added to a C4 plant carbon dominated soil

Application of tree leaves (C-3 plants) on maize (Zea mays L.) (C-4 plant) fields is an agroforestry management technology to restore or maintain soil fertility. The rate at which the tree leaves decompose is crucial for the nutrient supply to the crop. We studied the in situ decomposition of Sesbania sesban (L.) Merr. leaves or C-3 sugar for 4 - 8 days after application to a maize field in Kenya. By using the difference of around 10 parts per thousand in natural abundance of C-13 between the endogenous soil C (mainly C-4) and the applied C (C-3), we could calculate the contributions of the two C sources to soil respiration. The delta(13)C value of the basal respiration was from -15.9 to -16.7 parts per thousand. The microbial response to the additions of leaves and sugar to this tropical soil was immediate. Application of sesbania leaves gave an initial peak in respiration rates that lasted from one to less than 6 days, after which it levelled off and remained about 2 - 3 times higher (230-270 mg C m(-2) h(-1)) than the control respiration rates throughout the rest of the experiment (5 - 8 days). In the sugar treatment, there was no initial peak in respiration rate. The respiration rate was 170 mg C m(-2) h(-1) after 4 days. At the end of the experiments, after 4-8 days, as much as 14-17% of the added C had been respired and about 60% of the total respiration was from the added sesbania leaves or C-3 sugar. This non-destructive method allows repeated measurements of the actual rate of C mineralisation and facilitates decomposition studies with high temporal resolution in the field.