Respiration from C-3 plant green manure added to a C-4 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 Sesban ia sesban (L.) Merr. leaves or C-3 sugar for 4 - 8 days after application t o a maize field in Kenya. By using the difference of around 10 parts per th ousand 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 tw o C sources to soil respiration. The delta(13)C value of the basal respirat ion was from -15.9 to -16.7 parts per thousand. The microbial response to t he additions of leaves and sugar to this tropical soil was immediate. Appli cation of sesbania leaves gave an initial peak in respiration rates that la sted 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 respi ration rates throughout the rest of the experiment (5 - 8 days). In the sug ar treatment, there was no initial peak in respiration rate. The respiratio n 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 ab out 60% of the total respiration was from the added sesbania leaves or C-3 sugar. This non-destructive method allows repeated measurements of the actu al rate of C mineralisation and facilitates decomposition studies with high temporal resolution in the field.