The management of rice straw, fertilisers and leaf litters in rice cropping systems in Northeast Thailand - 1. Soil carbon dynamics

Declines in soil organic mailer (SOM) have occurred in most rainfed rice cr opping systems in the world. Such declines threaten the sustainability of t hese systems and steps need to be taken to reverse the decline. Paired soil samples collected from forest and cropped areas in Northeast Thailand reve aled an approximately 70% decline in both labile carbon (C-L) and total car bon (C-T) in the surface 10 cm as a result of cropping. C-L was found to be higher in the 20-40 cm soil layer in the cropped than the forest soil indi cating C leaching or soil mixing. In a field experiment it was found that C-L, and a calculated Carbon Manage ment Index (CMI), increased with annual applications of low rates (1500 kg/ ha dry matter) of leaf litter from Cajanus cajan, Phyllanthus taxodifolius, Acacia auriculiformis and Samanae saman. Five seasons of leaf litter appli cation increased the C-T pool by 24-37% and more than doubled C-L and soil C lability. The retention of rice straw improved C-T in the first year of t he trial but led to no significant increases in subsequent years. Higher ra tes of fertilisers did not result in increased soil C, despite increased yi elds. Crop residues, leaf litters and green manures with slow breakdown rates are needed to rehabilitate soil C. C-L and the CMI can be used effectively to monitor the rate of change in soil C.