We tested the Rothamsted Carbon Model (RothC) against three long-term (27-28 years) experimental sites on Thai upland soils in order to see how this widely used 'temperate' soil carbon turnover model performed in a typical farming region in the tropics. We were able to verify -over a much longer period than had been examined in previous studies -that RothC performs well in a tropical region in plots used for continuous cropping experiments of maize and cassava without organic matter application. However, the model overestimated soil organic carbon (SOC) in some plots to which large amounts of organic matter (rice straw or cassava stalks) were applied. This overestimate could not be attributed to errors in estimating either the amount of C input to the soil or the ratio of decomposable plant materials to resistant plant materials entering the soil. Among many factors affecting SOC dynamics (e.g. weather conditions, soil characteristics, etc.), which are different in tropical regions from temperate regions, we conclude that the activity of soil fauna might be a major factor which makes the performance of RothC worse where much organic matter was applied. We suggest that care should be taken when applying RothC to tropical soils with large amounts of added organic matter.

This study was carried out to analyze the effects of cattle manure application and no-tillage cultivation on changes in soil carbon stock and carbon balance in maize (Zea mays L.) fields in infertile tropical sandy soil in Khon Kaen, Northeast Thailand. The experimental field was treated for 3 years from August 1999 to June 2002 with conventional cultivation, cattle manure application and no-tillage cultivation. Cattle manure was applied at a rate of 20 Mg ha -2 (fresh weight) 5 days before sowing maize. Weeds in the notillage cultivation field were sprayed with herbicide and dead weeds were left in the field. Soil was sampled at 0-50 cm depth before plowing in the first-third years and 1 year after the third year of the experiment. The CO 2 flux from the soil surface was measured using the alkaline absorption method every 1-4 weeks throughout the period. The biomass of maize and weeds was stable under conventional cultivation, increased with cattle manure application, and decreased with no-tillage cultivation. Soil carbon stock in the 0-30 cm soil depth increased from 16.0, 11.0 and 13.3 Mg C ha -1 at the start of the experiment to 17.1, 18.2 and 15.9 Mg C ha -1 3 years later in the conventional cultivation, cattle manure application and notillage cultivation treatments, respectively. The CO 2 flux from the soil surface of the maize field was 10.1-11.5 Mg C ha -1 year -1 under conventional cultivation and 8.5-9.4 Mg C ha -1 year -1 under no-tillage cultivation over the 3 years of the experiment. With cattle manure application, the CO 2 flux was 10.5 Mg C ha -1 year -1 in the first year, which was not different from the other treatments; this increased to 13.3-15.3 Mg C ha -1 year -1 in the second and third years. Analysis of the carbon balance showed that the applied cattle manure increased the soil carbon stock and plant biomass, which consequently further increased carbon input to the soil. In the no-tillage cultivation treatment, carbon loss by CO 2 flux from the soil was low, although carbon input to the soil by returning maize residues and weeds was also low; hence, only a small amount of carbon input accumulated in the soil.

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