Modeling Soil Organic Carbon Dynamics

To quantitatively i~vestipte the effects of different uwiroemental f~ctors such as temperature water status~ clay fractior~ on organic matter decomposition, C02-C was measured froni the incubetion of wheat and rice straw as well as wheat and rice stubble mixed with the paddy soil over a 90- day period. Five temperature levels of 120C~ 18C 24C~ 30 0C and 360C were respectively set up. Four water statuses including 200glkg, 300g/kg, 500g/kg and submerged condition were involved for each temperature level. Under the incubation condition of 360C and 300g/kg, the influence of soil texture on the organic matter decay was investigated by incubating the mixture of wheat straw and paddy soil with different clay fractions over a 90-day period. Results show that the decay rate of straw was higher than that of stubble and roots. An increase of temperature within a lower temperature range enhanced greatly the decomposition while such an enhancement decreased within a higher temperature range. Under the condition of non- flooding water status, the temperature coefficient of organic matter decomposition (Q,~,) declined exponentially with the incubation time (t) which can be expressed as Q1(,=l.263EXP(2.881/t). The effect of water status on the organic matter decomposition shown that the decay rate at the water contents of 300g/kg?00g/kg was higher than those under the treatments of 200gIkg and the submerged. Within the first month period of incubation, carbon amount emitted by decomposition was negatively related to the clay fraction of the soil. The basic functions of different environmental factors are set up to elucidate their influence on decomposition. Using the data obtained from incubation experiment and the 揵lack box? theory, we established the model of organic matter decomposition with different variable factors with the aid of SYSTAT, Excel software. The model is as follows: Differential Model: z~~C SIxTI3xWI1x(k, xOM~1+k2xOM51) Integral Model: C,拁C0x { Fx (1 -exp(-k1 x SIxTI,x WI1xt)] +( 1-F) 41 -exp (-k2x SIxTI,x WI,xt)] OMN+k2x 0M5) the decomposition amount of the i day, SI: soil texture function, TI,~ WI1: the temperature and water functions of the i day, OMNI :liable component, 0M3 : non-liable component, C,: the amount of accumulated OM decomposition, C0: the amount of initial OM. F: die liable fraction, t the decomposing dLys, k1~ k2: the decomposition rate of liable component, OM~, : non-liabk component. 60 The model above is validated by the data of incubation experiment, the field experiment and the long-term soil improvement experiment. The organic carbon value measured has a good relationship with the simulated. The utility of this model can be exploited to forecast the organic carbon. For the paddy soil of Wanshi in Yixing, the result shows that the organic carbon content will drop from 8g/kg to about 6glkg in thirty years from 1991 without straw returning, rise from 8 g/kg to about 11 g/kg with half straw returning and from 8 g/kg to about I 6g/kg with whole straw returning. If the organic carbon content will be I 3g/kg in thirty years. there should be 60% straw returned every year. Under this condition, the carbon released from soil is about 3500kg/ha per year.