A system dynamic model to assess exploitability of agricultural residues and effects on soil organic carbon

Bio-based products are made from organic materials; the use of crop residues eliminates the competition with the food sector and lowers the costs. However, crop residues are also an important input of organic carbon in the soil, and are, therefore, important to maintain soil quality and productivity levels. The aim of this thesis is to evaluate the variation of Soil Organic Carbon (SOC) based on the amount of agricultural residues extracted during harvesting, by use of a dynamic model. To do so, a System Dynamic model of the turnover of C in soils, based on the RothC model, was implemented. The model was used to simulate the effects of wheat straw removal from the field on the SOC in a case study based in Ravenna (Italy). A one at a time (OAT) exploratory sensitivity analysis was also conducted to study the model behaviour. The results show an inverse linear relationship between the amount of residues extracted and the SOC at 10, 20 and 100 years of the simulation. Harvesting the crop residues can result in a decrease of the SOC of ~50%. The sensitivity analysis shows that particular care should be taken in determining the decomposition rate constant of the humus in the soil studied. An observed limitation of the RothC model is that it overestimates the SOC in semi-arid climate; however, this did not affect the results of the present case study. Furthermore, the work underlines the role of the soil carbon/nitrogen ratio as a necessary determinant of the SOC to be maintained in soil.