Quantitative description of soil organic matter dynamics - A review of approaches with reference to rice-based cropping systems

This review paper presents a historical overview of quantitative approaches used in describing soil organic matter (SOM) dynamics, with the objective of identifying processes relevant in describing long-term carbon and nitrogen dynamics in rice-based cropping systems. A number of existing SOM models have been evaluated based on their objectives and underlying concepts. Models are grouped into two broad classes of analytical and simulation models, and within the latter comprehensive and summary models, based on the level of detail in process descriptions. Most of the existing SOM models have been developed for aerobic, upland conditions and describe the processes of decomposition of organic matter, growth and death of microbial biomass, nitrification, immobilization, denitrification, NH4+-volatilization, leaching of NO3¿, crop uptake, erosion and runoff. However, in anaerobic lowland conditions, the rates and relative importance of the processes are different. This paper evaluates the suitability of existing models for application under alternate flooded and non-flooded conditions, corresponding to rotations of rice-upland crops. Since no existing SOM model describes the processes for such conditions and their effect on SOM quantity and quality, a simulation model is yet to be developed, including these processes, which can satisfactorily describe the changes in SOM and crop yield. This paper also identifies the processes to consider in developing a long-term simulation model for alternate flooded and non-flooded conditions.