Dynamics of soil organic matter formation in croplands — Conceptual analysis

Abstract

A conceptual analysis of soil organic matter (SOM) dynamics in croplands was made by the aid of a model. The state variables considered were plant residue (PR) and SOM. Multicompartments of PR and SOM were considered in the model, and a two-compartment case was used for illustration. The parameters considered are the input rate of PR and SOM, the disappearance rate constants of the PR and SOM variables, and the respiratory fraction of the transformation processes. The large mean residence time (MRT) difference found among SOM variables in literature greatly simplified the solution of the SOM model. In a time span of several cropping seasons to several decades, only the active SOM compartments with MRT less than a few decades will contribute to the significant changes of SOM status in croplands. The stable SOM compartment with the MRT of several hundred or several thousand years can be treated as a constant background of the SOM. The PR input parameter is directly related to the residue of the primary production of a cropland. The SOM input parameter is a fraction of the PR input parameter and is the humified fraction as it is implied in the SOM model. The mean age (MA) of a SOM is a function of time and is limited by the value of the MRT of the most stable SOM compartment. The MA of a cropland usually increases as the time of cultivation increases. The changes in MA of the SOM in a soil after cultivation reflect on changes of the relative size of the native stable SOM in a cropland.