Describing the transformation of organic carbon and nitrogen in soil using the MOTOR system

This article presents a framework (MOTOR 3.01, MOdular description of the Turnover of Organic matteR), which describes the transformation of organic carbon and nutrients in soil. The state of each component of the organic matter in soil is described by a vector and the transformations by a matrix of terms. Actual turnover is calculated by multiplication of these matrices, state vectors and a rate vector. The resulting system is powerful because it is modular in construction and any one part of it may be altered simply and quickly without reference to the rest of the calculation system. Once the flow of an element is described, related nutrients (nitrogen in relation to carbon, for example) and their isotopic tracers are handled directly without additional programming or without repeated simulations. A great benefit of this approach is that mixtures of substrate or substrates of very different qualities (C:N, fibre content, labelled and unlabelled) can be dealt with easily by inserting another residue pool. If necessary the kinetics of decomposition of this new pool can differ from the others; the fate of its decomposition products is also easy to define. Where nutrients such as N are in short supply, the transformations are no longer a linear function of inputs. A novel two-call algorithm is described which first identifies if shortages of N occur during decomposition as a whole and then modifies decomposition iteratively until supply and demand for N are in balance.The structure of the modules is explained and an example given of the system's application to test a model of the protection of organic carbon and nitrogen in soil during turnover. Parameters derived from a published dataset for carbon turnover are adapted to follow the turnover of 15N-labelled crop residues in soil and the resultant model tested against two independent datasets.