The ICBM family of analytically solved models of soil carbon, nitrogen and microbial biomass dynamics — descriptions and application examples

Abstract

Based on the Introductory Carbon Balance Model, ICBM, we present a set of analytically solved models. The original ICBM comprises ‘Young’ and ‘Old’ soil C, two decay constants and parameters for litter input,'humification' and external influences — in all five parameters. The new models describe soil C (and N) balances more in detail, but they are built around the same core concepts such as first-order decomposition kinetics and a minimum number of soil C and N pools. More complex processes, such as plant growth and mortality as well as weather influence are not explicitly included. However, these processes are allowed to influence the model predictions by modifying model parameter values. These modifications may be based on ‘best guesses’, parameter optimisations to available data, or independent ‘front-end models’, e.g. calculations of temperature influence on decomposer activity. Listed according to increasing complexity, the models are: (1) ICBM/N, which is ICBM with nitrogen added. It calculates net N-mineralisation and adds parameters for C/N ratios and soil organisms as well as organism efficieny — nine parameters in all; (2) ICBM/2N, which gives a more precise description of the initial stages of decomposition by splitting the ‘Young’ pool into two. The nitrogen part of the model has parameters for the C/N ratios of ‘labile’ and ‘refractory’ input of organic material, organism biomass and humification and also microbial growth efficiency for ‘labile’ and ‘refractory’. The model has 13 parameters in all, but can be run as a pure C model (ICBM/2) with only seven parameters; (3) ICBM/2BN, where organism biomass C and N is explicitly modelled. This model is usually run with daily, weekly or monthly steps and adds parameters related to biomass — 18 parameters in total — and can be run as a pure C model (ICBM/2B) with 13 parameters or even as a model with only one ‘Young’ pool (ICBM/B). We give examples of model applications, both short- and long-term, and show that the models relatively easily can be applied to various, more or less incomplete, data sets. The models do not require simulation techniques and are easily programmed in, e.g. electronic spreadsheets such as Microsoft Excel. By transformation of the time steps even dynamic driving variables, e.g. weather-related, can be applied without simulation. Model equations and ready-to-run programs (Excel, SAS) can be found at http://www.mv.slu.se/vaxtnaring/olle/ICBM.html.