Romul_Hum-A model of soil organic matter formation coupling with soil biota activity. II. Parameterisation of the soil food web biota activity

- A soil fauna food web (FW) module was elaborated for modelling soil organic matter (SOM) formation.
- A unique modelling approach used the ratio of faunal excrement and necromass to soil respiration to parameterise the contribution of FW fauna to SOM formation.
- The algorithms and functions for excrement and necromass output from FW micro- and mesofauna as a source of organic matter for SOM formation are described.
- Parameters were developed to predict available nitrogen output due to FW functioning.

A soil food web (FW) based approach to modelling the contribution of soil micro- and mesofauna to the formation of soil organic matter (SOM) was developed as an improvement to the soil organic matter model ROMUL, which is the predecessor of Romul_Hum. The main improvement is that the by-products (excrement and necromass) of soil micro- and mesofauna, and their important role as precursors to formation of stable SOM, that are not represented in any soil organic matter model, are now included in the Romul_Hum. First, parameters were compiled to explicitly and mechanistically model the foundational role of microbial communities in the soil FW by using a substrate C:N ratio based function that allows (a) the fungal to bacterial biomass ratio to vary from 3 to 15, (b) fungal C:N to vary from 8 to 30, and (c) bacterial C:N to vary from 5 to 8. These equations were used in model algorithms instead of fixed values that are used in the SOM models. Second, ordination was used on 15 published FWs biomass C datasets to define six FW patterns as combinations of two energy channels (fungal and bacterial) and three types of soil horizons (L, F + H, Ah or Ahe) in temperate and boreal forests. Third, FW analysis methods were used to quantify excrement and necromass C produced by soil micro- and mesofauna at different trophic levels. Soil heterotrophic respiration (expressed as the rate of SOM mineralisation), a core rate variable in all SOM models, was associated with the formation of the faunal by-products (excrement and necromass) using stoichiometric relationships between the production of faunal excrement, necromass and total FW biotic respiration. The ratios of “excrement mass/necromass C to soil heterotrophic respiration C” were key parameters incorporated into the Romul_Hum. The values of these ratios vary from 0.03 to 0.41 for the bacterial energy channels (with maximum in the F + H soil horizon) and from 0.02 to 0.17 in fungal energy channels (wih maximum in the Ah/Ahe soil horizon). The well established mechanistic algorithm for available nitrogen (N) of the FW methodology was used to develop functions for the “ratio of available N to heterotrophic respiration C” depending on substrate C:N ranging from 5 to 120. The resulting algorithms, functions and new parameters for excrement and necromass production, as well as parameterisation of available N production, are described for all six FW patterns. The newly developed FW module was incorporated in the simulation model of SOM formation, Romul_Hum, allowing for the calculation of faunal organic matter contributions to SOM formation. This novel approach to including soil faunal activity in modelling of soil C dynamics takes advanatage of the high degree of organisation within soil biotic communities in FWs and integrates the effects of microorganisms and soil fauna that govern the processes of organic residues transformation and mineralisation in the soil system.