By Cindy H. Shaw, Oleg Chertov, and Darrell Hoffman*
Soil fauna are key agents in different types of organic debris processing in the forest floor and mineral topsoil, resulting in humus forms. The humus form that develops at any given site is the result of the amounts and types of litter inputs (such as wood, leaves, roots) from local vegetation, temperature and moisture conditions, and the dynamics of soil biota including fauna, fungi, and bacteria. Humus forms are most broadly divided into three orders: mor, moder, and mull. Each order is associated with a set of physical properties that are easily observed and are related to processes occurring in the forest floor and the underlying mineral soil.
Mors are characterized by a distinct boundary between organic and mineral soil horizons, often with plentiful plant roots and fungal hyphae throughout the organic layers. Mulls develop when there is sufficient mixing of organic and mineral soil often resulting from the activities of soil fauna; they have a relatively thick, dark, mineral Ah horizon enriched with organic matter.
Moders are humus forms with properties that transition between those of mors and mulls. The orders can be further divided into groups described by horizon types (such as L, F, H, Ah), which indicate the stage of decomposition, or include descriptors for composition (such as woody, fungal mossy).
The activities of different organisms and the interactions among them have direct consequences for carbon dynamics, as they are the agents driving rates of mineralization and respiration, as well as stabilization, and sequestration. Although the actions of soil fauna mediate the cycling and storage of carbon, the direct effects of these organisms can be difficult to study, and until now have not been included in carbon models. In addition, most carbon models focus on predicting carbon emissions because of the great interest in greenhouse gases, and they neglect the formation and storage of soil organic matter, which is important in carbon sequestration.
Recently, a model developed by scientists in Russia, Germany and Canada, Romul_Hum (the soil module of the individual tree forest growth model EFIMOD), integrates knowledge of humus form development, soil fauna food webs, dynamics of fungi and bacteria, and the resultant formation and stabilization of carbon in soil organic matter. Romul_Hum accounts for the many belowground interactions (fungivory, bactivory, predator-prey relationships) between soil fauna, bacteria, and fungi and how these interactions are regulated by the qualities of incoming vegetative material. Unlike other models, which treat decomposers as a homogenous group, Romul_Hum uses variations in the ratios of fungal to bacterial biomass, and the ratios of carbon to nitrogen in the fungal and bacterial biomass. Based on published soil fauna data, different types of food webs are defined for combinations of decomposers and types of horizons in humus forms. An earthworm module was developed with parameters for for food palatability, ingestion and egestion, food consumption, lifespan, excretion, and assimilation efficiency. It is especially important to understand the effects of earthworms interacting with other soil fauna activities in Canada as earthworms are invasive to Canada’s large boreal forest (see blog: Earthworm invasions in northern forests). As earthworms spread through the Canadian boreal forest, they will change the carbon dynamics and carbon balance of the ecosystems.
Romul_Hum could be used to predict, understand, and quantify those changes. The types of changes to the forest carbon cycle from invasive earthworms is dependent on the species of earthworm(s) present. These changes can include mixing organic material in the forest floor with the mineral soil below, shifting the balance between fungal and bacterial biomass, and stabilizing carbon in soil organic matter as it moves through the gut of the earthworm, or as earthworms ingest and stabilize faeces produced by meso-fauna. Beyond these effects, earthworms can change the food and habitat available for other groups of soil fauna, such as nematodes, mites, and springtails, affecting the survival and success of meso-faunal populations.
By acknowledging and accounting for the complex interactions between soil faunal food webs and their habitat (humus forms), the modelling approach of Romul_Hum provides a means to evaluate how management, and potentially climate change, affects relationships between soil fauna biodiversity and soil carbon sequestration. In Europe, humus form classification is currently being refined within the HUMUSICA project. In Canada, humus forms are used as part of a system (Forest Floor Recovery Index) to evaluate the success of reclamation after mining.
*Cindy Shaw (firstname.lastname@example.org) is a Research Scientist, and Darrell Hoffman (email@example.com) is a Forest Soil Research Assistant, for the Canadian Forest Service at Natural Resources Canada. Oleg Chertov is a scientist who has worked in Russia, Germany, and Finland, and created the Romul_Hum models with Alex Komarov