Spatial arrangement of decomposition activity and population dynamics in soil: importance for trace gas (N2O) production. 

 

Aim

To establish a mechanistic understanding of the relationship between trace gas emission, decomposition, and decomposer population dynamics at the microscale where the limiting factors operate in the terrestrial environment.

 

Background

Decomposition of organic matter can produce compounds that are harmful if exported to the surroundings. This is especially evident in the terrestrial environment where physical-chemical conditions, organic matter supply, and livestock are regulated by man, and where and contamination of aquifers with NO3- and emission of N2O and CH4 to the atmosphere prevail. The marked variation at the micro- and mesoscale in the terrestrial environment strongly influences the emission of trace gases as well as the activity and dynamics of the decomposer organisms. Our laboratory has much experience with the effect of placement of organic resources on trace gas emission. Recently we have shown how decomposer populations in contrasting resources (dead or living plant material in the rhizosphere) affect each other via inorganic nitrogen, depending on the distance between these resources. N2O emission will prove a very attractive parameter to clarify the dynamics of nitrogen limitation is such systems. The present project therefore represents a symbiosis between knowledge on relevant key factors for N2O emission and knowledge on the dynamics of decomposition and decomposer populations in soil.

 

Project description

Focus is on the functional distance between contrasting organic resources and its importance for the emission of N2O. The functional distance is governed by physical placement, soil moisture affecting distance of diffusion, presence of spatially extended organisms such as fungal hyphae and plant roots, as well as mobile fauna elements. At the scale relevant to the present study the latter are likely to be found among nematodes, microarthropods and enchytraeids. In this physical space with the above-mentioned transport vectors between resources we will characterise key factors for N2O release, decomposition, and decomposer populations. Of particular interest is the minimum functional distance where populations on the different resources are still operating as separate entities.

 

Methodology

The investigation is a laboratory study of soil model systems with realistic physical structure. Characterisation of texture, volume weight, size distribution of pores, and of the water potential vs. water content relationship allows determination of the distance of diffusion versus water content. N2O production is determined for the whole system by measurement of emission. N2O production potential is characterised at greater detail within the single resource components of the system as dependent on limiting factors. The decomposition pattern can be described from respiration activity, accumulation of inorganic nitrogen and content of catabolic enzymes in the different compartments. Decomposer populations can be quantified by microscopy (bacteria, fungi), growth in dilution series (bacteria, protozoa), or enumeration following extraction/flotation (nematodes, microarthropods, enchytraeids). The detailed characterisation of populations can be done uning denaturing gradient gel electrophoresis (DGGE) or terminal restriction fragment length polymorphism (TRFLP) for microorganisms, whereas e.g. nematodes and microarthropods have to be characterised with the help of existing identification literature.

 

Copenhagen University contact person: Søren Christensen ([email protected])