Food web assimilation
As they feed, soil biota take-up carbon and nutrients, of which a part is used for growth and becomes part of their bodies [both alive (biomass) and dead (necromass)]. This is referred to as food web assimilation. All soil biota contribute to this process. Soil biota can either feed on solid or soluble organic matter or on each other. Depending on these who-eats-what relationships, food web assimilation can be further divided into five further sub-processes: microbial assimilation, bacterial and fungal feeding, mycorrhizal carbon translocation (for Carbon and Climate Regulation function only), predation and organic matter consumption.
In general, food web interactions provide information on soil biota diversity, the recycling of carbon and nutrients or the control of population size of specific soil organisms. In this way, food web interactions support all of the soil functions in one form or another. Food web assimilation, specifically, looks at how much carbon and nutrients are locked-up in biomass and flow from one feeding group to the next. This is particularly relevant for the Carbon and Climate Regulation and Nutrient Cycling functions. Food web assimilation, by bacteria and fungi mainly, also support the Water Regulation function by biological retention.
Measuring food web assimilation requires substantial technical and/or computational expertise. The flows of carbon and nutrients can be traced using stable isotopes[1], or estimated with food web models[2],[3]; both techniques need information on the abundance or biomass of various groups of soil biota. Alternatively, one could measure the individual sub-processes. For example, microbial assimilation can be measured with chloroform fumigation[4],[5]; a technique which kills the microbial community in a soil sample so the carbon and nitrogen stored in the microbial biomass can be determined.
[1] Morriën E et al. 2017. Soil networks become more connected and take up more carbon as nature restoration progresses. Nature Communications 8: 14349.
[2] De Ruiter PC et al. 1993. Calculation of nitrogen mineralization in soil food webs. Plant and Soil 157: 263–273.
[3] Holtkamp R. 2011. Modelling C and N mineralisation in soil food webs during secondary succession on ex-arable land. Soil Biology and Biochemistry 43: 251–260.
[4] Brookes PC. 1985. Chloroform fumigation and the release of soil nitrogen: A rapid direct extraction method to measure microbial biomass nitrogen in soil. Soil Biology and Biochemistry 17: 837–842.
[5] Vance ED. 1987. An extraction method for measuring soil microbial biomass C. Soil Biology and Biochemistry 19: 703–707.