Microbial biomass turnover

The Rothamsted Carbon Model (RothC) uses a five pool structure, decomposable plant material (DPM), resistant plant materials (RPM), microbial biomass, humified organic matter, and inert organic matter to assess carbon turnover (Coleman and Jenkinson 1996 Guo et al. 2007). The first four pools decompose by first-order kinetics. The decay rate constants are modified by temperature, soil moisture, and indirectly by clay content. RothC does not include a plant growth sub-module, and therefore NHC inputs must be known, estimated, or calculated by inverse modeling. Skjemstad et al. (2004) tested an approach for populating the different pools based on measured values. 

The parameters for characterising soil microbial activity used in the reviewed research results are total microbial biomass, diverse enzymatic parameters, carbon turnover parameters andffiycorrhization, ... 

Friedel, J.K., Gabel, D. and Stahr, K. 2001. Nitrogen pools and turnover in arable soils under different durations of organic farming II Source-and-sink function of the soil microbial biomass or competition with growing plants Journal of Plant Nutrition and Soil Science-Zeitschriftfur Pflanzenernahrung und Bodenkunde 164 421-429. 

Physical protection is exerted by occlusion of particulate organic matter (POM) inside aggregates. It is responsible for the physical separation of organisms active in decomposition and substrates, reduced oxygen availability in the substrate compartment, and reduced biomass turnover through protection from microbial grazers (Mamilov and Dilly, 2002). 

Jenkinson, D. S. and Ladd, J. N. (1981). Microbial biomass in soil measurement and turnover, in Soil Biochemistry volume 5, (E. A. Paul and J. N. Ladd, Eds.). New York Marcel Dekker, 415-471. 

Gregorich E. G., Voroney R. P., and Kachanoski R. G. (1991) Turnover of carbon through the microbial biomass in soils with different textures. Soil Biol. Biochem. 23(8), 799-805. 

Van Veen J. A., Ladd J. N., and Amato M. (1985) Turnover of carbon and nitrogen through the microbial biomass in a sandy loam and a clay soil incubated with uniformly carbon-14-labeled glucose and nitrogen-15 ammonium sulfate under different moisture regimes. Soil Biol. Biochem. 17(6), 747-756. 

We define microbial turnover of phosphorus in soil as the sum of all microbially mediated transformations and the related fluxes of phosphorus in the soil. The quantitative description found in the literature only encompasses the phosphorus flux through the soil microbial biomass (i.e. turnover of microbial phosphorus), and thus excludes phosphorus fluxes to the cell externally caused by microbial activity, indicated by the by-passing arrows in Fig. 7.1 ... 

Similarly, Joergensen and Castillo (2001) could not exclude the possibility that the fungi in the Nicaraguan soils they studied contained low ergosterol contents. Nevertheless, fungal-to-bacterial-biomass ratios are of interest because the two groups contribute differently to specific functions in the phosphorus cycle, react differently to environmental and nutritional factors, and in turn affect microbial phosphorus turnover (see below). 

Temporal fluctuations in microbial phosphorus (Fig. 7.2) have been used to estimate the annual phosphorus flux through the soil microbial biomass and the turnover time of microbial phosphorus under field conditions (Chen et al, 2003). This approach assumes that the sum of the fluctuations representing a decrease in microbial phosphorus is equivalent to the mean annual biomass turnover (McGill et al., 1986). Results depend on the frequency and time of sampling, but allow a qualitative compari-... 

Kouno et al. (2002) estimated the turnover times of soil microbial biomass carbon and phosphorus by following decreases in biomass and P following the addition to soil of C-labelled glucose and KH PO, or of ryegrass which had been double... 

Microbial phosphorus turnover also takes place in the absence of net changes in the microbial biomass. Isotope studies suggest that related phosphorus fluxes are important, but the underlying processes in soils are not well understood. Studies should test whether diffusion and efflux from microbial cells are important. Models would help to integrate different processes and factors related to gross phosphorus mineralization and microbial phosphorus turnover in general. 

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