Soil organic matter microbial turnover

Several authors have applied in situ pulse labeling of plants (grasses and crops) with C-CO2 under field conditions with the objective of quantifying the gross annual fluxes of carbon (net assimilation, shoot and root turnover, and decomposition) in production grasslands and so assess the net input of carbon (total input minus root respiration minus microbial respiration on the basis of rhizodeposition and soil organic matter) and carbon fixation in soil under ambient climatic conditions in the field. 

Grayston SJ, Vaughan D, Jones D (1996) Rhizosphere carbon flow in trees, in comparison with annual plants the importance of root exudation and its impact on microbial activity and nutrient availability. Appl Soil Ecol 5 29-56 Gregorich EG, Ellert BH, Monreal CM (1995) Turnover of soil organic matter and storage of com residue carbon estimated from natural 13C abundance. Can J Soil Sci 75 161-167... 

The increase in atmospheric COi because of fossil fuel emissions has been identified as a major driving force for global climate change. Soil organic matter (SOM) is expected to be an important sink for this carbon (Ciais et ciL, 1995 Schimel, 1995 Steffen et aL, 1998). However, at higher mean temperatures, this sink may act as additional source for CO2 if it is accessible to microbial decomposition. To understand these complex interactions between stabilization and decomposition of SOM, it is crucial to investigate not only the turnover and stability, but also the chemical nature of soil organic matter. 

Soil microbial biomass constitutes a significant carbon sink, in that it represents a significant portion of the active organic carbon pool. Turnover of active biomass is generally on the order of days, as compared to several years for the soil organic matter. Nutrients may be held tightly... 

The diverse activity of the biomass in soil organic matter studies integrates with many topics covered in other parts of this book. The reader is, therefore, referred to the appropriate chapters for further information on rates of decomposition and turnover, microbial polysaccharide and soil structure, phytotoxic compounds, enzyme activities, and mineral cycling. 

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. 

Thomsen I. K., Schjonning P., Jensen B., Kristensen K., and Christensen B. T. (1999) Turnover of organic matter in differently textured soils II. Microbial activity as influenced by soil water regimes. Geoderma 89(3-4), 199-218. 

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