Correspondence Comment on “Microbially Derived Inputs to Soil Organic Matter: Are Current Estimates Too Low?”
that virtually all the SOM is composed of microfaunal feces (e.g., (6)). In conclusion, the “current estimates” alluded to in the title are not of microbially derived material but merely of the living fraction, the microbial biomass and, as such, are not “too low”.
Simpson et al. (1) ably demonstrate the similarity between the NMR spectra of microbial material and of soil organic matter (SOM) and, in doing so, conclude that a considerable fraction of SOM is of microbial origin. However, their assertion that this is something new is misleading. Soil chemists have considered for a long time that material of both plant and microbial origin make up SOM. The authors appear to base this upon literature references that microbial biomass makes up less than 5% of SOM (2, 3). However, these latter references are referring to the living microbial fraction (which is what “biomass” means). While Simpson et al. later acknowledge the existence of both “dead and living” soil microbes, in both the Abstract and Introduction (and again in the Discussion) they confuse soil microbial biomass with microbially derived material or what can be called “necromass”. It is a fact that microbes within the soil grow and die, and this is part of the carbon cycle (4). It also appears that the “necromass” within soils generally exceeds the “biomass”, presumably due to the recalcitrance of microbial tissue. No one has said that only 1–5% of SOM is microbially derived. In practice, the proportions of microbially and plantderived may vary considerably. At one extreme we may have virtually 100% undecomposed plant material such as in the upper horizons of a sphagnum peat bog (5). At the other extreme we may have such highly processed material such
Literature Cited
10.1021/es8000932 CCC: $40.75
Published on Web 03/08/2008
2008 American Chemical Society
(1) Simpson, A. J.; Simpson, M. J.; Smith, E.; Kelleher, B. P. Microbially derived inputs to soil organic matter: Are current estimates too low? Environ. Sci. Technol. 2007, 41 (23), 8070– 8076. (2) Wardle, D. A. A comparative assessment of the factors which influence microbial biomass carbon and nitrogen levels in soil. Biol. Rev. Cambridge Philos. Soc. 1992, 67, 321–358. (3) Dalal, R. C. Soil microbial biomass - what do the numbers really mean? Aust. J. Exp. Agric. 1998, 38, 649–665. (4) Chapman, S. J.; Gray, T. R. G. Importance of cryptic growth, yield factors and maintenance energy in models of microbial growth in soil. Soil Biol. Biochem. 1986, 18, 1–4. (5) Comont, L.; Laggoun-Défarge, F.; Disnar, J. R. Evolution of organic matter indicators in response to major environmental changes: The case of a formerly cut-over peat bog (Le Russey, Jura Mountains, France). Org. Geochem. 2006, 37, 1736–1751. (6) Dawod, V.; Fitzpatrick, E. A. Some population sizes and effects of the Enchytraeidae (Oligochaeta) on soil structure in a selection of Scottish soils. Geoderma 1993, 56, 173–178.
Stephen J. Chapman Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, U.K. ES8000932
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