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Response to Comment on “Microbially Derived Inputs to Soil Organic Matter: Are Current Estimates Too Low?” We thank Dr. Chapman for his comments (1). Some confusion may arise from our use of the term “biomass”. We used the term biomass to represent “living and recently dead biological material” as is its commonly defined (2). However, after further review, the term appears to have more that one meaning in the literature depending on the context in which it is applied. Consequently, we acknowledge that in some cases the percentage of microbial biomass reported does refer to the active and/or living fraction only. In other cases, the term is not specifically defined and the techniques involved, for example DNA isolation, biomarkers, and fumigation procedues could be at least in part measuring cellular contents from dead or living cells. As Dr. Chapman correctly points out however, we do use “microbial biomass” and “microbial-derived material” intechangabley and, considering that in some cases microbial biomass refers to only the “living fraction”, our interchangability of these terms may be interpreted as an oversight on our part; we apologize for any confusion that this may cause. The NMR methods used in our study will evaluate the total cellular contribution from microbes and includes “contributions from living and dormant cells; as well as deceased cells that are intact or at the early stages of degradation”. This stated, the goal of this study is not in any way to criticize any previous work but simply to ask the question: “are microbial contributions to soil organic matter at this present time under estimated?”. Indeed we ask this question more in terms of the current understanding of soil organic matter and “humic substances” in general, rather than specific measurements of living microbial materials themselves. To our knowledge, the quantities of “total microbial contributions” (which would inlcude materials in living, dormant, and deceased cells), as measured here by NMR, are not known with any certaintly. It is this “total microbial contribution” that is determined in our study by NMR to represent a significant proportion of total soil organic matter. We feel that the main finding, that microbial cells, whether dead or alive can contribute in some soils more than half of the total soil organic carbon, is very significant, novel, and should not be underestimated. To clarify further, the NMR data suggest that, in some soils, the material from living, dormant, and desceased cells combined appears to account for over 50% of the extractable soil organic matter, and ∼45% of the humin (insoluble) fraction. This is interesting as Hayes states “In mineral soils humic substances can compose 70–80% of the total soil organic manner” (3). Yet, traditionally humic substances by definition “do not include biomolecules such as peptides, sugars, nucleic acid residues, fats etc.” (3). Thus it could be
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argued that finding a large proportion (>50%) of the alkalineextracted fraction from some soils (arguably considered by many as the operationally defined “humic substance fraction”) to be composed of recognizable contents from microbial cells (i.e., proteins, lipids, etc.) is novel and potentially of interest to many readers. Indeed this is in part reflected by an external reveiwer of the manuscript who stated “If these authors are correct, then we will need to drastically change the way we think about soil carbon. In particular, these estimates of soil microbial densities would suggest that soil carbon is a much more biogeochemically active phase than previously thought.” The larger than expected content of recognizable molecular categories in humic substances is in-line with the new paradigm that humic substances may not be a “distinct category” of materials but rather a complex mixture (4, 5). Considering the findings from our study it is likely this mixture includes large contributions from microbial cell contents that are lysed during a traditional humic substances extraction procedure (i.e., 0.1 M NaOH). Indeed in some of the soils studied in the manuscript, it appears that a large portion of the “humic fraction” may well be from microbial cellular contents. The questions of importance that arise from this study include the following. (1) What fraction of these cells is active? (2)What fraction is dormant but potentially active? (3) Are there large contributions from dead cells (necromass) in soil? If so how and why are these cells or cell contents “preserved” in soil? (4) Could this microbial fraction contain much of the soil nitrogen that was previously thought to be contained within humic substances? All of these questions are very important for a true understanding of soil organic matter, and carbon and nitrogen cycling, both in terms of climatic change and sustainable agriculture.
Literature Cited (1) Chapman, S. J. Comment on “Microbially derived inputs to soil organic matter: Are current estimates too low?”. Environ. Sci. Technol. 2008, 42, 3115. (2) Wikipedia contributors. Biomass. In Wikipedia, The Free Encyclopedia; http://en.wikipedia.org/wiki/Biomass; Accessed Janurary 2008. (3) Hayes, M. H. B.; Wilson, W. S. Humic substances in Soils, Peats and Waters: Health and Environmental Aspects; Royal Society of Chemistry: Cambridge, 1997; pp 3–5. (4) Kelleher, B. P.; Simpson, A. J. Humic substances in soils: Are they really chemically distinct? Environ. Sci. Technol. 2006, 40, 4605–4611. (5) Sutton, R.; Sposito, G. Molecular structure in soil humic substances: The new view. Environ. Sci. Technol. 2005, 39, 9009–9015.
Andre Simpson* and Myrna Simpson Department of Chemistry, Univerisity of Toronto, Toronto, Ontario Canada, M1C 1A4 ES800194M
10.1021/es800194m CCC: $40.75
2008 American Chemical Society
Published on Web 03/12/2008
