the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
18 Jan 2021
18 Jan 2021
Transformation of n-alkanes from plant to soil: a review
- 1Department of Geography, University of Zurich, CH-8057 Zurich, Switzerland
- 2Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, 1098XH, Netherlands
- 1Department of Geography, University of Zurich, CH-8057 Zurich, Switzerland
- 2Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, 1098XH, Netherlands
Abstract. Despite the importance of soil organic matter (SOM) in the global carbon cycle, there remain many open questions regarding its formation and preservation. The study of individual organic compound classes that make up SOM, such as lipid biomarkers including n-alkanes, can provide insight into the cycling of bulk SOM. While studies of lipid biomarkers, particularly n-alkanes, have increased in number in the past few decades, only a limited number have focused on the transformation of these compounds following deposition in soil archives. We performed a systematic review to consolidate the available information on plant-derived n-alkanes and their transformation from plant to soil. Our major findings were 1) a nearly ubiquitous trend of decreased total concentration of n-alkanes either with time in litterbag experiments or with depth in open plant-soil systems, 2) a decrease in either Carbon Preference Index (CPI) or Odd-over-Even Predominance (OEP) with depth, indicating degradation of the n-alkane signal or a shift in vegetation composition over time, and 3) preferential degradation of odd chain length and shorter chain length n-alkanes. The review also highlighted a lack of data transparency and standardization across studies of lipid biomarkers, making analysis and synthesis of published data time-consuming and difficult. We recommend that the community move towards more uniform and systematic reporting of biomarker data. Furthermore, as the number of studies examining the complete leaf-litter-soil continuum is very limited as well as unevenly distributed over geographical regions, climate zones, and soil types, future data collection should focus on underrepresented areas as well as quantifying the transformation of n-alkanes through the complete continuum of plant to soil.
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Carrie L. Thomas et al.
Status: final response (author comments only)
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RC1: 'Comment on soil-2020-107', Anonymous Referee #1, 10 Feb 2021
This straightforward paper reviews the available information on plant-derived n-alkanes and their transformation in soil. The major findings reported are known general trends such are n-alkane total content decrease, with ageing or with soil depth, a decrease in either Carbon Preference Index (CPI) or Odd-over-Even Predominance (OEP) with depth and the shifts in odd chain length and shorter chain length. The authors suggest the need for a more uniform and systematic reporting of biomarker data and the need to focus on underrepresented areas as well as in quantifying the transformation of n-alkanes through the complete continuum of plant to soil.
This is a very interesting work, well within the scope of SOIL journal. The review is pertinent and appropriately compiles the main findings described in the most relevant publications dealing with alkane biomarker distribution in soils. To the best of my knowledge, the review is novel and not previously published.
The paper is well structured, easy to read and follow, is written in good English. The references cited are adequate, update and pertinent.
I have no substantive concerns and therefore will recommend this work to be published in SOIL.
Only a few comments and minor corrections:
Write impersonal, mainly when this is a review paper
Check and Italicize the "n" for "normal" e.g. "n-alkanes" in the Abstract section
Paragraph 35. Apart from Kolattukudy et al., 1976, I think that the pioneering works of Eglinton et al., 1961a and b should be cited:
Eglinton et al (1962a) Nature DOI: 10.1038 / 193739a0
Eglinton et al (1962b) Phytochemistry DOI: 10.1016 / S0031-9422 (00) 88006-1
Paragraphs 55-65. Please, check the formulas (CPI, OEP & ACL) and normalize notation i.e. all in summation using the appropriate indexes (n, m)
Paragraphs 65-70. Here I missed some other relevant and general citations related to n-alkanes and other biomarkers diagenesis in soils and sediments.
Bourbonniere & Meyers (1996). DOI: 10.4319 / lo.1996.41.2.0352
Wiesenberg et al (2003). DOI: 10.1016 / j.orggeochem.2004.03.009
Meyers & Ishiwatari (1993). DOI: 10.1016 / 0146-6380 (93) 90100-P
Zhang et al (2006). DOI: 10.1016 / j.quascirev.2005.03.009
In addition, although the following references are cited in other parts of the MS, I will also recommend its inclusion in this introductory paragraph.
Bull et al (2000). DOI: 10.1016 / S0146-6380 (00) 00008-5
Otto & Simpson (2005). DOI: 10.1007 / s10533-004-5834-8Paragraph 330. Maybe it is worth to briefly mention other environmental aspects known to exert shifts in n-alkanes e.g. forest fires.
Almendros et al (1988). DOI: 10.1016 / 0016-7061 (88) 90028-6
González-Pérez et al (2008). DOI: 10.1016 / j.orggeochem.2008.03.014 -
RC2: 'Comment on soil-2020-107', Anonymous Referee #2, 16 Feb 2021
This paper aims to review plant n-alkanes and their transformation from plant to soil. The authors focused on data concerning the quantification of n-alkane but also on traditional indices (ACL, CPI and OEP). The study is based on 37 studies in which environmental, soil and vegetation data were taken into account. The authors observed the quantity, ACL, CPI and EOP from litterbag incubation, in plant-litter-soil continuum and in soil profile. They highlighted some trends: a decrease of n-alkanes concentration, CPI and EOP with time or depth. There are general trends however this study also showed a different evolution depending on the considered plant or considered site. This review is interesting despite the small dataset selected due to the limited number of suitable papers. The authors underlined the difficulties encountered in performing a meta-analysis on biomarkers and the need for accessibility of data and pointed to knowledge gaps and suggested recommendations. The manuscript depicts most of the outcome explaining the evolution of n-alkane pattern with time or in soil either due to degradation pathway or source shift. Perhaps the soil factors and the potential transfer of n-alkanes in soil could have been more detailed. Thus, I recommend this study for publication.
Nevertheless, I have a few remarks that must be answered. My main comment concerns the soil profiles. The authors described the trends, however for many results the data available are from surface to around ten centimeter. In many soils, these ten centimeters often only concern the organic layer. Accordingly, it should be clearly stated as it could be an important point. For example in figure 5a, we can observe important difference in n-alkane concentration in the first 20 cm and below. The distinction was better evidenced in figures 3 and 4. With the differences of scale, it is sometimes difficult to compare the evolution with depth (Figures 5 and 6). Perhaps it might be interesting to distinguish the trend between 0-20 cm and between 20 to deep soil in the discussion.
In the introduction, why do the authors specified that the study of lipids specifically could increase the overall understanding of SOM dynamics? Would it signify that the authors consider that all individual compounds have the same degradation and preservation pattern? However the authors mentioned potential preservation of n-alkanes. So the authors should improve how they would apply knowledge on alkane degradation on the whole SOM if they are better preserved.
L104: Replace where by when
L 155 and L169 to 172: Are the trends noticed really significant statistically? In fig 3a perhaps it is significant for coniferous forest, mixed forest, and in fig 3b too, except deciduous but for fig 4 it is even more difficult to know if the differences are significant. Could it be possible to apply statistical analysis?
L226-230: I totally agree with the authors about the limitation. We need data to evaluate trends. Thus, the “trajectory plant-litter-soil” mentioned is perhaps too large because for coniferous forest and grassland the data are from very shallow layer (fig 5a), perhaps even only litter. It is difficult to write that it is a trend from plant to soil.
L238: I do not remember that Jansen and Nierop (2009) discussed the production of alkane via alkene oxidation. The authors quote another citation for this possible source of alkanes.
L245-247: I do not understand how n-alkane degradation could result in an increase of n-alkane concentration.
L290-291: How could earthworm influence the dis2009 should be Zech et al., 2010.
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RC3: 'Comment on soil-2020-107', Anonymous Referee #3, 19 Feb 2021
With ‘transformation of n-alkanes from plant to soil’ Thomas and co-authors have chosen a topic for their review that is an extremely narrow field of research. This is acknowledged by the authors already in their abstract stating ‘only a limited number (of studies) have focused on the transformation of these compounds… in soil archives’. Moreover, there is according to my knowledge no discussion or controversy in the scientific community concerning transformation of alkanes from plants to soils. This likely explains why no questions are raised by the authors in or at the end of the introduction chapter. I therefore doubt that the chosen topic merits a review paper that shall attract attention and address a broader readership.
Moreover, the readers of ‘SOIL’ do not learn anything new and the manuscript contains flaws. The ‘major findings’ summed up by the authors (decreasing n-alkane concentrations and decreasing CPI) are trivial, known for a long time and described by more than 90% of the cited respective studies. The first part of the third ‘major finding’ (preferential degradation of odd chain length) is equal to major finding (2) just in other words and the second part of the third ‘major finding’ (preferential degradation of shorten chain length n-alkanes) is simply wrong and not supported by the majority of the studies cited by the authors (see ll. 164ff and l. 262). Actually interesting or striking features such as the accumulation of soil microbial-derived medium-chain n-alkanes or the increase of n-alkane concentrations at coniferous forest sites (Fig. 3b) are unfortunately not or insufficiently emphasized or wrongly explained (the increase can be simply explained with needles producing no n-alkanes but understory in coniferous forests contributing to the soil n-alkane pool). A review focussing on plant to soil transformation should not include subsoils or peat archives. Statements or citations like in l. 200 or alkane depth functions of peat archives like in Fig. 5 are not helpful and in the worst case misleading, because in steppe biomes there is high bioturbation in typically loose eolian sediments and in peat archives the vegetation may have changed. Apart from Fig. 5, also Figs. 3, 4 and 6 are hardly readable. Concerning Fig. 3b, I can hardly imagine (actually it cannot be) that fresh deciduous forest material and fresh mixed forest material contains no alkanes. Please check and correct your data and figures. Last but not least, it does not become clear what the knowledge gaps are. The authors encourage expanding the dataset to less researched geographic areas… I consider it to be rather unlikely that this approach will help increasing our understanding of plant to soil transformation of n-alkanes.
To sum up, the issues raised above demonstrate that the overall aim formulated by the authors at the end of the introduction (l. 68ff: ‘consolidation of the available information on the fate of n-alkanes in soils… better process understanding of degradation…’) is only inadequately achieved. Most importantly, soil microbial build-up of n-alkanes is insufficiently addressed.
Specific Comments
l. 48 and 50: I exemplarily checked both Marzi et al., 1993 and Hoefs et al., 2002 and found them to be inappropriately cited. Marzi and Hoefs use CPI and OEP, but not in the sense that their studies or results support what the authors cite them for, namely well preserved or highly degraded plant organic matter. Please be more specific with your citations.
Result chapter: numbering of subchapter makes no sense
l. 238: cannot be correct, oxidation of alcohols does certainly not produce n-alkanes. The succession of oxidation is aliphatic – aldehyde – alcohol – acid.
l. 283ff: I do not agree with the statement that ‘retaining the range of chain length and the most abundant chain length’ ‘evidences that there is limited change… no preferential degradation...’. Fig. 2a shows that all ACL lines increase.
Carrie L. Thomas et al.
Carrie L. Thomas et al.
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