Quantification of biomarkers as an estimation of soil organic matter turnover and sources under a crop rotation

Lipid biomarker analysis is an efficient tool for tracing organic matter sources in diverse environments. The quantification of biomarkers facilitates the location of soil organic carbon (SOC) from different sources in a soil profile. According to their structure, biomarkers from total lipid extracts (TLE) would exhibit different degrees of susceptibility to degradation, affecting thus their preservation in soils. Hence, it is crucial to better identify these biomarkers according to diverse stability scales. The aim of this study is to assess SOC contributions from aboveground and to develop a wider approach based on the allocation of C to quantitatively assess the sources of organic matter in low SOM content, highly weathered Mediterranean soils, following a C3-C4 rotation experiment.

Soil samples were taken from three depth intervals (0-5, 5-20, 20-40 cm) from a Mediterranean agricultural soil at “La Hampa” experimental station used for a crop rotation experiment with wheat (C3 plant) and maize (C4 plant). Lipids were extracted and quantified as described in [1].

The total lipid extracts were dominated by a homologous series of n-alkanols (saturated alcohols), short-, mid- and long-chain fatty acid methyl ester (FAME), branched FAME, unsaturated (mono- and polyunsaturated) FAME and sterols. Short-chain FAME, monounsaturated FAME were the most abundant fractions of free lipids. Mono-unsaturated alkanoic acids (Cn:1 FA) were detected in considerable amounts in all samples, namely various isomers of C16:1, C18:1, C20:1 and C22:1; these are believed to be mainly synthesised by soil bacteria. A significant increase of these compounds in rotation plots leads to an effective microbial consumption of labile organic matter in the surface soil [2]. Regarding FAME, the observed chain lengths ranged from C13 to C32, showing a unimodal distribution maximising at C16 and C18. These compounds are attributed also to microbial products, supporting our findings from the high proportion of the monounsaturated compounds found. In general, and in relation with all compounds, the abundances increased up to 20% compared with the control plots representing the initial content.

These results indicate that, only after three years of crop rotation, a considerable contribution of soil organic carbon is inherited from bacterial activity. The combination of extractable lipids has been shown to validate the use of TLE as a proxy for source and other information on vegetation change and soil processes. This work will bring a discussion on the use of these compounds for tracing the impact of crop rotation on carbon storage.

Acknowledgement: Ministerio de Ciencia Innovación y Universidades (MICIU) for INTERCARBON project (CGL2016-78937-R). L. San Emeterio also thanks MICIU for funding FPI research grants (BES-2017-07968). Mrs Desiré Monis is acknowledged for technical assistance.

[1] M. San-Emeterio, L., Bull, I. D., Holtvoeth, J., and González-Pérez, J. A.: Compound-specific isotopic analysis of fatty acids in three soil profiles to estimate organic matter turnover in agricultural soils., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18526, https://doi.org/10.5194/egusphere-egu2020-18526, 2020.

[2] Tu, T. T. N., Egasse, C., Anquetil, C., Zanetti, F., Zeller, B., Huon, S., & Derenne, S. (2017). Leaf lipid degradation in soils and surface sediments: A litterbag experiment. Organic Geochemistry, 104, 35-41.