The objective of this study was to determine the origin of organic matter incorporated in Amazon forest soils subjected to vegetation fire by analyzing the aliphatic biomarkers (n-alkanes) present in lipid extracts of soil samples

COSTA, F. de S.

COSTA, J. B. da

DICK, D. P.

LARA, L. Z.

LIMA, M. O.

PERALBA, M. do C.

English

Repository Open Access to Scientific Information from Embrapa

Biomarker analysis in soils of the Amazon rainforest after vegetation fire Janaina Berne da Costa1, Deborah Pinheiro Dick1, Maurifran Oliveira Lima1, Larissa Zacher Lara1, Maria do Carmo Peralba,1 Falberni de Souza Costa2 1Federal University of Rio Grande of Sul (UFRGS), Institute of Chemistry, 91501-970 Porto Alegre, RS- Brazil  2Embrapa, Centro de Pesquisa Agroflorestal do Acre, 69900-970 - Rio Branco, AC - Brazil E-mail contact: bernejana@gmail.com  1. Introduction Biomarkers, also known as molecular markers, refer to compounds that have a relationship with their precursors and indicate the occurrence of a particular process within an organism. Recently, the evaluation of biomarkers in plants or soil has become a more common procedure, since the abundance of these compounds are sensitive to changes in the soil organic matter (SOM) caused by the use and management of soil.1 Soil organic matter is made up, among other moieties, by a lipid fraction, which is composed of insoluble organic substances in water. These compounds can be extracted with organic solvents such as dichloromethane and methanol, and may separated and identified by gas chromatography coupeld with mass spectrometry (GC-MS). The content of this fraction comprises much of the soil aliphatic structures and may give information about the source of the organic matter present in soil (microbial origin or higher plants) and is therefore called biomarkers. Studies on soils biomarkers can contribute to the understanding of the dynamics and stabilization of organic matter, which is of great importance from an environmental point of view.2 The objective of this study was to determine the origin of organic matter incorporated in Amazon forest soils subjected to vegetation fire by analyzing the aliphatic biomarkers (n-alkanes) present in lipid extracts of soil samples. 2. Materials and methods The study area is located at Embrapa-Acre in Rio Branco, Acre/Brazil and consists of primary forest which was partially burned in 2011. Samplings were conducted in September 2012 (one year after burning) and September 2014 (three years after burning) in primary forest areas (PF) and burnt forest (BF) at two depths: 0-5 and 100-150 cm. The contents of C and N of the soil samples were determined by dry combustion. Approximately 4 g of sample was used for lipids extraction. The samples were extracted via soxhlet with dichloromethane/methanol (3:1 v/v) for 24 hours. Completed the extraction step, the lipid extract was concentrated in rotary evaporator. The extracts containing lipids were then subjected to fractionation using preparative liquid chromatography atmospheric pressure. It was used a column of silica and alumina and the n-hexane solvent for elution of n-alkanes biomarkers. The distribution of n-alkanes was determined by GC/MS using an Agilent 6890N gas chromatograph connected to a Waters Quattro Micro GC tandem Quadrupole mass spectrometer, equipped with Agilent HP-5 column (30 m length, 0.25 mm i.d, 0.1 µm film thickness) and automatic injector. The oven temperature programme was 70°C (2 min hold), followed by rating of 30°C/min from 70 to 100°C and 40°C/min from 100 to 308°C (held 8 min). The GC-MS was operated using Selected Ion Monitoring (SIM) mode. 3. Results and discussion 3.1. n-alkanes (0 - 5 cm) The mass chromatograms (m/z 71) for the n-alkanes for soils studied at the depth of 0-5 cm, one year after burning are shown in Figure 1A and Figure 1B. Gas chromatographic analyses of saturated hydrocarbons did not show a similar n-alkane distribution in soil of PF (Figure 1A) and BF (Figure 1B). Two types of n-alkane distribution can be distinguished. In the first type, which occurred with PF soil, a bimodal distribution profile was observed and it showed the presence of a series of n-alkanes ranging from C17-C31, and a predominance of even over odd with maximum C18 and C19. This distribution profile is characteristic of a SOM with contribution from both microbial  and higher plants origin. The second type of n-alkane distribution, presented by BF, showed an unimodal distribution profile with a presence of n-alkanes ranging from C16-C31 with maximum at C16, C17 and C18. This is a typical profile of SOM from microbial origin. After fire, the samples showed a change in the distribution profile: the largest n-alkanes chain (C29-C31) related to higher plants has been reduced by the burning of biomass (Figure 1B)     Figure 1A. Profile of n-alkanes for soil of PF (0-5 cm).                             Figure 1B. Profile of n-alkanes for soil of FB (0-5 cm).  3.2. n-alkanes (100 - 150 cm) The chromatograms of n-alkanes one year after burning of PF and BF at the depth 100-150 cm showed similar distribution profile as the corresponding surface layers ( Figures 2A and 2B). Nevertheless, the lipids abundance was smaller than in the 0- 5 cm layer. This behavior corroborates with total C data (data not shown), that decreased steadly along the profile.   Figure 2A. Profile of n-alkanes for soil of PF (100-150 cm).                       Figure 2B. Profile of n-alkanes for soil of FB (100-150 cm). Conclusions The SOM of the primary forest comprises contributions of microbial moieties and of higher plants along the whole profile. This similar distribution pattern of n-alkanes indicates that a percolation of SOM fragments produced at the surface to the deeper layers occurred. The higher annual precipitation of 2580 mm year-1 and the lower content of clay (20%)  favor this proccess. After vegetation burning, a lipids fraction original from higher plants are fragmented in smaller chains, enriching thus the proportion of C16 to C24 n-alkanes.  This effect was also observed at the deeper layer, suggesting that the percolation of SOM along the profile is a constant and intense profile. It is therefore inferred that in this environment, in oposite to subtropical Brazilian soils, the alterations of SOM composition due to changes of forest management are very fast and have a great impact in deeper soil layers.  References [1] Killops SD, Killops VJ. 2005. Introduction to Organic Geochemistry, 2nd ed. Blackwell Publishing, Oxford, UK, 393 p.   [2] Jandl G, Baum C, Blumschein A, Leinweber P. 2012. The impact of short rotation coppice on the concentrations of aliphatic soil lipids. Plant Soil 350:163–77.   Acknowledgement - The authors thank CNPq for financial support.        

Biomarker analysis in soils of the Amazon rainforest after vegetation fire.

http://www.alice.cnptia.embrapa.br/bitstream/doc/1055093/1/26154.pdf

Biomarker analysis in soils of the Amazon rainforest after vegetation fire.

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