Soil organic matter in soil depth profiles: Distinct carbon preferences of microbial groups during carbon transformation

https://doi.org/10.1016/j.soilbio.2007.09.016Get rights and content

Abstract

This study investigates how carbon sources of soil microbial communities vary with soil depth. Microbial phospholipid fatty acids (PLFA) were extracted from 0–20, 20–40 and 40–60 cm depth intervals from agricultural soils and analysed for their stable carbon isotopes (δ13C values). The soils had been subjected to a vegetation change from C3 (δ13C≈−29.3‰) to C4 plants (δ13C≈−12.5‰) 40 years previously, which allowed us to trace the carbon flow from plant-derived input (litter, roots, and root exudates) into microbial PLFA. While bulk soil organic matter (SOM) reflected ≈12% of the C4-derived carbon in top soil (0–20 cm) and 3% in deeper soil (40–60 cm), the PLFA had a much higher contribution of C4 carbon of about 64% in 0–20 cm and 34% in 40–60 cm. This implies a much faster turnover time of carbon in the microbial biomass compared to bulk SOM. The isotopic signature of bulk SOM and PLFA from C4 cultivated soil decreases with increasing soil depth (−23.7‰ to −25.0‰ for bulk SOM and −18.3‰ to −23.3‰ for PLFA), which demonstrates decreasing influence of the isotopic signature of the new C4 vegetation with soil depth. In terms of soil microbial carbon sources this clearly shows a high percentage of C4 labelled and thus young plant carbon as microbial carbon source in topsoils. With increasing soil depth this percentage decreases and SOM is increasingly used as microbial carbon source. Among all PLFA that were associated to different microbial groups it could be observed that (a) depended on availability, Gram-negative and Gram-positive bacteria prefer plant-derived carbon as carbon source, however, (b) Gram-positive bacteria use more SOM-derived carbon sources while Gram-negative bacteria use more plant biomass. This tendency was observed in all three-depth intervals. However, our results also show that microorganisms maintain their preferred carbon sources independent on soil depth with an isotopic shift of 3–4‰ from 0–20 to 40–60 cm soil depth.

Introduction

Plant debris and organic compounds in soils are mainly degraded and transformed by soil microbes, which thereby strongly influence stabilization and destabilization of soil organic matter (SOM) (Nannipieri et al., 2003) and soil carbon dynamics (Gleixner et al., 2002). Soils that have undergone vegetation change e.g. from C3 (δ13C value ≈−29‰) to C4 plants (δ13C value ≈−12‰) or vice versa are well suited to study soil carbon dynamics as their SOM will be naturally labelled with either a C3 or C4 isotopic signature (Balesdent and Mariotti, 1996). This enables us to distinguish between new C4-derived carbon and remained older carbon originating from the former C3 vegetation using stable isotope measurements (Gleixner et al., 1999).

As phospholipids are only found in viable cells, they represent excellent biomarker for living microbial biomass. Furthermore their corresponding phospholipid fatty acids (PLFA) can be associated to distinct groups of microbes. For example, Gram-positive bacteria are generally characterized by saturated and in particular branched PLFA (iso/anteiso but also methyl branching). Similarly, monounsaturated and cyclopropyl-substituted PLFA are signature fatty acids for Gram-negative bacteria (Frostegård and Bååth, 1996; Zelles, 1999).

Combined structural and isotopic analyses of microbial PLFA give great insight in a wide range of substrates used as microbial carbon sources (Abraham et al., 1998; Hanson et al., 1999; Ruess et al., 2005) but also into the microbial control of biogeochemical processes (Boschker et al., 1998). From 13C analysis of PLFA there is evidence that not all available carbon substrates are equally used by soil microbes as microbial groups may use different carbon sources. This emphasizes the distinct functionalities of different microbial groups in soil, e.g. Gram-positive and Gram-negative bacteria (Waldrop and Firestone, 2004). Furthermore, 13C isotope analyses of PLFA in soils that have undergone C3/C4 vegetation change also allow differentiation between recent plant biomass and older SOM microbial carbon sources. However, not only different microbial carbon sources in general, but also preferred carbon sources of distinct microbial groups might be identified in this way. In addition, different microbial carbon sources in different soil depth intervals might be found due to changes in microbial community structure which also depends on soil depth (Fierer et al., 2003). As microbial activity has a strong impact on soil carbon cycling more knowledge on microbial substrate usage and transformation of soil carbon is needed for a better understanding on SOM dynamics.

Therefore the goal of this work was to study soil microbial carbon sources using PLFA as microbial biomarkers and to identify microbial groups responsible for the decomposition of different types of carbon substrates in soil. We used soils that were exposed to a C3/C4 vegetation change and also studied the influence of soil depth on microbial carbon sources.

Section snippets

Study site and soil sampling

Soil samples were taken from the long-term field experiment “Eternal Rye” in Halle/Saale (Sachsen-Anhalt, Germany), following the September 2001 harvest. This experimental site was established in 1878 with continuous rye (C3 plant) cropping. In 1961 one part of the C3 cultivated plot was changed to continuous maize (C4 plant) cropping (Merbach et al., 2000), we use the plot remaining in C3 cultivation for comparison of δ13C values. The soil is classified as Haplic Phaeozem derived from sandy

Organic carbon concentration of soil organic matter

The concentration of organic carbon of C3 cultivated soil in the plough horizon (0–20 cm soil depth) is 1.3%, and for C4 cultivated soil 1.2% (Fig. 1A). With increasing soil depth organic carbon concentrations decrease for C3 (0.8% at 20–40 cm, 0.3% at 40–60 cm) and C4 cultivated soils (0.8% at 20–40 cm, 0.4% in 40–60 cm). Estimated annual addition of plant litter to soils differ (C4-input: 0.02 kg C m−2: C3 input: 0.05 kg C m−2), and this is reflected in higher organic carbon content in 0–20 cm for

Discussion

The organic carbon content of both C3 and C4 cultivated soils decreases with soil depth and in top soils (0–20 cm) it was found to be higher for C3 (1.3%) than for C4 cultivated soils (1.2%) (Fig. 1A). These values are typical for agricultural soils but they also depend on soil type and soil management (Gleixner et al., 1999; Balesdent et al., 2000; Schnellhammer and Sirch, 2001).

For the 20–40 cm as well as 40–60 cm soil depth interval the organic carbon content of maize cultivated soil was found

Conclusions

Our study gives new insight to soil microbial carbon sources and how they may vary with depth. Characteristic PLFA from different soil microorganisms, Gram-negative and Gram-positive bacteria as well as soil fungi, could be identified. Our results show that with increasing soil depth more carbon derived from decades old SOM and less recent carbon derived from plants is used as carbon source by all microorganisms. However, with increasing soil depth different carbon sources for the two groups of

Acknowledgements

The authors would like to thank Steffen Rühlow and Stefanie Lenk for excellent technical assistance as well as Heike Geilmann and Willi Brand from the Isolab group of MPI-BGC for 13C isotope analysis of bulk material. Susan Trumbore is greatly acknowledged for improving the manuscript. We also thank the Deutsche Forschungsgemeinschaft (DFG) for financial support of project Gl 262/4 and of the priority program SPP 1090.

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    Present address: Department of Earth System Science, University of California, Irvine, 2101 Croul Hall, Irvine, CA 92697-3100, USA.

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