The role of microarthropodal groups in carbon turnover as revealed by 13C fatty acids analysis – method development and impact of heavy metal remediation

The most powerful tool to explore nutrient turnover in complex systems such as soils are stable isotope fractionation and labelling studies. This has been extensively used when investigating microbial carbon cycles by targeting the carbon stable isotopes in microbial phospholipid fatty acids (PLFA). However, exploring the role of mesofauna during carbon turnover in soil ecosystems has long been limited due to the small size and weight of those organisms and therefore issues in the detection and quantification of carbon stable isotope ratios. Only recently, carbon stable isotope analysis of fatty acids (FA) have been established and opened the window to include mesofauna into carbon turnover studies. We have used this new possibility after refining the available FA method to discover the role of microarthropods together with the microbial PLFA analysis. This study is to our knowledge the first, which has used ¹³C labelled plant material and has followed its incorporation into microbial PLFAs and microarthropodal FAs in a greenhouse experiment containing heavy metal contaminated and remediated soils.

Total microbial biomass and ¹³C incorporation into microorganisms was significantly increased and the PLFA pattern shifted after remediation of heavy metal contaminated soil. In accordance, the abundance of the microarthropodal groups Gamasina, Oribatida and Collembola were also increased, while Astigmata were not affected. The relative FA patterns of those groups differentiated significantly among each other, but were not influenced by soil treatment, meaning that the altered microbial PLFA pattern was not transferred into microarthropodal FA. However, the amount (nmol FA) per individuum was elevated in the heavy metal contaminated soil. In contrast, incorporation of ¹³C into FA was lower in contaminated soil in Gamasina, Astigmata and Collembola. ¹³C incorporation of Oribatida was at a constantly high level over the different soil treatments.

These first results revealed, that the relative FA pattern of the microarthropodal groups was not affected by changes in the microbial PLFA pattern due to soil treatments. Differences in the absolute FA amount per individuum and the ¹³C uptake were rather governed by the life and reproduction strategies, with higher fattiness and low abundance under adverse environmental conditions (contaminated soil), constant ¹³C incorporation in K-strategist (Oribatida) and higher ¹³C incorporation of mainly r-strategist (Gamasina, Astigmata and Collembola) under improved conditions.