Trophic structure and major trophic links in conventional versus organic farming systems as indicated by carbon stable isotope ratios of fatty acids

D. Haubert^1*, K. Birkhofer², A. Fließbach³, M. Gehre⁴, S. Scheu⁵ and L. Ruess⁶

Abstract
Using bulk tissue and fatty acid ¹³C analysis we investigated major trophic pathways from soil microorganisms to microbial consumers to predators in conventional versus organic farming systems planted for the first time with maize.

Organic farming led to an increase in microbial biomass in particular that of fungi as indicated by phospholipid fatty acids (PLFAs). Microbial PLFAs reflected the conversion from C₃ to C₄ plants by a shift in δ1¹³C of 2‰, whereas the isotopic signal in fatty acids (FAs) of Collembola was much more pronounced. In the euedaphic Protaphorura fimata the δ¹³C values in maize fields exceeded that in C₃ (soybean) fields by up to 10‰, indicating a close relationship between diet and vegetation cover. In the epedaphic Orchesella villosa δ13C values shifted by 4‰, suggesting a wider food spectrum including carbon of former C₃ crop residues.

Differences in δ¹³C of corresponding FAs in consumers and resources were assessed to assign food web links. P. fimata was suggested as root and fungal feeder in soybean fields, fungal feeder in conventional and leaf consumer in organically managed maize fields. O. villosa likely fed on root and bacteria under soybean, and bacteria and fungi under maize. Comparison of δ¹³C values in FAs of the cursorial spider Pardosaagrestis and O. villosa implied the latter as important prey species in soybean fields. In contrast, the web-building spider Mangora acalypha showed no predator–prey relationship with Collembola.

The determination of δ¹³C values in trophic biomarker FAs allowed detailed insight into the structure of the decomposer food web and identified diet-shifts in both consumers at the base of the food web and in top predators in organic versus conventional agricultural systems. The results indicate changes in major trophic links and therefore carbon flux through the food web by conversion of conventional into organic farming systems.


Source
Oikos (2009) 118: 1579-1589
DOI: 10.1111/j.1600-0706.2009.17587.x


Author Locations and Affiliations
(1) Institute of Zoology, Darmstadt, University of Technology, Schnittspahnstraße 3, DE_64287 Darmstadt, Germany
(2) Department of Animal Ecology, Justus Liebig University, Heinrich-Buff-Ring 26-32, DE_35392 Gießen, Germany
(3) Research Institute of Organic Agriculture (FiBL), Ackerstraße, CH_5070 Frick, Switzerland
(4) Department of Analytical Chemistry, Centre for Environmental Research (UFZ) Leipzig-Halle, PO Box 2, DE_04301 Leipzig, Germany
(5) Institute of Zoology and Anthropology, Georg-August-University, Berliner Straße 28, DE_37073 Go¨ttingen, Germany.
(6) Ruess, Institute of Biology, Humboldt University, Invalidenstraße 42, DE_10115 Berlin, Germany
* Corresponding author, E-mail  haubert@bio.tu-darmstadt.de


Posted March 2011