Martin Potthoff

The determination of δ13C in soil microbial biomass using fumigation-extraction

Abstract The determination of the isotopic composition of the microbial biomass C in soil is an important tool to study soil microbial ecology and the decomposition and microbial immobilization of soil organic C. We discuss advantages and disadvantages of different methods to determine 13C/12C in soil microbial biomass and propose a new procedure that is based on the UV-catalyzed liquid oxidation of fumigated and non-fumigated soil extracts combined with trapping of the released CO2 in liquid nitrogen and subsequent determination of δ 13 CO 2 -C by a gas chromatograph connected with an isotope ratio mass spectrometer (IRMS). This method was evaluated using test solutions with known isotopic composition and soil extracts. Additionally, the method was compared with an off-line sample preparation technique combined with isotope analysis by a dual-inlet IRMS and an on-line analysis using an elemental analyser connected with an IRMS. All methods applied obtained comparable results and there were no significant differences between the δ 13 C values measured. The off-line preparation procedure had the highest precision but it was also the most labour-intensive. The choice of the most suitable method depends mainly on the number of samples that have to be analysed, the salt concentration of the extracts and the differences of δ 13 C that have to be detected. The application of this method with liquid oxidation and subsequent GC-IRMS analysis showed that microbial biomass C of a grassland soil was 13C-enriched by 2‰ δ 13 C PDB compared with the total soil organic C. The addition of maize straw resulted in a rapid immobilization of maize C in the microbial biomass.

Soil biota in vineyards are more influenced by plants and soil quality than by tillage intensity or the surrounding landscape

Tillage is known for its adverse effects on soil biota, at least in arable agroecosystems. However, in vineyards effects might differ as tillage is often performed during dry periods or only in every other inter-row allowing species to re-colonise disturbed areas. We examined the response of earthworms (lumbricids), springtails (collembola) and litter decomposition to periodically mechanically disturbed (PMD) and permanently green covered (PGC) vineyard inter-rows and assessed whether site effects are altered by the surrounding landscape. In commercial vineyards in Austria we sampled earthworms by handsorting, springtails by soil coring and pitfall trapping and installed litter decomposition bags. Earthworm species diversity increased with plant biomass under PMD but not under PGC; earthworm density was unaffected by tillage but increased with plant biomass mainly at high soil quality (soil fertility index). Springtail species diversity was unaffected by tillage; springtail densities (mainly larger species) were reduced under PGC. Litter decomposition was little affected by investigated parameters. Landscape heterogeneity affected the functional diversity of surface springtails, but did not influence soil-dwelling springtails, earthworms or litter decomposition. We conclude that effects on soil biota of periodical tillage in vineyards need not necessarily be detrimental and will be modified by plant biomass and soil quality.