Heiner Flessa

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.

Use of microcalorimetry to study microbial activity during the transition from oxic to anoxic conditions

Abstract. Microbial heat production is a nonspecific measure for microbial activity irrespective of O2 availability in soils. In a series of long-term batch microcalorimeter experiments with closed ampoules, we examined the microbial activity in glucose-amended soil aggregates from different soil depths of a clay forest soil during the transition from aerobic to anaerobic conditions. Furthermore, the influence of the soil aggregate size on the long-term metabolic heat production was examined. Heat output curves showed a distinct pattern for soil samples from different soil depths and aggregate sizes and led to the following conclusions: 1. Microbial biomass and microbial activity strongly decreased with increasing soil depth as well as increasing soil aggregate size despite relatively constant organic C concentrations. 2. The transition from aerobic to anaerobic conditions led to a considerable drop in microbial activity. However, based on the energy balance, 10–26% of the heat production during the aerobic phase is attributable to anoxic or partly anoxic metabolism. 3. After O2 exhaustion, a lag phase of low but constant heat output was observed, followed by a peak of anaerobic metabolic activity. Heat production during the lag phase was hypothesised to be an indicator for the biomass of facultatively anaerobic microorganisms in the soil.

Effect of fertilization history on short-term emission of CO2 and N2O after the application of different N fertilizers - a laboratory study

Increasing organic carbon (OC) stocks in soils reduce atmospheric CO2, but may also cause enhanced N2O emissions. The objective of this study was to determine whether there are any differences in N2O and CO2 emissions from sandy arable soils with different soil OC and total nitrogen stocks due to the annual application of either farmyard manure (S-FYM) or mineral fertilizer (S-MIN) over 27 years. A laboratory incubation was performed to test the short-term effects of the application of different fertilizers [farmyard manure (FYM), KNO3 (MIN) and biogas waste (BW)] on N2O and CO2 emissions. The CO2 emission rates indicated that OC availability in the soil was higher after BW application than after FYM application. N2O emission for 53 days following fertilizer application amounted to 0.01% (MIN), 0.21% (FYM) and 24% (BW) of the total amount of N applied. The high emissions induced by BW were attributed to the combination of a high availability of OC and ammonium in the fermented waste. Fertilization history, which caused higher soil OC stocks in S-FYM, did not influence N2O emissions. The results suggest that characterization of C and N pools in organic fertilizers is required to assess their impact on N2O emissions.

Anthropogene Störung des Stickstoff-Kreislaufs

ZusammenfassungStickstoffverbindungen akkumulieren derzeit in den Böden und in der Atmosphäre und dringen in die Grundwasserspeicher vor. Dies hat schon heute und wird mehr noch in der Zukunft Veränderungen der Ökosysteme und eine Beeinträchtigung der Wasser- und Luftqualitäten und des Klimas zur Folge haben. Einige dieser Wirkungen entfalten sich nur langsam, sind aber, wenn die Prozesse einmal angestoßen sind, auch kaum beeinflußbar.Ökologische Vorsorge kann sich nicht auf die Minimierung der Exposition von Organismen gegenüber naturfremden Stoffen beschränken, sondern muß auch natürliche, aber anthropogen überhöhte Stoffkonzentrationen und deren Wirkungen sowohl auf Einzelorganismen als auch auf die Ökosysteme berücksichtigen.AbstractNitrogen compounds accumulate in soils and the atmosphere and penetrate into groundwater reservoirs. This already affects terrestrial ecosystems, water and air quality, and the climate now and will even influence these parameters more in the future. Some of the effects only develop slowly, but the underlying processes can barely be influenced.Ecological precaution cannot be limited to a minimization of the exposure of organisms by xenobiotics, but should be equally concerned with the effects of anthropogenically enhanced concentrations of natural compounds upon organisms and ecosystems.

Einfluß der N-Versorgung und der CO2-Konzentration auf die Feinwurzelbildung und die Veratmung wurzelbürtigen Kohlenstoffs der Buche

Increasing atmospheric CO2 concentrations and nitrogen deposition may change below ground allocation of assimilates in trees. We conducted an 13C labelling experiment with beech to quantify the effects of elevated [CO2] and tree internal N stocks on the allocation of assimilates to the fine roots and on below ground respiration. Our data show that both elevated [CO2] and reduced tree internal N stores led to an increased investment of assimilates into root growth and root respiration. Root activity (expressed as below ground respiration per mass unit fine roots) was also increased under elevated [CO2], but lowered for trees with reduced N stocks growing under ambient [CO2]. The measured changes in root activity were due to changes in total root growth rather than changes in specific root activity.