Sunday, 9 July 2006 - 9:50 AM
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Contribution of Roots, Rhizodeposits and Soil Organic Matter to CO2 Efflux from Maize Rhizosphere as Revealed by 13C Natural Abundance.

Martin Werth1, Irina Subbotina2, and Yakov Kuzyakov1. (1) Univ of Hohenheim, Institute of Soil Science and Land Evaluation, Emil-Wolff-Str. 27, Stuttgart, 70593, Germany, (2) State Univ of Rostov on Don, Dept of Soil Science and Agrochemistry, Rostov on Don, 344006, Russia

This contribution provides the verification of a simple procedure on quantitative separate estimation of 1) root respiration, 2) rhizomicrobial respiration, and 3) microbial respiration from soil organic matter (SOM) decomposition in non-sterile soils. The method is based on growing C4 plants on C3 soil or vice versa. Hence, the d13C values of SOM, maize roots, microbial biomass, and total CO2 efflux from the soil are used to determine the three fractions of CO2. These contributions of root respiration, rhizomicrobial respiration and respiration derived from SOM decomposition to total soil CO2 efflux can be calculated according to isotopic mass balance of microbial biomass and CO2.

Maize was grown on a C3 soil in the laboratory for 40 days and trapping of total CO2 was started nine days after germination. Samples of bulk soil, maize roots, microbial biomass (chloroform-fumigation-extraction method), and total CO2 (trapped in NaOH) were taken five times during the growth period and analyzed for d13C values at an isotope ratio mass spectrometry unit.

At the end of the experiment, d13C values of maize roots and total CO2 efflux were significantly different, but very close at –15.8 and –16.9‰, respectively. The d13C value of SOM was –26.8‰ and the one of microbial biomass was –23.7‰. Using these d13C values according to the suggested approach, root respiration contributed to 91%, rhizomicrobial respiration to 4%, and SOM decomposition to 5% of the CO2 efflux. Compared to recent studies, which used different approaches, rhizomicrobial respiration and SOM decomposition were underestimated by our approach. This could be due to 1) a 13C fractionation between microbial biomass and its CO2 or 2) a small portion of active microbial biomass in the rhizosphere contributing to the CO2 efflux. Checking the first reason for underestimation by incubation of C3 soil and trapping CO2, we found a fractionation of 3‰ between microbial biomass and respired CO2, leading to an accumulation of the heavier isotope in the CO2. Correcting the calculation by this fractionation, rhizomicrobial respiration increased by 6%. Checking the second reason for underestimation, we calculated that only a hypothetical portion of 60% active microbial biomass led to a rhizomicrobial respiration of 43%, which is comparable to reported results of other studies. However, this portion of active microbial biomass is non-realistic, since other studies showed that less than 10% of total microbial biomass is active and the remainder is dormant.

The suggested method based on 13C natural abundance for partitioning of total CO2 efflux from the soil into three sources fails due to the small part of active microbial biomass in the bulk soil and rhizosphere. This led to a discrepancy between d13C of microbial biomass and of microbially respired CO2.


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