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Kinetic analysis of microbial growth and enzyme mediated activities for assessment of soil carbon mineralization

Evgenia Blagodatskaya1, Sergey Blagodatsky1, Yakov Kuzyakov1
1 Agraroekosystemforschung, Uni Bayreuth

O 1.4 in Funktion von Ökosystemen

02.04.2009, 10:00-10:15, H8

Decomposition of organic substrates in soil is governed by microorganisms and is enzymes mediated process. Like the other enzymes-catalysed reactions the multistage process of CO2 production during decomposition of soil organic matter (SOM) is described by Michaelis-Menten kinetics, which relates functional properties of soil microbial community (maximal mineralization rate Vmax, substrate affinity of microbial enzymes Ks) with substrate availability and with decomposition rate of SOM. We used classical physiological approaches based on (i) microbial growth kinetics and (ii) enzymes affinity to the substrate to show the shift in functional properties of the soil microbial community after amendments with substrates of contrasting availability. Specific growth rate (µ) of soil microorganisms was estimated by fitting the parameters of the equation           v(t) = A + B * exp(µ*t), to the measured CO2 evolution rate (v(t)) after glucose addition, and where A is the initial rate of non-growth respiration, B - initial rate of the growing fraction of total respiration. Kinetic parameters of substrate-induced respiration and substrate availability in soil were monitored during the growth of maize, and decomposition of glucose or maize straw. Input of small amounts of easily available substrates (i.e. glucose or root exudates) significantly increased the specific growth rates of soil microorganisms by up to 13 and 20%, respectively. This increase, showing the shift from K to r strategies, was confirmed by a 50% decrease in the affinity of microorganisms to the easily available substrates. In contrast, maize straw lowered specific growth rates by 16–30% and increased the affinity of microorganisms to the substrate by 23–131% compared with untreated soil. After maize straw addition, the shift of the microbial community to K strategy was accompanied by an increase in the fast-growing responsive fraction of microbial biomass (as estimated by kinetics of substrate-induced respiration). The generation time of the fast-growing fraction of microbial biomass in soil was 1.8 to 2.8 hours, which was 100 to 1000 times faster than that of the whole microbial community. Easily available carbon (C) in soil amended with maize straw comprised only half of that extracted by 0.05 M K2SO4. Therefore, we conclude that C extracted by 0.05 M K2SO4 from the soil with maize straw residues had a lower availability for microorganisms as compared with glucose. Adding N to the soil prolonged the intensive mineralization period of plant residues, decreased specific microbial growth rates and increased the amount of easily available C in the soil. This indicates a more efficient use of maize residues by K-selected microorganisms after the removal of N limitation. Combination of two complementary physiological approaches based on microbial growth kinetics and substrate affinity showed contrasting effects of easily and less available substrates on the shift of growth strategies (r vs. K) of the whole microbial community. These approaches are also suitable for estimation of microbial availability of indigenous C in the soil and showed a good potential for soil ecological studies.

Letzte Änderung 22.02.2009