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| Titel |
Microbial respiration and kinetics of extracellular enzymes activities through rhizosphere and detritusphere at agricultural site |
| VerfasserIn |
Sebastian Löppmann, Evgenia Blagodatskaya, Yakov Kuzyakov |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250091770
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| Publikation (Nr.) |
 EGU/EGU2014-6079.pdf |
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| Zusammenfassung |
| Rhizosphere and detritusphere are soil microsites with very high resource availability for
microorganisms affecting their biomass, composition and functions. In the rhizosphere low
molecular compounds occur with root exudates and low available polymeric compounds, as
belowground plant senescence. In detritusphere the substrate for decomposition is mainly
a polymeric material of low availability. We hypothesized that microorganisms
adapted to contrasting quality and availability of substrates in the rhizosphere and
detritusphere are strongly different in affinity of hydrolytic enzymes responsible for
decomposition of organic compounds. According to common ecological principles
easily available substrates are quickly consumed by microorganisms with enzymes
of low substrate affinity (i.e. r-strategists). The slow-growing K-strategists with
enzymes of high substrate affinity are better adapted for growth on substrates of low
availability.
Estimation of affinity of enzyme systems to the substrate is based on Michaelis-Menten
kinetics, reflecting the dependency of decomposition rates on substrate amount. As
enzymes-mediated reactions are substrate-dependent, we further hypothesized that the largest
differences in hydrolytic activity between the rhizosphere and detritusphere occur at
substrate saturation and that these differences are smoothed with increasing limitation
of substrate. Affected by substrate limitation, microbial species follow a certain
adaptation strategy. To achieve different depth gradients of substrate availability
12 plots on an agricultural field were established in the north-west of Göttingen,
Germany: 1) 4 plots planted with maize, reflecting lower substrate availability with
depth; 2) 4 unplanted plots with maize litter input (0.8 kg m-2 dry maize residues),
corresponding to detritusphere; 3) 4 bare fallow plots as control. Maize litter was grubbed
homogenously into the soil at the first 5 cm to ensure comparable conditions for the
herbivore and detritivore communities in the soil. The kinetics (Km and Vmax) of
four extracellular hydrolytic enzymes responsible for C- and phosphorous-cycle
(β-glucosidase, β-xylosidase, β-cellobiohydrolase and acid phosphatase), microbial biomass,
basal respiration (BR) and substrate-induced respiration (SIR) were measured in
rhizosphere, detritusphere and control from 0 – 10 and 10 – 20 cm. The metabolic quotient
(qCO2) was calculated as specific indicator for efficiency of microbial substrate
utilization.
We observed clear differences in enzymes activities at low and high concentrations of
substrate. At substrate saturation enzyme activity rates of were significantly higher in rooted
plots compared to litter amended plots, whereas at lower concentration no treatment effect
could be found. The BR, SIR and qCO2 values were significantly higher at 0 –
10 cm of the planted treatment compared to litter and control plots, revealing a
significantly higher respiration at lower efficiency of microbial substrate utilization in the
rhizosphere.
The Michaelis-Menten constant (Km) decreased with depth, especially for β-glucosidase,
acid phosphatase and β-xylosidase, indicating higher substrate affinity of microorganisms in
deeper soil and therefore different enzyme systems functioning. The substrate affinity
factor (Vmax/Km) increased 2-fold with depth for various enzymes, reflecting a
switch of predominantly occurring microbial strategies. Vmax/Km ratio indicated
relative domination of zymogenous microbial communities (r-strategists) in 0 – 10
cm depth as compared with 10 – 20 cm depth where the K-strategists dominated. |
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