The dependence of soil microbial activity on recent photosynthate from trees S. G. Göttlicher, K. Steinmann, N.R. Betson, P. Högberg Plant Soil (2006) Stabile Isotopes 2009/10, Barbara Gratzer

Aim of study The factors limiting activity and growth of microorganisms in native soils are poorly understood Record effect of direct plant below-ground photosynthate based C-flux to soil microorganisms (especially in forests with large trees and considerable soil heterogeneity)

Introduction - Microorganisms Metabolise different fractions of soil organic matter (SOM) and recent litter with symbiotic mycorrhizal fungi and others in the rhizosphere Functional distinction between heterotrophic MO and those directly supplied with C from recent photosynthates (and therefore are limited by C)

Introduction Soil microbial biomass and activity can be assessed e.g. by substrate induced respiration to measure amount of C in living biomass The induced respiration carries the isotopic signature of the added C 4 -substrate, while the basal respiration carries the isotopic signature of the C 3 -system.

Content of study The importance of labile C inputs from trees on respiratory activity for soil microorganisms was surveyed by comparing plots with and without girdled pine trees (Pinus sylvestris) to additions of C 4 - sucrose in order they were enabled to differentiate between utilization of endogenous C 3 -soil C- sources and exogenous C 4 -sucrose

Methods Plots: 3 girdled and 3 controls plots 900 m 2 with 120 trees each Bark was removed 0,3m long circular section 2 cylinders in each plot 1 (control) to measure basal respiration 1 to measure SIR field C 4 -sucrose injections Gas sampling (CO 2 respiration rate)

Methods Keeling plot (to assess isotopic composition of soil respired CO 2 ): two-component isotope mixing model (δ 13 C of atmospheric and soil-respired CO 2 ) m

Explanation for additional C 3 -C metabolisms after addition of C 4 - sucrose Increased use of C already present in the microbial biomass OR accelerated (beschleunigt) decomposition of SOM

Results Girdled and non-girdled plots respiration rate after sucrose application was ca. double that of respiration rate measured in the field Non-girdled plots: increase in respiration of endogenous C 3 -C was significantly higher Girdled plots: %C 3IR (C 4 -sucrose-induced increase in C 3resp ) decreased This means: induced respiration was totally C 4 - C based

Results Different patterns of soil respiration after sucrose addition in girdled and non-girdled plots show terminating the C-flux from the top to the root-soil system not only reduces root and mycorrhizal respiration, but also influences soil-MO on a long-term basis.

Reserves of C 4 -sucrose C become exhausted in the absence of a continuous plant phloem-C flux to the soil, amplifying the C-limiting conditions experienced by MO MO switch from decomposing endogenous SOM to more labile sucrose added What do results indicate?

Discussion Without direct supply of photosynthate to soil, amount of C 4 -sucrose C was reduced to zero. Why do MO not exhaust all their C sources where C 4 -sucrose C is present? Hypothesis: “metabolic alertness”  if a food event occurs, they invest more energy This “metabolic alertness” is reduced after absence of supply with C

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