| Utilization of low molecular weight organic substances (LMWOS) in soil is regulated by
microbial uptake from solution and following incorporation of into specific cell cycles.
Various chemical properties of LMWOS, namely oxidation state, number of carbon (C)
atoms, number of carboxylic (-COOH) groups, can affect their uptake from soil solution
and further microbial utilization. The aim of the study was to trace the initial fate
(including the uptake from soil solution and utilization by microorganisms) of three
main classes of LMWOS, having contrast properties - sugars, carboxylic and amino
acids.
Top 10 cm of mineral soil were collected under Silver birch stands within the Bangor
DIVERSE experiment, UK. Soil solution was extracted by centrifugation at 4000 rpm during
15 min. Soil was spiked with 14C glucose or fructose; malic, succinic or formic acids; alanine
or glycine. No additional non-labeled LMWOS were added. 14C was traced in the dissolved
organic matter (DOM), CO2, cytosol and soil organic matter (SOM) during one day. To
estimate half-life times (T1 ∕2)of LMWOS in soil solution and in SOM pools, the single and
double first order kinetic equations were fitted to the uptake and mineralization dynamics,
respectively.
The LMWOS T1 ∕2in DOM pool varied between 0.6-5 min, with the highest T1 ∕2for sugars
(3.7 min) and the lowest for carboxylic acids (0.6-1.4 min). Thus, initial uptake of LMWOS
is not a limiting step of microbial utilization. The T1 ∕2 of carboxylic and amino acids in DOM
were closely related with oxidation state, showing that reduced substances remain in soil
solution longer, than oxidized.
The initial T1 ∕2 of LMWOS in SOM ranged between 30-80 min, with the longest T1 ∕2 for
amino acids (50-80 min) and the shortest for carboxylic acids (30-48 min). These T1 ∕2values
were in one-two orders of magnitude higher than LMWOS T1 ∕2 in soil solution, pointing that
LMWOS mineralization occur with a delay after the uptake. Absence of correlations between
LMWOS T1 ∕2 in SOM with C oxidation state, number of C atoms or number of -COOH
groups in LMWOS demonstrates that intercellular metabolic pathways are more
important.
Mineralization of LMWOS amounted for 20-90% of total applied amount. Maximum
mineralization was found for carboxylic acids and minimum for sugars, whereas 14C
incorporation into cytosol and SOM pools followed the opposite trend. There were close
positive correlation between the portion of mineralized C and substance oxidation state, but
negative with the amount of C incorporated into the cytosol and SOM pools. This shows that
substance properties affect the final partitioning of LMWOS-C between mineralized and
utilized pools.
Thus, initial uptake of LMWOS from soil solution and final partitioning of LMWOS-C
between the mineralized and microbially utilized pools are related to their chemical
properties. In contrast, LMWOS mineralization dynamics is regulated by intercellular
metabolization pathways. |