Microbial utilization and mineralization of [ 14C]glucose added in six orders of concentration to soil

Abstract

The substrate availability for microbial biomass (MB) in soil is crucial for microbial biomass activity. Due to the fast microbial decomposition and the permanent production of easily available substrates in the rooted top soil mainly by plants during photosynthesis, easily available substrates make a very important contribution to many soil processes including soil organic matter turnover, microbial growth and maintenance, aggregate stabilization, CO 2 efflux, etc. Naturally occurring concentrations of easily available substances are low, ranging from 0.1 μM in soils free of roots and plant residues to 80 mM in root cells. We investigated the effect of adding 14C-labelled glucose at concentrations spanning the 6 orders of magnitude naturally occurring concentrations on glucose uptake and mineralization by microbial biomass. A positive correlation between the amount of added glucose and its portion mineralized to CO 2 was observed: After 22 days, from 26% to 44% of the added 0.0009 to 257 μg glucose C g −1 soil was mineralized. The dependence of glucose mineralization on its amount can be described with two functions. Up to 2.6 μg glucose C g −1 soil (corresponds to 0.78% of initial microbial biomass C), glucose mineralization increased with the slope of 1.8% more mineralized glucose C per 1 μg C added, accompanied by an increasing incorporation of glucose C into MB. An increased spatial contact between micro-organisms and glucose molecules with increasing concentration may be responsible for this fast increase in mineralization rates (at glucose additions <2.6 μg C g −1). At glucose additions higher than 2.6 μg C g −1 soil, however, the increase of the glucose mineralization per 1 μg added glucose was much smaller as at additions below 2.6 μg C g −1 soil and was accompanied by decreasing portions of glucose 14C incorporated into microbial biomass. This supports the hypothesis of decreasing efficiency of glucose utilization by MB in response to increased substrate availability in the range 2.6–257 μg C g −1 (=0.78–78% of microbial biomass C). At low glucose amounts, it was mainly stored in a chloroform-labile microbial pool, but not readily mineralized to CO 2. The addition of 257 μg glucose C g −1 soil (0.78 μg C glucose μg −1 C micro-organisms) caused a lag phase in mineralization of 19 h, indicating that glucose mineralization was not limited by the substrate availability but by the amount of MB which is typical for 2nd order kinetics.