The Activation Of Soil Microbial Biomass By Trace Amounts Of Substrate

Very small inputs of labile substrates(e.g.glucose and root extract)can cause several times more triggered C(i.e.the differences in CO₂-C evolved from the soil amended with trace amounts of substrate compared to that evolved from the same non-amended soil)to be evolved during a short period(from hours to days)from soil than was contained in the added substrates.This is termed the triggering response.Some studies suggest that the triggering response is an apparent priming effect which is induced by trace amounts of substrate,accelerating the metabolism of the soil microbial biomass to maximize the utilization of potential substrate inputs.However,to date,there have been very few studies on the triggering response.Evaluating the factors,dynamics of microbial biomass and C pools involved will help to provide a more complete understanding of the triggering response.To undertake this,¹³C isotope labeling was employed to evaluate the dynamics of the triggering response,microbial biomass and triggered C sources by studying the effects of soil type,substrate concentration,substrate type and soil depth.The main results were as follows:(1)The triggering response,C partitioning and microbial biomass dynamics in two soils were investigated.The cumulative triggered C from a Alfisols and a Vertisol,amended with ¹³C-labeled glucose(10?g glucose C g^-1soil),was equivalent to 149%and 195%respectively of the added glucose-C.The strongest evolution of triggered C occurred from 3 to 5 hours after glucose amendment,whereas the largest absolute increase in triggered C was from 24 to 72 hours.The initial rapid triggering stage was caused by glucose mineralization,then the decomposition of microbial biomass carbon(MBC)and labile soil organic carbon(SOC)dominated thereafter.All microbial indicators(ATP,MBC and biomass ATP)recovered by the end of the incubation.The overall taxonomic composition of the soil bacteria was little influenced,but the co-occurrence association and complexity were significantly increased by the last stage(72 hours)of the triggering response.(2)The differences in microbial uptake,mineralization and triggering intensity were revealed by adding different amounts of glucose.The triggered C was higher than added C only when glucose was applied at 1.5%or 3%of MBC,equivalent to156%and 113%of added C respectively.Most of the glucose uptake occurred within2 hours and mineralization was within 24 hours.In addition,the ATP concentrations in the soils given the two glucose rates increased more than 200%at 5 hours and MBC decreased significantly at 24 hours.All other treatments with glucose amendments at 6-30%of MBC did not show the triggering response despite higher microbial uptake of glucose and a longer period(0-72 hours)of rapid glucose mineralization.The increases in ATP with higher glucose additions,equivalent to12-30%of MBC,were smaller than those with lower rates of additions,whereas MBC increased significantly after incubation for 168 hours.(3)The differences in triggered C and microbial biomass dynamics induced by complex and simple substrates in different soil depths(0-10 cm,10-30 cm and 30-60cm)were determined.All soils amended with root extract or glucose at different soil depths at 3%MBC showed the triggering response.The triggered C decreased with soil depth and increased with substrate complexity.Root extract at 3%MBC induced the equivalent of 400%of added C to be evolved from the 0-10 cm depth after 240hours.However,none of the treatments amended with substrates at 30%MBC induced the triggering response.The increases in ATP and decreases in MBC involved in the triggering response all peaked at 72 hours.The rates of change increased with the amount of triggered C and were 9.10-18.8%and 5.60-16.2%respectively.Significant changes in ATP and MBC were found in 0-10 cm soil amended with root extract at 3%MBC at 72 and 240 hours.The MBC and ATP changes following substrate addition at 30%MBC peaked at 24 hours and increased steadily after 72hours.All treatments of the three soil layers showed a mean biomass ATP concentration of 10.2±1.8?mol g^-1biomass C.The above results indicated that the size of the triggering response and changes in microbial biomass are influenced by substrate complexity and soil depth.(4)The different C sources involved during the course of the triggering response in the different soil layers were evaluated.The ¹³C abundances in MBC(5.27-6.71atom%)of different soil layers were significantly higher than in SOC(1.94-2.82atom%)after 56 days of pre-incubation with high ¹³C abundance maize and glucose.Addition of trace amounts of glucose to the pre-incubated soils caused 2.3-6 times more triggered C evolved than was contained in the triggering glucose,which increased with soil depth.The atom%¹³C-CO₂values derived from model calculations agreed more closely with measured values when assumed to be entirely derived from MBC at day 1 and 5 and from ¹³C-maize and ¹³C-glucose at day 10.In addition,MBC decreased significantly at day 5.These results indicated that the triggered C evolved during the triggering response was initially derived from MBC and dominated by mineralization of ¹³C-maize and glucose thereafter.In conclusion,the triggering response is mainly due to the rapid activation of soil microorganisms through mineralization of MBC,so as to utilize the potential substrate inputs.The study of its mechanism is of great significance in advancing our understanding of the process of soil C turnover.