A new approach to determine soil microbial biomass by calorimetry

MARÍA EUGENIA SESTO CABRAL; E. ELIZABETH SIGSTAD

Lugo

Congreso; XVI ISBC (Internacional Society for Biological Calorimetry); 2010

Sociedad Internacional de Calorimetría Biológica y Universidad de Lugo

Several methods were developed to determine soil microbial biomass (SMB). The substrate induced respiration (SIR)1 is based on the activation of the soil microbial community by the addition of glucose and the resulting initial maximal respiration is used to estimate the SMB. As soil respiration could be successfully measured by calorimetry2 in this work we present an approach to determine SMB using this technique. Soil samples were collected and stored at 4ªC until used. Soil (1g) amended with 2 mg glucose g-1 at 60% of water holding capacity was placed in the calorimeter at 25ºC. Thermal power (P1) was recorded for 2 h. Then, a vial containing a solution of 0.5 M NaOH was introduced and values of thermal power (P2) were recorded again. After 2 h, the vial was removed and values of thermal power were recorded for 2 more hours. Values of P were converted to specific thermal power (p). The specific rate of CO2 (rCO2 / nmol g-1 h-1) evolution was calculated by pasting twice in a worksheet of an Origin 6.0 computer program the p – t curve obtained. From one, the p values due to the CO2 evolution were eliminated. Then, this curve was subtracted from the original one and further integrated to obtain q in mJ g-1. This heat value was divided by the heat of neutralization of CO2 with NaOH and by the time of measurement. Values of rCO2 were converted into SMB / g g-1 by using the molar volume and a factor of 25.33. Conventional SIR method was also performed3. Log values of p1 were linearly correlated with log values of SMB as previously found4. Strikingly the equation relating both parameters was not significantly different from the previously reported. Calorimetric SMB values were 1.3 times lower than those obtained by SIR