Calorimetry versus respirometry for the monitoring of microbial activity in a tropical soil

Abstract

Microcalorimetric and respirometric methods were applied to investigate the microbial activity in a tropical red Latosol (RL) soil. The combination of both methods to collect data enabled knowledge of environmental factors related to the characteristics of microbial activity of the Oxisol soil. The soil sample, with 35% of moisture, was assayed over a range of glucose additions, varying from 2.0 to 10.0 mg, causing an increase in enthalpy from −2069±60 to −2455±73 kJ mol −1, with a lowest value at 2.0 mg and similar values in the range of 4.0–10.0 mg of glucose. Samples with different degree of moisture content: 17, 28 and 35%, gave the enthalpic values: −2200±64, −2411±70 and −2455±73 kJ mol −1, respectively. The 60% of maximum water holding capacity, 28, and 35% of moisture content in the soil resulted in very close enthalpic values for microbial activity. Species of aerobic bacteria and fungi were isolated from the RL. Bacteria degrade glucose as follows: 1st, 2nd and mixture of them, gave the enthalpic values −2185±87, −413±16 and 921±35 kJ mol −1, respectively, whereas, 1st and 2nd fungi isolated from PDA cultures gave the enthalpic values −1183±45 and −759±30 kJ mol −1, respectively. The mass of carbon dioxide liberated was determined from 15 day duration of the experiments. These respirometric results were: 4.03 for soil glucose-amendment; 3.34 for 1st bacterium; 3.25 for 2nd bacterium; 3.12 for the 1st fungus; 3.09 for the 2nd fungus; and 3.14 for the mixture of 1st and 2nd bacteria, respectively. The efficiency of bacteria and fungi in glucose consumption was evaluated in soil, showing the largest values for bacteria in comparison to fungi, when similar soil samples were assayed in the glucose microbial degradation.