Comparison of heat output and CO2 respiration to assess soil microbial activity: a case of ultisol soil
Glucose-induced microcalorimetry and carbon dioxide (CO<sub>2</sub>) production are two widely applied methods to assess microbial activity in soil. However, the links among them, microbial communities and soil chemical properties based on large number of soil samples are still not fully understood. Seventy-two soil samples of different land uses were collected from an ultisol soil area in south China. The best correlation between the rate of heat output and the rate of CO<sub>2</sub> respiration occurred in 8–16 h reaction (R<sup>2</sup> = 0.64), followed by 0–8 h (R<sup>2</sup> = 0.50) (P < 0.001). However, the correlations decreased sharply after 16 h. The heat output per biomass unit (Q<sub>T</sub>/MBC) was well correlated with the total phospholipid fatty acids (PLFAs) (R<sup>2</sup> = 0.56) and bacterial PLFAs (R<sup>2</sup> = 0.53) (P < 0.001). In contrast, these links were not apparent between soil respiratory quotient (qCO<sub>2</sub>) and the total PLFAs and microbial communities. Redundancy analysis further confirmed that Q<sub>T</sub>/MBC was a more comprehensive indicator to assess soil microbial activity and soil quality than qCO<sub>2</sub>, showing a good negative correlation to soil organic carbon, total nitrogen (N) and mineral N, and pH. This work is very helpful to better guide the application of calorimetry and CO<sub>2</sub> respiration in assessing microbial activity in soils.
