Temperature sensitivity of soil microbial activity modeled by the square root equationas a unifying model to differentiate between direct temperature effects and microbial community adaptation

Numerous models have been used to express the temperature sensitivity of microbial growth and activity in soil making it difficult to compare results from different habitats. Q10 still is one of the most common ways to express temperature relationships. However, Q10 is not constant with temperature and will differ depending on the temperature interval used for the calculation. The use of the square root (Ratkowsky) relationship between microbial activity (A) and temperature below optimum temperature, A=ax(T-T-min), is proposed as a simple and adequate model that allow for one descriptor, T-min (a theoretical minimum temperature for growth and activity), to estimate correct Q10-values over the entire insitu temperature interval. The square root model can adequately describe both microbial growth and respiration, allowing for an easy determination of T-min. Q10 for any temperature interval can then be calculated by Q10=[(T+10 - T-min)/(T-T-min)](2), where T is the lowest temperature in the Q10 comparison. T-min also describes the temperature adaptation of the microbial community. An envelope of T-min covering most natural soil habitats varying between -15 degrees C (cold habitats like Antarctica/Arctic) to 0 degrees C (tropical habitats like rain forests and deserts) is suggested, with an 0.3 degrees C increase in T-min per 1 degrees C increase in mean annual temperature. It is shown that the main difference between common temperature relationships used in global models is differences in the assumed temperature adaptation of the soil microbial community. The use of the square root equationwill allow for one descriptor, T-min, determining the temperature response of soil microorganisms, and at the same time allow for comparing temperature sensitivity of microbial activity between habitats, including future projections.