The responses of microbial temperature relationships to seasonal change and winter warming in a temperate grassland

Abstract

Microorganisms dominate the decomposition of organic matter and their activities are strongly influenced by temperature. As the carbon (C) flux from soil to the atmosphere due to microbial activity is substantial, understanding temperature relationships of microbial processes is critical. It has been shown that microbial temperature relationships in soil correlate with the climate, and microorganisms in field experiments become more warm-tolerant in response to chronic warming. It is also known that microbial temperature relationships reflect the seasons in aquatic ecosystems, but to date this has not been investigated in soil. Although climate change predictions suggest that temperatures will be mostly affected during winter in temperate ecosystems, no assessments exist of the responses of microbial temperature relationships to winter warming. We investigated the responses of the temperature relationships of bacterial growth, fungal growth, and respiration in a temperate grassland to seasonal change, and to 2 years' winter warming. The warming treatments increased winter soil temperatures by 5-6°C, corresponding to 3°C warming of the mean annual temperature. Microbial temperature relationships and temperature sensitivities (Q ) could be accurately established, but did not respond to winter warming or to seasonal temperature change, despite significant shifts in the microbial community structure. The lack of response to winter warming that we demonstrate, and the strong response to chronic warming treatments previously shown, together suggest that it is the peak annual soil temperature that influences the microbial temperature relationships, and that temperatures during colder seasons will have little impact. Thus, mean annual temperatures are poor predictors for microbial temperature relationships. Instead, the intensity of summer heat-spells in temperate systems is likely to shape the microbial temperature relationships that govern the soil-atmosphere C exchange.

Temperature adaptation of soil microbial respiration in alpine, boreal and tropical soils: An application of the square root (Ratkowsky) model

“…The square root model has been widely used for soil microbial growth (Nicola & Bååth, 2019; Nottingham et al, 2019; Pietikäinen et al, 2005; Rinnan et al, 2009; van Gestel et al, 2013). Similar results have earlier been shown for respiration in a few soils (Birgander et al, 2018; Kätterer et al, 1998; Pietikäinen et al, 2005). The present study, with its large sample size, clearly suggests that the square root equation is valid for soil heterotrophic respiration and a good descriptor of the temperature relationship of microbial respiration in soils from tropical to alpine environments, covering most of all temperature regimes globally.…”

“…Although similar results were found, T min for respiration in cold, temperate and tropical climate was −11.1°C, −5.3°C and −2.4°C, which is in the higher range of that suggested for bacterial growth in these three climatic zones, −10°C to −15°C, −5°C to −10°C and 0°C to −5°C. Pietikäinen et al (2005) compared two soils showing that T min was a few degrees higher for respiration than for growth, and Birgander et al (2018) compared one soil during different seasons with a mean T min for respiration of 5.1°C and for bacterial growth of 7.9°C. Thus, the pattern of a higher T min for respiration than growth in soil appears to be repeatable.…”

“…Second, the optimum temperature ( T opt ), which describes the point of maximum activity, is another important parameter from the MMRT model. However, T in situ has been shown to be substantially lower than T opt for microbial growth in most environments (Birgander et al, 2018; Donhauser et al, 2020; Rinnan et al, 2009; van Gestel et al, 2013, 2020), and thus it will not be necessary to invoke decreasing rates above T opt for predictive purposes, as discussed by Bååth (2018). If the T in situ becomes close to or higher than T opt , the soil microbial community will adapt to the new conditions, increasing T min and T opt , and T in situ will then still be lower than T opt (Donhauser et al, 2020; Rousk & Bååth, 2011), as also indicated by the increase in T min with MAT (Figure 3).…”

“…Any seasonal effects on T min of microbial respiration were not considered. A recent study showed that there was no significant difference in T min for bacterial growth between winter and summer (Birgander et al, 2018), suggesting that there may be no seasonal effects on T min . Climate variables (i.e., MAT and MAP) for both forest and alpine grassland ecosystems were derived from the Worldclim database using GPS locations and A RC MAP (Environmental Systems Research Institute, Redlands, CA, USA).…”

“…Moreover, T min allows for a direct descriptor of the temperature adaptation of the microbial community (Bååth, 2018; Nottingham et al, 2019), with lower T min in cold areas and vice versa, making it possible to include temperature adaptation of the microbial community in future predictions of effects of global temperature increase. The square root model has been widely used for pure culture growth of bacteria and/or fungi (Corkrey et al, 2014; Ratkowsky et al, 2005), as well as for microbial communities in soil using short‐term incubations to identify temperature adaptation of microbial growth (e.g., Birgander et al, 2018; Pietikäinen et al, 2005; Rinnan et al, 2009; van Gestel et al, 2020). A recent study, across a mean annual temperature (MAT) range from 6°C to 26°C along a tropical elevation gradient, showed that soil microbial growth is adapted to long‐term temperature differences and T min for microbial growth increases by 0.3°C with each increase in MAT by 1°C (Nottingham et al, 2019).…”