Abstract/Summary

Understanding the mechanisms by which soil respiration responds to climate change is critical to predicting and mitigating future global warming. Changes in temperature and moisture are known to influence soil processes like soil organic matter decomposition and heterotrophic respiration which are mediated by soil microorganisms. For better predictions of microbial respiration, there is a need to understand the importance of microbial community composition and incorporate the processes they mediate in predictive climate models. However, the accuracy of such models could be enhanced by accounting for high frequency temperature fluctuations (e.g. daily maximum and daily minimum temperature). This study explored the complex mechanisms by which soil microbial communities react to temperature change both in the laboratory and the field. The overall aim was to examine how the legacy effect of soil temperature conditions affect the temperature sensitivity of soil respiration. In a pilot study, sieved or intact soil cores from arable, grassland and woodland land use types were incubated for 42 days and soil CO2 flux measured over time. The results showed that physical disturbance did not significantly influence soil respiration. Differences in soil respiration rate between land use types were due to contrasting soil water holding capacity and the quantity and stoichiometry of soil organic matter. While our climate warms, a reduced Diurnal Temperature Range (DTR) has been observed over the last 50 years as daily minimum temperature has increase more than daily maximum temperature, due to global climate change. However, the relative importance of these short-term diurnal temperature oscillations, compared to the overall increase in daily average temperature, is unclear. Especially since most laboratory measurements of soil respiration are made at constant temperatures. The same grassland soil used in the pilot experiment was incubated at four different temperature regimes including constant incubation at temperatures representing the daily minimum (5°C), daily mean (10°C), daily maximum (15°C), and diurnal oscillation between average daily minimum and maximum (5-15°C) temperature of the area the soil was collected from for 17 weeks and CO2 flux measured over time. CO2 released from the oscillating incubation was similar to that from the maximum incubation temperature, not the average incubation temperature. Daily maximum temperature dictates the composition of soil microbial community. Changes in soil biological and chemical properties due to temperature change was consistent with apparent thermal acclimation. It was therefore concluded that daily maximum rather than daily average temperature determines the flux of CO2 from soil, thus challenging the justification for researchers performing experiments by incubating samples at the daily mean temperature without oscillation. It was unclear whether the importance of daily maximum temperature was due to its influence on intracellular or extracellular enzyme activity rates. Extracellular enzymes are considered to catalyse the rate limiting step in organic matter decomposition. To investigate the temperature sensitivity of extracellular and intracellular enzymes, two extracellular enzymes (β-glucosidase and chitinase), intracellular enzyme activity (glucose-induced respiration), and basal respiration were assayed at a range of temperatures (5°C, 15°C, 26°C, 37°C and 45°C) after the same grassland soil used in the previous experiments was pre-incubated at 5°C, 15°C, or 26°C. The aim was to assess whether both extracellular and intracellular enzyme activities are equally sensitive to temperature and whether pre-incubation temperatures, which should create a gradient of acclimation, influences these processes. The result revealed that pre-incubation temperatures influenced the temperature sensitivity of both extracellular and intracellular enzyme activities, and that these two enzyme-mediated processes were not equally sensitive to temperature. While intracellular enzyme had higher temperature sensitivity in the range 15°C - 26°C, extracellular enzymes had higher temperature sensitivity in the range 26°C - 37°C. Thermal acclimation was also observed and attributed to temperature-induced changes in stoichiometry (C/N ratio) of soil organic matter and soil chemistry (pH). Finally, attempt was made to observe the impact of soil warming on the thermal adaptation of the soil microbial community under field conditions. The aim was to examine whether the addition of soil organic matter through cover crop residue incorporation mitigates the effect of simulated climate change (winter warming) on the resilience of the soil microbial community to wet and dry cycles. Custom designed Open Top Chambers (OTCs) were deployed and demonstrated to effectively, but inconsistently, warm soils above the ambient temperature. However, the laboratory component of the work could not proceed due to COVID-19 pandemic. Overall, I can conclude that (i) soil microbial communities thermally adapt to changing temperature, (ii) changes to daily maximum temperature are more important than changes in daily average temperature, and (iii) this is because extracellular enzymes activity is the rate limiting step in organic matter decomposition and this step is more sensitive to temperature changes during warm days than cool days. (v) Temperature driven changes in soil chemical and biological properties influenced the thermal adaptation of soil microbial respiration.