Soil microbial responses to simulated climate change drivers.
Climate change is one of the biggest environmental challenges being experienced in the 21st century and is expected to continue to cause drastic alterations to the hydrological, biological and ecological ecosystems. Soil, the second largest carbon pool after the oceans, is a major factor in the global response towards climate change. The ability of soil to act as a sink or source of carbon as climate change increases can be influenced by soil microbial activity. Soil microbial activity is a key driver of terrestrial ecosystem functions and is extremely sensitive towards climate changes. Therefore, the main objective of this study was to investigate the effects of individual and/or interactive global change factors on soil microbial activity and diversity under elevated or ambient temperature incubations during the spring and summer seasons. This was accomplished by the addition of carbon dioxide (CO2), methane (CH4) or simulated rainfall to soil over a 60-day period using Screen Aided Carbon Dioxide Control experiments. Soil microbial dehydrogenase, urease, arylsulphatase and β-glucosidase activities were determined using standard enzyme assays over the 60-day period. In spring, the soil dehydrogenase and arylsulphatase activities increased by 28.07% and 28.48%, respectively, after the addition of elevated CO2 under elevated temperature. Lower dehydrogenase activities were observed at day 60 for most plots during summer while β-glucosidase activity was unaffected by the addition of single or multiple global change drivers during spring. In summer however, all treatments resulted in 28.05 - 36.39% higher β-glucosidase activity by day 15, compared to day 0. Urease enzyme activity was higher during spring at both temperature conditions indicating that moisture limitation and temperature change constrained the urease enzyme production during the summer period. Neither the combination of elevated CO2 and rainfall nor the combination of elevated CO2, rainfall and methane induced substantial changes to the enzyme activities during both seasons, suggesting an antagonistic effect of the combination of these global change factors. However, differences observed from a combination of elevated CO2 at higher temperature clearly reflect a potential for interaction that will affect soil enzymes and subsequent nutrient cycling. This study also investigated the changes in the soil bacterial RuBisCo gene (cbbL), important for CO2 fixation and the corresponding changes in soil organic carbon (SOC), upon exposure to various single or multiple global change drivers. Lowest cbbL gene copy numbers were observed during summer, while, during spring, the cbbL gene copy numbers increased (90.9 – 93.09%) by day 60 compared to day 0, under elevated temperatures. The combination of global change drivers did not result in a substantial variation in cbbL gene copy numbers across seasons suggesting a counteractive effect of the factors, similar to changes in soil microbial enzyme activity. No direct correlation between changes in copy number and SOC was observed, although lower SOC in summer at elevated temperature did result in overall lower cbbL gene copies. Denaturing Gradient Gel Electrophoresis, used to investigate changes in soil microbial community structure, revealed seasonal variability changes in microbial diversity during spring. Soil moisture was a key factor in determining microbial responses during both seasons, with the elevated rainfall treatments able to counteract the adverse effects of elevated temperature during the spring season, with communities in these plots appearing more robust. Increased temperatures and lower soil moisture during the summer period had a negative effect on microbial diversity; however, sequence analysis of excised bands revealed the dominance of thermotolerant bacterial species. A combination of all the global change factors did not induce substantial change in community structure during spring at both temperature regimens. During summer at elevated temperature, growth of certain microbial species were inhibited by a combination of all the global change factors, highlighting the interactive effect between temperature, greenhouse gases and soil moisture. Furthermore, the loss of methanotrophic bacteria, (Methylosinus and Methylocystis) during both seasons can negatively impact greenhouse gas flux and consequently the carbon cycle at large. In this study, seasonal changes linked to variations in soil moisture, substrate availability and temperature strongly influenced soil microbial responses towards climate change. Considering that climate change is a multifactorial process, this study also clearly highlights the necessity for multi-factor global change studies, especially across different seasons in order to accurately predict the fate of soil ecosystem as climate changes continue to increase. Climate change studies often disregard microbial contributions and if carbon sequestration strategies are to be successful, we must fully understand microbial responses under various environmental conditions. An in depth understanding of factors that can lead to changes in soil microbial community activity and structure which influence nutrient and greenhouse gas cycling is essential towards enhancing knowledge of climate change mitigation strategies. Despite the drastic increases in greenhouse gases, temperature and/or rainfall simulated in the present study, it was evident that certain species of soil microorganisms were still able to survive and mediate biochemical activities that are beneficial to the community as a whole.