Effects of temperature on CO2 emissions from soils with different water repellency levels.

Abstract

Carbon dioxide (CO2) is a major greenhouse gas with a high contribution to global warming and climatic changes. Since soils store a significantly large amount of carbon, even a minor fluctuation in temperature can substantially influence the soil organic matter (SOM) dynamics and CO2 emissions. Soil water repellency (SWR) that reduces spontaneous wetting and water distribution in soils due to the presence of hydrophobic organic materials can influence SOM decomposition and CO2 emissions. It is not clear how the temperature changes can influence CO2 emissions from soils with different SWR levels. Therefore, this research aimed to examine the effects of temperature on CO2 emissions from soils with different SWR levels. Soil samples were collected from 0-5 and 5-10 cm depths from a non-repellent (NR) Swietenia mahogani forest in Mapalana and an extremely water-repellent Casuarina equisetifolia (CE) forest in Hambanthota, Sri Lanka. A grassland soil was mixed with 10% CE litter powder to create a moderately water-repellent (MWR) soil. These soil samples were separately incubated at 60% water holding capacity and 20- 80°C temperature range with 10°C increments for 8 h. The CO2 emissions (NaOH trapping), SWR [Water Drop Penetration Time (WDPT) and Molarity of Ethanol Droplet tests], and SOM content (Walkley-Black method) were measured in incubated samples. The CO2 emissions increased up to maximum at 40°C and beyond that decreased in all tested soils. The highest CO2 emissions (0- 5 cm depth) were observed in NR soils; however, there were no significant differences in CO2 emissions between the three soil types (p>0.05). Results confirmed that the level of SWR did not affect the increasing and then decreasing trend of CO2 emissions from soils with increasing temperature. There was a negative correlation (R2=0.68) between CO2 emissions and WDPT only in MWR soil. The SOM contents showed an overall decreasing trend with increasing temperature in all the three soil samples due to faster decomposition rates facilitated by elevated temperatures.