Robert Lombardo

Julie Ma

Sarah Park

The Effect of Moisture Level of Soil on the Respiration Rate of Soil Bacteria

We tested the effect of soil moisture on the respiration rate for soil bacteria. Cellular respiration is a series of chemical pathways used to create ATP through the catabolism of certain organic compounds. An increase in the amount of water received by the bacteria helps it draw out nutrients in the soil and is a fundamental part of the process of respiration. Therefore, we hypothesized that the moisture level of the soil determines the respiration rate of the soil bacteria.

We obtained soil and set aside a small amount to be massed later to find its initial moisture level. Various levels of soil were measured out into a gas chamber, and different amounts of water were added. A CO₂ gas probe was used to measure the CO₂ levels for 5 minutes for each sample. A linear regression was used to find the respiration rate, which was the slope. The process was repeated multiple times. Five days later the set aside sample was massed to find its initial moisture, which was added on to the previous moisture level. A graph of the respiration rate versus the water concentration was made, and a trendline of best fit was added. Since bacteria needs water to respire but too much would halt the process, it was predicted that an increase in the moisture level of the soil will lead to increased respiration of the soil bacteria until saturation.

Our results indicated that the respiration rate of the soil bacteria increased with an increase in moisture level until 0.249 mL/g and then started to decrease. Our hypothesis was correct because the trendline y = -15.195x² + 7.5685x + 0.1542 followed our prediction. The respiration rate per gram of soil bacteria was highest when the amount of moisture in the soil was 0.249 mL/g, the maximum of the parabola. The R² value was 0.5833, meaning 58.33% of the variability of the respiration rate per gram of soil bacteria can be explained by the amount of moisture in the soil.

Results of the Iovieno et al. 2008 on the CABLE web site demonstrated similar results. An increase in moisture made the dormant bacteria active and more excited. It also showed that too much water killed some of the soil bacteria and therefore, decreased the respiration rate. Tang et al. 2006 studied the relationship between soil respiration and both soil temperature and moisture in forests in a region of China. It agreed with our results because although we did not test temperature, our results also had higher respiration rates for more moisture and lower rates for less. As a result of our studies and those from the CABLE web site, we could also conclude that respiration rate changes with seasons due to temperature and moisture changes. Summer would have the highest respiration rates due to the high moisture levels and temperature, and winter would have the lowest respiration rates due to low moisture levels and temperature.