Understanding and predicting the responses of terrestrial ecosystems to global climatic changes is an on-going challenge. Soil CO2 efflux is a critical ecosystem response as it directly affects the concentration of atmospheric greenhouse gases and future climate changes. Decomposition of soil organic matter (SOM) is sensitive to the change in soil temperature and may induce large releases of soil carbon. Change in temperature sensitivity of soil CO2 flux across 3000 m elevation gradient was studied at San Jacinto Mountains of southern California. Soil samples were collected from eight sites at 300 m elevation intervals over a 25 km horizontal distance with three representative sampling points for canopy and interspace and depth increments of 0-5 cm and 5-15 cm, at each location for a total of 96 samples. Soil samples were analyzed for particle size distribution, water holding capacity, organic matter content, and total carbon and nitrogen content using standard procedures. Air-dried and sieved (2 mm) soil samples weighing 50g were placed in glass jars and water content adjusted at 40 percent water holding capacity and incubated at 25°C. At regular intervals, soil-jars were incubated at 13, 19, 25 and 31 °C by placing them in temperature controlled circulating water-bath with an 8 hour period. Instantaneous soil CO2 fluxes at these four temperature intervals were measured using LI-7000 CO2 analyzer. Measurements were made at five day interval for first three weeks and then measured at 10 day interval. Over time, variations in estimated respiration at a reference temperature (25°C) and the temperature sensitivity of respiration with space and across 3000 m elevation gradient were described using Q10 value.
Results/Conclusions
Differences in SOM content with space and soil depth changed with elevation. With an increase in elevation from 270 m to 3000 m, values of SOM increased from 2.9 percent to 6.6 percent of soils under canopy within 0-5 cm depth. However, the highest value of SOM (9.5%) was observed at the mid-elevation (1500 m) for the same space and depth. Maximum difference in SOM content between canopy and interspace was observed at the site located at 1500 m elevation. Temperature sensitivity of soil CO2 flux was also dependent on the space and distinctly varied with soil depth within each site. Results revealed that temperature sensitivity over time was not only controlled by total organic matter content, site elevation and substrate quality played a major role in soil respiration-temperature dynamics.
