CO2 flux from soil is significant in most ecosystems and can account for more than 2/3 of total ecosystem respiration. In many cases CO2 fluxes from soil are estimated using eddy covariance techniques or the classical chamber method with measures of bulk concentrations and isotope composition of CO2. Whereas most of these studies estimate flux from the soil surface, we analyzed its concentration and isotope composition directly in soil profiles down to 8.5m depth. This experiment was conducted in Sumter National Forest in summer of 2016. The samples were collected from 3 different land use history sites: a) reference hardwood stands, mainly of oak and hickory that are taken to be never cultivated; b) cultivated plots, which were also used growing cotton prior to the 1950’s but for the last 50 years for growing corn, wheat, legume, sorghum, and sunflowers; c) pine stands, which had been used for growing cotton from beginning of the 19th century and then was abandoned in 1920s and planted with loblolly pine. We have analyzed 3 replicates of each land use. There were measured in the field CO2 and O2 concentration and collected gas samples were analyzed for Δ14C, δ13C and δ18O.
CO2 concentration in all types of land use has a maximum about 3m depth, approximately the same depth as the minimum of O2 concentration.
Isotope analyses revealed that carbon isotopic composition tend to become lighter with the depth for all three types of land use: in cultivated site it changes from -18%o at 0.5m to -21%o at 5m; in pine site from -22%o to -25%o and in hardwood from-21.5 -24.5%o correspondently, the O2 isotopic composition does not change significantly.
Based on analysis of Δ14C the turnover rate of CO2 is getting slower as depth increases. At the first 50 cm the exchange rate is the fastest on cultivated site, likely due to annual tilling, and concentration of 14C is actually equal to atmospheric. However, the turnover rate of Δ14C in soil CO2 slows down significantly as depth increases and reaches 140%o at 5m, which corresponds to a turnover rate of about 30 years. The forest sites have the same trend in the Δ14C changes; however the exchange rates are faster. For the hardwood site the turnover rate changes from about 5-10 years in the top 0.5m to about 15 years at 5m. The pine site has a slightly slower turnover rate than hardwood site but faster than cultivated site.
Cherkinsky, Alexander, Zachary Brecheisen, Daniel deB. Richter, Hong Sheng (2017): Estimation of soil respiration rates and soil gas isotopic composition for the different land use of Ultisols from Calhoun CZO. American Geophysical Union 2017 Fall Meeting, New Orleans, Louisiana, 11-15 December 2017.
This Paper/Book acknowledges NSF CZO grant support.