Tuesday, February 6, 2007

pCO2 Profiles in Soils of Irrigated Orchards and Their Relation to Carbon Sequestration.

Gil Eshel1, Boaz Arad1, Pinchas Fine2, Uri Mingelgrin3, and Guy Levy4. (1) Institute of Soil, Water and Environmental Science, Agricultural Research Organization, P.O.B. 6, Bet Dagan, 50250, Israel, (2) Institute of Soil, Water and Enviromental Science, Agricultural Research Organization, P.O.B. 6, Bet Dagan, 50250, Israel, (3) Institite of Soil, Water and Environmental Science, Agricultural Research Organization, P.O.B. 6, Bet Dagan, 50250, Israel, (4) ISRAEL,Min. of Agric., Agricultural Research Organizaiton, Po Box 6, Bet Dagan, 50250, ISRAEL

In semi-arid and arid regions, attempts to sequester carbon in soils in organic forms has met with little success.  However, in such regions, sequestration of carbon in the inorganic fraction of the geosphere may prove a viable alternative. Soils from semi-arid and arid regions commonly contain calcium carbonate (CaCO3), whose dissolution and re-precipitation are closely associated with carbon exchange between the soil and the atmosphere because the two reactions consume and release CO2, respectively. Manipulation of conditions governing CaCO3 dissolution and re-precipitation (e.g., causing pCO2 in the soil profile to change) may, thus, aid in the enhancement of inorganic carbon (IC) sequestration and contribute to reduction in CO2 emission to the atmosphere.  In an ongoing study, we monitored the pCO2 to a depth of 5 m in two adjacent orchards in the coastal plain of Israel, one irrigated with fresh water (FW) and the other with treated effluent (TE). In general, we measured in both orchards higher pCO2 values than the commonly reported values of one to two orders of magnitude higher than atmospheric pCO2. The pCO2 values obtained in our measurements were in the range of 1.5 kPa at a depth of 0.5 m to 15 kPa at depths of 3-5 m (mainly next to the trees). Such levels may affect reactions (e.g., enhancement of inorganic carbon dissolution) that take place in the soil and its chemical properties (e.g., pH). As expected, we found that soil pCO2 was affected by soil moisture, and the distance from the trees. However, irrigation water quality (fresh water vs. treated effluent) did not produce a significant effect on soil pCO2.  The relationship between the pCO2 and some related soil properties (e.g., CaCO3, clay and organic matter content) and the implications of the data obtained on soil carbon exchange with the atmosphere are discussed.