Impact of Irrigated Agriculture on Soil C Storage and Atmospheric CO2

In arid regions inorganic C (IC) can comprise more than 90% of the total C in the soil. The link of this C pool to atmospheric CO2 and climate change relates primarily to the precipitation/dissolution of the carbonate minerals in the near surface environment. The impact of changes in soil IC on atmospheric CO2 depends on local environmental and hydrological conditions. Under most environmental conditions, dissolution of these minerals leads to net removal of CO2 from the atmosphere. Practices favoring dissolution of carbonates include irrigation with surface waters, and irrigation with water in large excess of plant transpiration. Accumulation of IC in the soil is favored by lower irrigation water applications relative to transpiration (leaching < 30% of applied water), irrigation with ground waters at elevated CO2 concentrations, application of gypsum, and use of nitrate fertilizer. The net effect of irrigation on a global scale, neglecting the effect of fertilizer addition, is to increase soil IC by 30 Tg C/y as well as to release an almost equal amount of C to the atmosphere. Addition of acidifying fertilizers (NH4) reduce IC accumulation and increase CO2 emissions above 30 Tg C/y.There is conflicting evidence regarding actual changes in C storage as a result of irrigation. Liming practices in humid regions throughout the world are estimated to have no net effect on inorganic soil C but release up to 85 Tg C/y to the atmosphere.