The effect of irrigation on the carbonate status of Chernozems of Central Precaucasus (Russia)

Non-irrigated and irrigated Ordinary Chernozems of Stavropol Upland, Central Precaucasus (Russia), were examined. A number of morphological and instrumental methods was used for detailed investigation of the carbonates in the soils. This approach allowed us to reveal the essential change of the carbonate status of the irrigated Chernozems as a result of changing the soil moisture regime and the predominance of leaching processes under 30 years irrigation by fresh water. The changes included distinct enhancement of spatial variability and instability of carbonate-illuvial horizon deposition, the highest data scatter on carbonate content within this horizon, the signs of increased mobility of carbonate material and the change of the mechanism of the carbonate accumulations. Carbonates moving with soil solutions and mobile migrational forms of carbonate accumulations as well as segregating forms and lithogenic concretions were transformed under the irrigation. Irrigation for 30 years resulted in unfavorable reclamative changes of the Ordinary Chernozems carbonate status: decalcification of the upper part of the solum, disappearance of the mobile carbonate accumulations from the upper horizons, the initial point of the compact carbonate-hardened horizon in the low part of profile and the increase of the calcium and magnesium content in the soil solution. The problem studied is complicated because of the inherent variability of carbonate parameters and the multitude of factors having an influence on carbonate status. Much more work is needed because of the importance of the topic for sustainability of agricultural production systems, for prediction and overcoming of unfavorable consequences of irrigation, for the up-to-date problem of the greenhouse effect and the response of carbonate reserves to climate changes.