ИСТИНА

Organic carbon sequestration and food security may seem to be poorly correlated. However, the accumulation of organic carbon, independently on its effect on the climatic change, is of primary importance for the productivity of soils in agriculture, and thus is basic for providing food security. Though plants do not receive nutrients directly from humus, the presence of organic matter is of major importance for plant nutrition, because it ensures cation exchange capacity of soils, their structure and other physical properties, and the presence of a long-term supply of food for microorganisms in soils. In arid regions the organic matter in soils is not stable. If the input of organic residues is low, the rate of mineralization of organic matter is high; carbon releases to the atmosphere or precipitates as pedogenic carbonates. The process of C transfer from soil OM to carbonates can be described thermodynamically as an irreversible process that leads to increasing soil alkalinization and calcium carbonate accumulation (Volobuev, 1975). Predicted temperature increase would probably speed up the irreversible fixation of C in the form of calcium and sodium carbonates, because the rate of organic matter mineralization increases. The increase in organic matter input results in the intensification of the process of humus mineralization and consequent formation of carbonates. If we consider greenhouse concentration control in the atmosphere to be an absolute priority, the fixation of C in the form of pedogenic carbonates is a good mechanism for controlling the atmospheric CO2 concentration (Lorenz & Lal, 2012). However, from the point of view of soil fertility, the humus to carbonates transformation is a negative process. First, it decreases the energetic status of soils that is a good expression of its fertility. Second, the process of humus formation requires also fixation of significant amounts of N and P (Lal, 2004), which are consequently lost in the course of organic matter mineralization. One of the tasks for providing food security in arid regions is to maintain the stability of soil organic matter. The possible mechanisms for that may include non-tillage conservation agriculture, regulation of moisture regime in irrigated soils and the use of soil-protecting crop rotations.