## 3 Results and Discussion  
Data on SOC stocks are provided in the Figure 3. In general, the SOC stocks were higher in irrigated Light and Typical Calcisols than in reference types of this Calcisols. This could be caused by the effect of irrigation on soil biological productivity and additional accumulation of organic matter. The alteration of key soil properties under irrigation has been reported previously [22; 23]. In opposition, irrigated Dark Calcisols demonstrate decreasing SOC stocks. This could be caused by leaching of water-soluble fractions of organic matter in conditions of water saturation, while in mature non-irrigated soils there is a lack of water. Another reason for Dark Calcisol decarbonization is weak slope erosion; these soils are located in transitional zones from adyr to valley on elevated slope positions. The Light and Typical Calcisols are located on flatter territories with less pronounced or completely absent erosion. The gravimetric content of SOC in topsoil humus or the humus arable horizon is provided in Figure 4. These data demonstrate that the highest organic matter and total nitrogen content are typical for Dark Calcisols. If one compares SOC and TN data in pairs of soils investigated, it is evident that there is an essential decrease in both these biogenic elements in irrigated soils. Thus, the SOC stock (volumetric content) does not directly connect with its mass concentration in the fine earth and depends on the alteration of soil density and thickness of the whole solum and SOC-enriched horizons. It is very interesting that there are remarkable changes in C/N ration in all the investigated irrigated soils in comparison with the non-irrigated reference soils. This corresponds well with the data of [PERSON] [24] regarding the old, irrigated \"tugay\" soils of Uzbekistan. The profile distribution of the C/N ratio is completely different in mature and irrigated soils. Thus,topsoils of irrigated soil demonstrate more a narrow ration than the deepest ones. The volumetric content of TN was essentially higher in investigated soils of the Fergana Valley (this research) than in Calcisols and Cinamonic soils of the adjacent Namangan Valley [25], which illustrates the essential role of local climatic factors in the variability of the soils' chemical properties. The degree of soil organic matter depends on numerous factors: quality and quantity of the organic precursors, component composition of the fine earth, and climatic conditions [26]. The qualitative index of humification degree is the ratio of carbon of humic acids to carbon of fulvic acids--the CHA/CFA ratio [19]. The highest ratio value indicates the highest humification degree. These indexes, for the soil investigated, are provided in Table 1. It is evident that irrigation results in a decrease in humification degree. It is quite logical, because the reference non-irrigated soils are drier, and the reactions of polymerization in that state are more pronounced [26]. Moistening of the soil climate under irrigation results in an increase in fulvic acids and, by this, a decreasing CHA/CFA ratio. This tendency was revealed for all three pairs of soils investigated. Thus, we can speak about reorganization of the systems of humic acids under irrigation towards a decreasing humification degree in all irrigated soils. It is remarkable that the gravimetrical content of both groups of humic acids increased in Typical and Light Calcisols and decreased in Dark ones. This in good correspondence with the fact the stocks of humus in irrigated Dark Calcisols decreased and, conversely, showed incremental increase in the cases of Typical and Light Calcisols. Data on the humus state of the soils investigated are more or less similar to results from soils of Azerbaijan [27]. When studying the content of free amino acids in Light Calcisols, 14 to 20 free amino acids were detected and identified. Data on amino acids content and composition are presented in Figures 5 and 6.