EFFECT OF DIFFERENT LAND-USE TYPES ON THE HUMUS STATUS AND AGROCHEMICAL PROPERTIES OF IRRIGATED LIGHT SEROZEM SOILS IN THE ARID ZONE OF KAZAKHSTAN (A CASE STUDY OF THE KYZYLORDA REGION)

Abstract

Light Serozem soils constitute the principal soil resource supporting irrigated agriculture in the arid regions of southern Kazakhstan. Their inherently low organic matter content, rapid mineralization rates, and susceptibility to secondary salinization make them highly sensitive to changes in land use. Therefore, assessing changes in humus status and agrochemical properties is essential for maintaining soil fertility under irrigated farming systems. The objective of this study was to evaluate the influence of different land-use types on the humus status and agrochemical properties of irrigated Light Serozem soils in the arid zone of Kazakhstan using the Kyzylorda Region as a case study. The investigated soils included those under perennial alfalfa cultivation, irrigated cropland planted with melon crops, and land abandoned from agricultural use for more than 15 years. All soils were classified as light loams dominated by the fine-sand fraction. The highest soil organic carbon (SOC) content in the surface horizon was recorded under perennial alfalfa (0.96%), whereas irrigated cropland and abandoned land contained 0.72% and 0.65% SOC, respectively. Alfalfa cultivation was also associated with the highest content of non-hydrolysable organic residue, a greater proportion of humic acids, and a predominance of humic fractions strongly bound to the mineral soil matrix. In all land-use systems, the ΣHA/ΣFA ratio was 0.6, indicating the predominance of fulvic acids in the humus composition. Under both perennial alfalfa and irrigated cropland, nitrate nitrogen and available sulfur reached their maximum concentrations in the middle part of the soil profile, indicating downward redistribution and accumulation of these nutrients under irrigation. In contrast, available phosphorus and exchangeable potassium were concentrated mainly in the upper horizons. Exchangeable calcium and magnesium dominated the exchange complex, whereas exchangeable sodium remained low throughout the profiles. All soils exhibited a strongly alkaline reaction, with pH values ranging from 8.76 to 9.07. The results demonstrate that long-term cultivation of perennial alfalfa promotes soil organic carbon accumulation and the formation of more stable humus compounds. Conversely, long-term unmanaged fallow did not restore a stable pool of soil organic matter even after more than fifteen years of abandonment.