Humus of the brown soils of the mountain pastures of Uzbekistan

Abstract

The aim of the study: assessment of humus content and its distribution in the profile of brown soils of mountain pastures in Uzbekistan. Location and time of the study. Soil pits were cut in the Republic of Uzbekistan in seven agricultural regions, widely used in agriculture: Tashkent region, Bustonlik district; Jizzakh region, Zomin district; Samarkand region, Nurabad district; Navoi region, Nurata district; Kashkadarya region, Kitab district; Surkhandarya region, Sherabad district; Namangan region, Papal district. The study was carried out in 2019-2021. Methods. Field studies, sampling and sample preparation were carried out in accordance with the generally accepted methods: soil organic carbon content was measured by bichromate digestion, carbonate content was determined by acidimetric method and soil inorganic carbon content was calculated by subtracting from the total content of carbonates. Silty fractions were isolated by centrifugation. Humus differentiation into reserves and their estimation was carried out according to the method proposed by one of the authors. Main results. The studied brown soils of mountain pastures of Uzbekistan are characterized by loamy granulometric composition, with a clay fraction not exceeding 20%. The content of carbonates is low (2.5–9%), with the maximum amount in the carbonate horizon. Soils are slightly leached. The total humus content in the upper horizon varies from 1% to 6.6%. The dependence of soil humus content on the slope exposure was noted: the soils of the more humid northern and western slopes contain more humus than the southern and eastern ones. In addition, the humus content is significantly affected by erosion. To assess the nature of changes in the organic matter of the brown soils during their agricultural use, humus of the near, labile and potential reserves (as percentage of the total content) was isolated for the first time. It was revealed that humus of potential reserve prevails in soils (35.5-90%). Its vertical distribution is characterized by a sharp decrease in the sub-humus part, while the humus of the near and labile reserves is distributed evenly in the profile, without a pronounced dependence on external conditions and the degree of disturbance. The proportion of insoluble organic substances in the total humus content ranged from 0.39 to 86%, of sparingly soluble organic substances - from 3 to 100%. There is a relationship between the amount of sparingly soluble and insoluble organic matter in the upper horizon and the degree of soil erosion. Conclusion. The distribution of humus and its reserves in the brown soils of the mountain pastures of Uzbekistan is uneven and strongly affected by erosion. The loss of near and labile humus reserves leads to a decrease in the content of total humus. Further study of humus features, humus accumulation and restoration in brown soils is important for the development of recommendations for the rational use, anti-erosion protection and increasing the productivity of mountain pastures in Uzbekistan.