Methodological Approaches to Ground Monitoring and Baseline Levels of the Content and Stocks of Organic Carbon in Arable Light Chestnut Soils, Volgograd Oblast

For the test monitoring site on the territory of the Oroshaemaya Experimental Station in the south of the Volga Upland (Volgograd oblast, Russia), the estimates of the baseline contents and stocks of soil organic carbon (C_org) and soil bulk density and characteristics of their spatial variability and vertical distribution patterns within the upper 40 cm encompassing the entire humus layer are presented. The specified indicators characterize arable land in crop rotation under dry and irrigated farming practices and a 50-year-old fallow at seven dynamic monitoring plots with light chestnut soils. The studied soils have two types of profile horizonation: P–BMK–BCAnc (agrochestnut soils) and Pca–BCAnc (carbonate-accumulative agrozems); they are silty loamy and develop from yellowish brown sandy silt loams. In the WRB system, they are classified as Haplic Kastanozems (Loamic, Aric). The map of the coefficients of multitemporal soil line, retrospective analysis of remote sensing data for 40 years, and the results of field soil survey have been used to select the monitoring plots. Each plot 30 × 30 m includes nine individual sampling points, at which soil sampling from thin (5 or 10 cm) layers has been performed to a depth of 40 cm. The coefficient of spatial variation in the C_org content increases from 5–10 to 30–40% with increasing depth of the layer. On the arable field, a uniform vertical distribution of C_org within the plow layer is substantiated. On the fallow land, an accumulative type of the vertical distribution of C_org with a maximum in the surface light-humus horizon with the formed sod layer is observed. The obtained estimates of the contents and stocks of C_org are comparable with the available materials on light chestnut soils of the Volga–Don interfluve. The chosen plots are representative for monitoring. For the layer of 0–30 cm, the minimum significant difference in C_org stocks is, on average, 3.4 t/ha with a variation from 2.7 to 4.6 t/ha. The C_org stocks at different dynamic monitoring plots are significantly affected by the vertical distribution of soil bulk density values, which depends on the character of anthropogenic impact on soils.