Changes in the organic matter forms in chernozems of the Kamennaya Steppe under different land uses, locations, and hydromorphism degrees

The soils of the Kamennaya Steppe (Voronezh oblast) were studied. The rate of changes in the contents of C org and the particular forms of organic matter (labile, microbial, and stable) were revealed in the quasi-natural soils of the fallows and shelterbelt and in the arable soils (rainfed farming for 12, 55, 85, and 115 yrs and irrigated farming for 40 yrs) of different positions on the watersheds and slopes. The effect of the increased soil moistening in the recent decades was also studied. In the upper 50 cm of the fallow soils that were not plowed since 1882, the relative C org accumulation in the recent 30 yrs has amounted to 5%. The soils of the shelterbelt planted in 1903 were similar to the fallow soils. As compared to the soil of the unmown fallow, the C org loss from the 1-m soil layer under the shelterbelt and the 12-year-old cropland were less than 9%; the losses from the plowed soils (used for 55–115 yrs) were 21–27% on the watersheds and 37–46% on the slopes. In the first decade, the rate of the Corg losses in the 0- to 20-cm layer of the cultivated chernozem was 120 g C/m 2 . With the increasing duration of the soil plowing (from 55 to 115 yrs), the C org losses decreased from 45 to 28 g C/m 2 per yr in the watershed soils and from 51 to 35 g C/m 2 per yr in the soils on the slopes. The maximum loss of C org was found for the soils on slopes, waterlogged soils, and irrigated soils. In the slope soils, the C org loss due to erosion was 9–18% of the total. In the upper horizons of the old agrogenic soils, compared to the soil of the unmown fallow, the C ha /C fa increased, since the content of fulvic acids (FA) faster decreased than that of the humic acids (HA); the C content of the nonhydrolyzable residue was reduced. The slope and waterlogged soils differed from the watershed soils in the smaller amounts of HA and FA and in the greater content of humin carbon. In the 0- to 20-cm layer of the soils studied, the rate of the basal respiration (BR) was 0.2–0.5 μg C/g soil per h, the content of the microbial biomass (C micr ) was 326–1073 μg C/g, and the share of C micr amounted to 1.0–1.9%. These values were minimal in the irrigated soil and maximal in the fallow ones. A high correlation coefficient ( r = 0.88–0.92) was found between the C micr content and the BR, between the contents of C org and HA, and between the contents of C org and mobile C. The correlation coefficient between the contents of C org and FA and C org and humin C was 0.67.