ИСТИНА

The effect of soil organic matter (SOM) on formation of inter-particle bonds was studied in soils with high organic and no mineral carbon content in the topsoils of different origin under various land uses. The first set of soil samples was collected in the Alekhin Central Chernozem Reserve (Kursk region, Russia): 1a – Chernozem under native steppe vegetation and 1b - Chernozem under 67-year-old bare fallow. The second set of samples was sourced from the Midlands of Kwa-Zulu Natal, South Africa: 2a – Umbric Ferralsol under pine plantation and 2b - cultivated Plinthic Ferralsol under long-term maize monoculture. The texture of fine (<0.25 mm) fraction (laser diffraction method, Analysette; Fritsch, Germany) of samples of 1a is silt and 1b is silty loam, 2a - silt, 2b –silty clay loam. The soil carbon content (CHN analyzer; Vario EL, Germany) in the native soil 1a – 1b – 2a – 2b was 6.70 - 3.14 - 18.73 - 3.61% respectively. Upon oxidation (50% H2O2, 20-40 days) it went down to: 0.52 - 0.08 - 2.55 – 0.42% respectively. The rheological behaviour of the capillary-wetted soil pastes was characterized by amplitude sweep test with the modular compact rheometer MCR-302 (Anton-Paar, Austria). The storage modulus in linear viscous-elasticity range (LVE-range) and crossover of storage modulus as well as the loss modulus were determined. The stability of structural bonds in the untreated sample 1a is double the structural stability of 1b. The structural stability of 2a is triple that of 2b. The sample treatment with H2O2 in all cases except 2a consistently improves the storage modulus in the LVE-range. In the absence of SOM the interaction between soil particles becomes more rigid, the soil is less plastic and rupture occurs at lower loads. The sample 2a is very high resistance to stress in native state and loses it upon the SOM removal. We link it to the change in particle size distribution upon the H2O2 treatment. In contrast to other samples, the 2a shows significant reduction in coarse silt fraction (from 44.5 to 23.2%) and an increase in clay content (from 10.8 to 36.5). The 50-10µm fraction of untreated sample consists to a large extent of clay-SOM-clay micro-aggregates resistant to 400 J ml-1 of ultrasonic dispersion energy. SOM oxidation leads to disaggregation and changes in particle size distribution. We connect the observed differences to the type of organic matter inputs. Unlike the herbaceous vegetation, pine plantations (soil 2a) lead to significant accumulation of plant litter on the soil surface. The dissolved organic matter (DOM) is leached out of the litter layer by percolating water. The sorption of DOM on the mineral surfaces contribute to particle aggregation. This work was in part sponsored by the Russian Foundation for Basic Research , grant 16-04-01111 and the Green Landscapes and Thutuka programmes of the NRF, South Africa. Keywords: soil, structure, organic matter, rheology, amplitude sweep test