Turnover of organic matter in differently textured soils. I. Physical characteristics of structurally disturbed and intact soils

Summary

Soil type effects on organic matter turnover are most often ascribed directly to differences in soil clay content. Since soil texture determines the physical characteristics of soil, aggregation and water holding capacity may be more relevant to address in the search for controls of organic matter turnover. Most studies of microbial processes in soils are based on structurally disturbed soil, where the abiotic conditions for the microbial activity may be quite different from those in intact soils. In this study, basic physical characteristics were determined for structurally disturbed and intact soil samples from differently textured soils. Bulk soil was retrieved from 0–20 cm depth at six locations along a textural gradient in an arable field on Weichselian morainic deposits in Denmark. The samples (NA1 to NA6) ranged in clay from 11 to 45% and in silt from 7 to 15%. Clay and silt-sized organomineral complexes were isolated from NA2 soil by ultrasonic dispersion and sedimentation in water. The clay and silt fractions were added individually and in varying proportions to NA1 soil, providing three clay-amended (CL2, CL4 and CL6) and three silt-amended (SI2, SI4 and SI6) soils. All 12 soils were crushed in air dry state to <2 mm, mixed, re-moistened and exposed for 17 months to freeze/thaw and dry/wet cycles as well as tillage to induce regeneration of soil structure. Intact soil cores were then equilibrated at four water matric potentials on ceramic plates (-30, -100, -500 and -1500 hPa) and analyzed for volumetric water and air content, and air diffusivity and permeability. Cores of undisturbed, but recently tilled topsoil from each sampling location in the field (RE1 to RE6), were included as reference samples for the experimentally manipulated (disturbed) soils. The CEC of the soils was closely related to clay content. For the clay-amended soils, CEC also correlated to organic matter content. Cores of disturbed and undisturbed soils with <20% clay were similar in bulk density. At higher clay contents, disturbed soils were less dense than undisturbed ones. All SI soils and the NA1 soil showed similar pore space distribution, while clay-amended (CL) soils resembled their corresponding NA soils. In contrast to undisturbed RE soils, the disturbed NA, CL and SI soils had a much greater volume of large pores (>100 m). Air diffusivity and permeability measurements showed disturbed soils to have a less continuous and more tortuous pore system than undisturbed reference samples. Water-filled pore space at a critical level of air diffusion potential was significantly higher for undisturbed than for disturbed samples, especially in soils high in clay. Drop cone measurements showed disturbed soils to be structurally weaker than undisturbed ones. Intact and structurally disturbed soils were found to differ significantly in physical properties even after 17 months of soil structure regeneration. Water-filled pore space seems to reflect the potential of available water and aeration status to regulate aerobic microbial activity of structurally disturbed soil, but not of intact field soil.