Organic matter and soil tilth in arable farming: Management makes a difference within 5-6 years

Management-induced depletion in soil organic carbon (SOC) may create critical tilth conditions for arable farming. We investigated the short-term effect of crop rotation and addition of animal manure on SOC fractions, the dispersibility of soil clay, the length of fungal hyphae, wet aggregate stability, tensile strength of dry aggregates, and the pore size distribution, gas diffusivity and permeability of undisturbed bulk soil. SOC fractions were measured in whole-soil samples and in 1-2 mm air-dried aggregates. Plough layer soil was sampled 5 and 6 years after the start of a field experiment with different cropping systems at two loamy sand soils (Foulum, approximately 9% clay and Flakkebjerg, approximately 14% clay). A soil drop test was performed in the field to evaluate in situ soil friability. A system dominated by small grain cereals not receiving animal manure served as a reference treatment ('CEREAL'). This system was compared to the same crop sequence but with application of animal manure ('CEREAL + MANURE', only at one location), and to a diversified crop rotation including grass/clover but without addition of animal manure ('CEREAL + GRASS'). A part of each field plot was compacted by a medium-sized tractor. The content of SOC was lowest for the CEREAL system at both locations. Hot-water extractable SOC displayed the same pattern. The carbon fractions in aggregates responded similarly to cropping systems as those in whole-soil samples. Clay dispersibility was highest in the CEREAL system. The length of fungal hyphae was enhanced by the versatile crop rotation. Soil compaction tended to increase clay dispersibility. Our results confirmed agronomic observations that the tilth in the Flakkebjerg soil was problematic and worse than in the Foulum soil, but generally only trends were found in amelioration of the poor mechanical tilth characteristics. In contrast, soil (macro)porosity was significantly higher for the CEREAL + GRASS system compared to the other two systems, and the CEREAL + MANURE and CEREAL + GRASS systems had more tortuous pore systems compared to the CEREAL system and better resisted compaction than the latter. We conclude that only 5-6 years of differentiated soil management significantly affected SOC fractions, the dispersibility of clay, and the growth of fungal hyphae. As only trends were found in the mechanical aspects of soil tilth, the tilth-forming agents may serve as early indicators of changes in soil tilth. Our results also indicate that soil pore characteristics are affected by short-term management and probably provide the basis for later significant changes also in mechanical tilth characteristics.