Soil functions and in situ stress distribution in subtropical soils as affected by land use, vehicle type, tire inflation pressure and plant residue removal

Soil deformation due to compaction and shearing with heavy machinery is still one of the largest threats to soil functionality and thus to crop performance on arable soil. By determining the in-situ stress input and propagation due to tractor and harvester wheeling on two south Brazilian soils, a Typic Hapludox and a Typic Hapludalf, we aimed to investigate the consequences of different wheeling conditions on soil composition and functions. Stress state transducer (SST) sensors were installed in three to four soil depths to measure stress impact and propagation and to quantify the effect of different contact pressures by different agricultural machines, tire inflation pressures, and absence or presence of plant residues on soil under no-tillage (NT, each Hapludox and Hapludalf) and natural grassland (NG, only Hapludalf). Field measurements of principal stress σ₁ were amended by laboratory analyses to quantify changes of soil precompression stress σₚ, air permeability, and water retention before and after the passage of a harvester.Wheeling with heavy load in the Hapludox under NT even ruptured the “no-till pan”, resulting in lower σₚ, while the Hapludalf under NT was compressed more and became more stable. The biologically stabilized Hapludalf under NG suffered strongly from wheeling by reduced σₚ, water retention, and air permeability. Reasons for this behavior are clayey (Hapludox) vs. sandy loam soil texture (Hapludalf) as well as higher bulk density in the Hapludalfs than in the Hapludox. By comparing the evaluated factors, the most pronounced impact was found for tire inflation pressure. Lower pressure strongly diminished contact pressure and σ₁ in the soil. Straw had a similar, but less striking effect. The harvester, being heavier than the tractor, caused higher stress input and altered soil physical properties profoundly in all sites. Subsequent passes lead to further compaction, though of decreasing intensity with each additional pass. The effect was the stronger, the closer to the surface, but the surface layer itself showed a quite irregular result due to lug/interlug interactions.A low contact pressure, e.g. by low tire inflation pressure, is an efficient measure to avoid harmful soil compaction. Keeping plant residues also helps in preserving soil structure, but is less efficient in the short-term.