Soil characteristics and root growth in a catena across and outside the wheel tracks for different slurry application systems

Compaction of subsoils is a steady increasing challenge as induced by the increase in weight as well as power of field machinery. We performed a field experiment with two different machinery systems used in slurry application. A tractor-trailer combination (TT) was compared to a tricycle-like self-propelled machine with traction on all three wheels (SP). The maximum wheel load was 88 and 114 kN for the TT and SP treatments, respectively. The wheels on the TT machinery were equipped with 710–800 mm wide tires inflated to 300 kPa, while the width and inflation pressure of the most heavily loaded tire on the SP machine were 1050 mm and 250 kPa. The machines were driven across the soil in three replicate plots in a randomized block experiment. Traffic took place at a soil water content close to field capacity. Immediately following the experimental traffic, cores of undisturbed soil were sampled at 30, 50 and 70 cm depths in a catena across the width of the wheel tracks. The cores were drained to − 100 hPa matric potential prior to measurement of air permeability. For both machinery systems, we measured significant differences in soil properties across and outside the width of the wheel tracks. The volume of pores > 30 µm, ε100, and/or the air permeability, ka, indicated a loosening of the soil at or closely outside the edge of the tire for both the SP and the TT treatment. The bulk density, ρb, was higher and ε100 lower for TT than SP at 30 and 50 cm depth for soil under the tire, at the tire edge and also some decimetres from the tire edge. Accordingly, ka was also lowest for TT, and the permeability normalized to a unit volume of ε100 indicated that the effect on pore permeability was due to the increased ε100 rather than to differences in tortuosity. The rooting depth of a spring barley crop was lower beneath and at the edge of the wheeled tracks than outside. This may reflect the effect of experimental traffic on soil properties although it cannot be excluded that the effect on the topsoil while establishing the barley crop has played a role as well. We consider that the results reflect the effects of repeated wheeling and/or consistent differences in traction and horizontal stresses. We encourage further studies of wheeling effects including the soil close to and outside the wheel tracks.