Cumulative effect of annually repeated passes of heavy agricultural machinery on soil structural properties and sugar beet yield under two tillage systems

The aim of this study was to determine potential cumulative effects of repeated passes with current heavy agricultural machinery on topsoil (0-0.3m) and subsoil (below 0.3m) physical properties of a Luvisol as affected by long-term tillage (annual mouldboard ploughing to 0.3m depth (MP), shallow-mixing conservation tillage to 0.1m depth (SM) with a wing-bladed rigid tine cultivator). Moreover, sugar beet yield was determined. Wheeling was conducted with a six-row self-propelled sugar beet harvester representing contemporary heavy agricultural machinery (wheel load 7.8-11.7Mg, average ground contact pressure 100-145kPa). Wheeling was applied once per year over three consecutive years after harvest of sugar beet, cereal and cereal, and moreover, independent from regular plot management with light experimental machinery. Soil moisture at wheeling (0-0.6m depth) was around 100% field capacity in most years, which was secured by irrigation before wheeling if necessary. Repeated wheeling negatively affected penetration resistance, macropore volume (equivalent diameter >50μm) and air permeability of topsoil (0.05-0.1m, 0.18-0.23m) and subsoil (0.4-0.45m) layers, while biopore number and surface water infiltration remained unaffected. SM compared to MP tillage increased penetration resistance while decreasing macropore volume and air permeability in the 0.18-0.23m layer, whereas reverse effects occurred in 0.4-0.45m depth. Sugar beet yield was decreased by wheeling and SM tillage compared to the control treatments. No significant interactions between wheeling and tillage occurred in any parameter investigated. Conclusively, SM tillage did not provide better subsoil resistance against compaction compared to MP treatment under wheeling and soil conditions prevalent in our experiment. Repeated wheeling with heavy agricultural harvest machinery is obviously at risk to exceed the bearing capacity of susceptible soils. Although (i) under regular harvest conditions just small parts of arable fields (except headlands) are wheeled with high loads, (ii) harvest is by far not every year conducted under high soil moisture, and (iii) effects in the subsoil were small, such risks have to be taken into account. Reduction of tillage depth to <0.1m is not recommended for high yielding sugar beet crops grown on loessial soils.