Effect of mechanical stress on structure and productivity of a loess-derived Luvisol with conventional and conservation tillage

In Germany farmers are committed to caring for the land by a soil protectio n law. Yet vehicles with ever increasing axle load endanger productivity an d environmental quality of arable soils. In spring of 1995 a field experime nt was startet on a wet silty Luvisol to test the effect of single mechanic al loading on soil and crop characteristics, when managed by mouldboard plo ughing (PL) or conservation tillage (CT). CT soils are considered to be mor e resistant against compactive stresses and to recover from degeneration mo re rapidly than PL soils. Beside an unwheeled control the loading treatment s were light (2 X 2.5 t; number of wheel passes times wheel load); medium ( 2 X 5 t) and high (6 X 5 t). In 1995 even light loading of the PL soil caused a significant yield declin e by 50% in spring barley, but this happened on CT soil only with high load ing. In subsequent years with winter wheat and winter barley yield decline was less distinct. Loading of PL soil reduced total root length (from 4 to 1 km m(-2)) and rooting depth (from 70-90 to 40-70 cm), but on CT soil only root length was diminished by high loading. A tillage-traffic pan (30-35 c m) hindered subsoil rooting in FL, which was favored in CT by earthworm cha nnels. High loading caused compaction to at least 50 cm depth. Within the p an of the PL soil, penetration resistance attained 5 MPa and bulk density 1 .65 g cm(-3). In the CT soil the zone of maximum compaction was closer to t he surface (15-25 cm), In PL soil the saturated hydraulic conductivity and the O-2-diffusion coefficient gradually decreased with loading, but in CT s oil only with heavy loading. The compacted top soil was broken in subsequen t years by ploughing (PL: 25 cm) or rotary implements (CT: 5-8 cm), With FL , structure in the pan layer and subsoil did not recover, and rooting depth was still limited. Some restoration, however, was indicated with CT. Here transmitting properties increased in time, which was attributed to the reco nstruction of root and earthworm channels, as demonstrated by computer tomo graphy. We conclude that in silty soils compacted layers below ploughing depth will hardly be regenerated by internal processes. CT soils are less susceptible to loading, but high stresses are harmful too. Therefore recommending CT a s a measure for protecting soil from compaction would not be enough, consid ering the present development towards heavy field machinery.