Intensive tillage regimes may cause detrimental effects on soil pore characteristics and structural stability. Conservation tillage has been suggested as an effective measure to avoid such adverse effects as well as to increase soil organic matter in soil surface layers. However, the accumulation of soil organic matter may also promote nitrous oxide emissions and nutrient losses through strong links between the carbon and nitrogen cycles. The TRACE-Soils project aims to identify mechanisms underpinning trade-offs and synergies of soil carbon sequestration, greenhouse gas emissions and nutrient losses in agricultural soils across Europe. Soil structure is a driving factor in the transport of water and gas through soil, and influences, among others, the prevalence of oxic and anoxic soil processes. While it is common knowledge that soil structure is highly responsive to soil management practices, knowledge on the effects of different tillage strategies on soil structure in relation to trade-offs and synergies of soil carbon sequestration is still lacking. We conducted a range of laboratory measurements on intact soil cores (100-cm3) taken from seven long-term agricultural experiments (12 years or longer) with a common experimental setup located along a pedoclimatic gradient in Europe. We measured oxygen diffusivity and air permeability at -100 hPa matric potential and deducted air-filled and water-filled pore space at -100, -60 and -30 hPa matric potential as well as dry bulk density from balance-measurements. The soil samples were taken from two depths – 0–0.1 m and 0.1–0.2 m – after harvest, prior to any further field operations. We tested for differences (p <.05) in tillage regime (inversion tillage, reduced tillage, no tillage), depth and their interaction across the long-term experiments, as well as for confounded location-effects. Location and soil depth generally had the strongest effect on the assessed soil characteristics, while the effect of tillage practice and the interaction between tillage and depth often was much smaller or not significant. The effect of tillage practice was pronounced but generally interacted with depth. For instance, a higher water-filled pore space at -100 hPa was found for no tillage (63%) compared to inversion tillage (58%) at 0–0.1 m depth (across locations). The effect of depth and/or treatment could not be generalised to all individual long-term experiments assessed. We observed strong interaction effects between depth and experimental location. Moreover, we observed an interaction between tillage practice and experimental location on pore conductivity characteristics, except for oxygen diffusivity. These results show that the effects of location and sampling depth generally overshadowed the long-term effect of tillage on soil structural characteristics in the topsoil when soil was sampled after the growing season. Stronger effects of tillage may have occurred if samples had been taken closer to the time of tillage.
