Long-term soil quality and C stock effects of tillage and cover cropping in a conservation agriculture system

Conservation and regenerative agriculture are widely considered as approaches aiming at addressing challenges in relation to climate change and soil quality. In that respect, no tillage (NT) and use of cover crops (CCs) are seen as key elements in achieving the sustainability goals of these approaches, but the long-term individual and interactive effects of these elements remain unknown. We examined the impact of tillage (NT and mouldboard ploughing) combined with a fodder radish CC in continuous cereal cropping. Soil was sampled in the 0–10 and 10–20 cm layer after two decades of treatments in the CENTS experiment at AU Viborg (Denmark) situated on a sandy loam with 9% clay. We assessed soil structural quality, SOC stocks, clay dispersibility (CD), wet stability of aggregates (WSA) and soil pore characteristics. Neither tillage nor cover cropping affected the SOC stock in the 0–20 cm soil layer. No tillage improved CD, WSA and plant available water capacity in 0–10 cm depth as compared to ploughing. The marked improvement in CD and WSA of NT soils could not be explained by SOC per se, but rather positive effects ascribed to absence of disturbance. In contrast, soil porosity, especially in 10–20 cm depth, the fraction of soil volume represented by >30 µm pores and gas diffusivity decreased, and NT soils resulted in a less good soil structural quality. The inclusion of CCs improved soil structural quality and the functionality of the soil macropore system. Hence, CCs have the potential to alleviate negative effects of NT on pore characteristics at macroscale. Furthermore, we found that the positive effects of NT on CD and WSA and of CCs on pore characteristics at macroscale were much more pronounced after long- (20 yrs NT; 13 yrs CC) than after medium-term (10 yrs NT; 5 yrs CC) underlining the value of long-term conservation agriculture experiments.