From Soil Legacy to Wheat Yield Decline

Winter wheat is one of the most important crops cultivated globally. The yield of winter wheat is stagnating in several areas of the world since the 1990s. Continuous winter wheat cultivation is associated with a marked yield decline that is often attributed to the proliferation of the soil-borne pathogen <em>Gaeumannomyces tritici</em> (take-all). However, it is unlikely that this yield decline is solely due to take-all, but rather it is the outcome of more complex soil-microbe-plant interactions that drive plant-soil feedbacks in the rhizosphere of winter wheat. The soil legacy of the preceding crop is a major determinant of the growth and development of the succeeding plant, and it can be expected that the rotational position of winter wheat will influence the productivity of the following winter wheat. The aim of this thesis was to analyze plant-soil feedbacks in successive winter wheat rotations and better understand the observed yield decline. We specifically aimed at investigating how the rotational position of winter wheat affects the microbial communities and the associated nutrient cycling and enzymatic activity in the soil, and how the root system of winter wheat responds to these changes, thereby affecting plant growth and productivity. Finally, we looked into the potential of compost and plant growth-promoting rhizobacteria to compensate for the growth reduction in continuous winter wheat rotations and provide farmers with a sustainable toolbox to safeguard plant productivity and food production. We developed and used novel mesocosms for growing winter wheat and employing isotopic tracers to enable the quantification of important rhizosphere processes and assess above- and belowground carbon allocation, nitrogen uptake and water uptake from various soil layers. We found that there was much higher initial nitrate availability in the soil of winter wheat after oilseed rape at the germination and tillering growth stage compared to continuous winter wheat, especially in the subsoil. This was associated with nitrogen immobilization by the microbial community, which was associated with distinct root plastic responses and reduced winter wheat growth early in the growing season. Soil legacy of the preceding crop also had a strong influence on soil enzymatic activity and nitrogen cycling as indicated by changes in the activity of nitrogen-related enzymes and the abundance of microbial nitrogen-cycling genes. We also found a higher and sustained belowground allocation of freshly assimilated carbon at the flowering and grain ripening growth stages of winter wheat growing after oilseed rape that was available for microbial use, revealing a higher rhizodeposition in non-successive winter wheat rotations. Non-successive winter wheat converted more of the freshly assimilated carbon into biomass and achieved higher yields than continuous winter wheat. Winter wheat after oilseed rape exhibited distinct patterns in shaping its microbial community, with a higher abundance of taxa involved in important nutrient mineralization processes and capable of conferring plant protection against important soil pathogens. Application of both compost and plant growth-promoting rhizobacteria caused a positive plant-soil feedback in continuous winter wheat cultivation, compensating the growth reduction and yield decline. This work thus demonstrated that yield decline in successive winter wheat rotations is dependent on several key rhizosphere processes and that soil legacy of the preceding crop is an important driver of the productivity of the succeeding plant. By understanding this phenomenon, we can design resilient, productive and multifunctional farming systems that can cope with the increasing adversities of climate change without compromising yield production and food security.