Preceding crop history modulates the early growth of winter wheat by influencing root growth dynamics and rhizosphere processes

Self-succession of winter wheat (WW) in crop rotations results in substantial yield decline. This decline has been mostly attributed to the soil-borne fungus Gaeumannomyces graminis var. tritici (Ggt; take-all) causing earlier root senescence. A broad shift in the soil microbial community has also recently been proposed to confound this effect even in years without significant Ggt infestation in the field. We aimed to establish a mechanistic basis for the relationship between rotational position of WW and yield decline at an early wheat growth stage. To this end, an outdoor experiment with 1 m deep rhizotrons was set up using a sandy loam soil. WW was grown in soil after oilseed rape (KW1), soil after one season of WW (KW2) and soil after three successive seasons of WW (KW4). The plants were grown until the beginning of stem elongation (BBCH 30). At harvest, both shoot and root dry weight were markedly affected by the preceding crop, with a pronounced reduction of plant biomass of KW2 (-43%) and KW4 (-45%) compared to KW1. At BBCH 30, KW1 soil had much lower mineral N compared to KW2 (-49%) and KW4 (-39%). Non-purgeable organic C, a readily available energy source for soil microorganisms, was further reduced in successive WW rotations compared to KW1. Increased NH4+ and NPOC concentrations were found in root-affected soil compared to root-free bulk soil, indicating a strong hotspot for organic N mineralization in the rhizosphere. At the same time, the markedly higher shoot N concentration led to a lower C:N ratio of 31 for KW1 compared to KW2 and KW4, which had a C:N ratio of 46 and 44, respectively, suggesting a better exploitation of soil mineral N sources by KW1. In contrast, microbial biomass C and N were higher in KW2 and KW4 compared to KW1, pointing to enhanced microbial N immobilization in KW2 and KW4. The higher C:N ratio of WW straw compared to oilseed rape residues that are returned to the soil following harvest, obviously stimulated immobilization of soil N in microbial biomass, thereby limiting the availability of N for WW growth in KW2 and KW4. Root growth traits exhibited a strong response to WW rotational position, with higher root tissue density, root mean diameter and lower specific root length for KW1 compared to KW2 and KW4. Root length density (RLD) was overall higher in KW1 compared to KW2 (-29%) and KW4 (-31%), especially at 0-30 cm soil depth. Interestingly, higher RLD values for KW1 were also observed at the lowest depth of 60-100 cm compared to KW4, suggesting a strong effect of rotational position on nutrient accessibility in the subsoil. Successive WW invested more in acquisitive root traits that did not compensate for the reduction of biomass production. Our results highlight the effect of rotational position of WW on soil and plant properties and provide guidance for management-based adaptations at field level to improve WW productivity.