Bacillus seed coating mitigates early growth reduction in successive winter wheat without altering rhizosphere bacterial and archaeal communities

The soil legacy of successively grown winter wheat (WW) often leads to lower plant growth and yield. In this study, we assessed the effect of Bacillus pumilus seed inoculation on the early growth of successively grown WW. We conducted an outdoor experiment using newly designed temperature-regulated rhizotrons. WW was grown in soil from two rotational positions, i.e., first WW after oilseed rape (W1) and second WW after oilseed rape (W2), until the end of tillering. We measured several plant and soil biochemical parameters. In addition, amplicons of the 16S rRNA gene were sequenced to account for bacterial and archaeal community shifts in the rhizosphere, and functional genes involved in the nitrogen cycle were quantified to estimate possible changes in N cycling due to B. pumilus inoculation. B. pumilus seed coating significantly compensated for the early growth reduction of W2, and this effect was primarily linked to changes in root plasticity with a higher root length density and a smaller specific root length in inoculated W2 compared with non-inoculated W2. There was a higher LAP activity in the rhizosphere of inoculated W2 plants than in the rhizosphere of non-inoculated W2 plants and this was followed by a reduction in soil NO₃^-, most probably due to an enhanced plant N uptake capacity. This was also shown in the increased potassium content of the inoculated W2 plants compared with their non-inoculated counterparts. B. pumilus seed coating did not influence the bacterial and archaeal alpha and beta diversity, but differential abundance analysis identified differences in the relative abundance of certain taxa between non-inoculated and inoculated W2. While B. pumilus seed coating significantly improved root growth and nutrient uptake in W2, this was not accompanied by a higher absolute abundance of bacterial or archaeal genes involved in N-cycling. Our study suggests that certain plant-beneficial microbes can reverse the negative plant-soil feedback in successive WW rotations and provides strong evidence of B. pumilus seed coating to promote WW productivity under such rotations.