Preceding crop and seasonal effects influence fungal, bacterial and nematode diversity in wheat and oilseed rape rhizosphere and soil

Crop rotation can have major influences on yield, which may be the result of changes in the composition of the rhizosphere microbiome. In particular there is evidence that yields of both oilseed rape and wheat can be influenced by the frequency in which they are grown in rotation with each other. In the current study we investigated the effect of preceding crops (either wheat or oilseed rape) on wheat and oilseed rape yield, with associated changes in the rhizosphere and bulk soil communities of fungi, bacteria and nematodes using terminal restriction fragment length polymorphism (TRFLP) of rRNA genes. Yield of wheat and oilseed rape were reduced by 11 and 10% respectively when grown two years consecutively. Rhizosphere populations were significantly different to bulk soil populations for all groups of organisms. Seasonal shifts in the communities were observed in the rhizosphere for all groups. Communities of fungi, bacteria and nematodes were all significantly influenced by the preceding crop in the wheat rhizosphere, while just the nematode population was affected by preceding crop in the oilseed rape rhizosphere. In particular when two consecutive crops of oilseed rape were grown, relative abundance of members of nematodes within the genus Eumonhystera increased markedly. The fungal foliar pathogen Mycosphaerella graminicola, the teleomorph of Zymoseptoria tritici which causes septoria leaf blotch in wheat, was identified in the rhizosphere of wheat and was significantly more abundant in wheat grown after oilseed rape. We conclude that overall, preceding crop had less impact on community composition than season or crop type, but that specific changes in communities at particular plant growth stages may have substantive impacts on crop growth.