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Question
Does a reduction in size class of soil biota aggravate the impacts of soil compaction on plant communities?

Location
Controlled mesocosm study with a European ruderal plant community exposed to soil compaction and soil biota communities of different size classes.

Methods
We evaluated the individual and combined effects of soil compaction and soil biota communities of different size classes (<2000, <212, <20 ?m, natural control) on the structure and productivity of a ruderal plant community, and the establishment of a neophyte (Lupinus polyphyllus) in a full-factorial mesocosm study.

Results
Soil compaction reduced the productivity of the plant community, but the negative effect decreased in magnitude as the size class of the soil biota community was reduced. Consistently, shoot growth of the dominant grasses was most reduced by soil compaction when bigger soil biota were present; the shoot growth of forbs showed an opposite pattern. In contrast, legumes seem to profit from soil compaction independent of the size class of the soil biota community, and were detrimentally affected only in the natural control soil. Establishment of the neophyte L. polyphyllus was best under natural soil conditions and was not affected by soil compaction.

Conclusions
A reduction in soil biota size due to intensified management may change the impacts of soil compaction on plant communities. The results indicate that soil compaction has the most detrimental effect on primary productivity in soil communities where a range of size classes of soil biota is present and the functional complexity of the soil biota community is therefore high.