AM -arbuscular mycorrhiza; Chl -chlorophyll; F% -mycorrhiza fungi colonisation of the root system; OS1 -control treatment; OS2 -alfalfa (Medicago sativa L.) mulch treatment; OS3 -rye (Secale cereale L.) straw mulch treatment; OS4 -black foil covered treatment; POD -peroxidase; PS II -photosystem II; ROS -reactive oxygen species.