Effect of cropping systems and arbuscular mycorrhizal fungi on soil microbial activity and root nodule nitrogenase

Forage legumes are used to enhancement soil fertility of the agro ecosystem. Understanding effect of them on agro ecosystem soil status during when these legumes growing and after that is essential. In one experiment the effects of inoculation with the arbuscular mycorrhizal fungi (AMF), Glomus mosseae, and mixed cropping systems (MCS) on forage biomass yield, nitrogen production, nitrogenase activity and after harvesting on soil microbial activity were studied at various mixed cropping ratios of berseem clover (Trifolium alexandrinum L., B) to Persian clover (Trifolium resupinatum L., P) (B:P = 1:0, 3:1, 1:1, and 1:3). In the second experiment, the effect of treatments on soil microbial activity were studied by soil collection after clover harvesting and 8-week soil incubations in the laboratory. MCS had positive effects on root and shoot dry weight. The effects of AMF on plant yield were positive. AMF affected the fraction root and the vertical root distribution. Plants colonized by AMF showed shorter roots than control plants. At cut 1, with the AMF colonization, the greatest nitrogenase activity (79.61 μmol C2H4 g dwt−1 h−1) of root nodule was observed with B:P = 3:1. At cut 2, the Persian clover plants colonized by G. mosseae in the mixed crop (1:3) had a higher nitrogenase activity (77.38 μmol C2H4 g dwt−1 h−1). The greatest nitrogen accumulation in the aboveground biomass, 23.5 mg g−1 forage dry matter, was obtained with mixed cropping (B:P = 1:1) in the presence of the AMF colonization. Microbial activity measured as substrate-induced respiration and activities of dehydrogenase, alkaline phosphatase, and acid phosphatase enzymes responded positively to AMF colonization; with the greatest activities for B:P = 1:3.