Influence of arbuscular mycorrhizal inoculation on maize straw degradation and soil microbial biomass carbon and nitrogen, and enzyme activity in di-chamber split-root device

A large number of plant residues enters into soil on a daily basis in nature. The decomposition of these residues forms the foundation of nutrient cycles, especially carbon/nitrogen cycle. Several physical, chemical and biological factors (and interactions) contribute to the degradation processes. Among biological factors, micro-organisms which act as consumers and decomposers directly accelerate degradation processes or indirectly use acids and enzymes to do so. As micro-organisms, mycorrhizas have been recognized with special importance due to their especial microhabitat requirements. Arbuscular mycorrhizal (AM) fungi can form mutualistic symbiosis with more than 80% of higher plant species. The contribution of AM to plant residue degradation processes have varied at different hierarchical levels (plant root, mycorrhizas and soil mycelium), of course with mentioning accompanying bacteria. Most experiments have been carried out in pot or single compartment device conditions, which have made it difficult to clarify different effects of mycorhiza symbiosis on plant residue degradation. In this study, a di-compartment split-root device was used to quantitatively compare the changes in degradation processes in mycorrhizosphere and rhizosphere conditions. In the experiment, maize straw was used as representative plant residue and two different AM fungi (Glomus mosseae and Glomus etunicatum) inoculated. Samples were respectively harvested in 20 d, 30 d, 40 d, 50 d and 60 d after inoculation, analyzed for soil enzymatic activity, soil microbial biomass carbon and nitrogen, soil respiration and qCO₂ and the mechanism of how mycorrhizal inoculation accelerated maize straw degradation discussed. The results showed that by inoculation with two different AM fungi, maize straw degradation mass and coefficient in mycorrhizosphere compartment were higher than in root compartment. Mycorrhizal inoculation enhanced carbon degradation but limited nitrogen degradation. It also decreased carbon/nitrogen ratio which facilitated further degradation. For soil biological performance, it was noted that catalase, protease, acid phosphatase, microbial biomass carbon, nitrogen and soil respiration were more enhanced in mycorrhizosphere compartment than in root compartment. This initiated the next step of forming more active microbial community. The increased indices involved in the degradation process were the main reasons behind mycorrhizal acceleration of degradation. Different ability to accelerate maize straw degradation lied in function diversity of AM fungi. More AM fungal species and soil types were suggested for consideration in future studies.