The Effect Of Co-inoculating AM Fungi And Rhizobia On Nitrogen And Carbon Allocation In Soybean/Maize Intercropping System

Soybean/maize intercropping is one of the most common agricultural cultivation systems. The previous studies showed that intercropping soybean improved maize growth in soybean/maize intercropping systems, but the mechanism is still uncertain. Soybean plants can form symbiotic nitrogen fixation with rhizobia, simultaneously, arbuscular mycorrhizal(AM) fungi can infect roots of both soybean and maize to form symbiotic mycelium network, which have the potential to transfer and redistribute the fixed nitrogen between soybean and maize. However, how much do AM fungi and rhizobia contribute to intercropping advantage, how is nitrogen transfered and distributed in the intercropping systems, and how many photosynthetic carbon do soybean and maize separately invest to rhizobia and AM fungi? These series of questions are seldom reported. In the present study, soybean and maize in intercropping system were inoculated with AM fungi and/or rhizobia at the different nutrient treatments in two-chambered-pots separated by 37 ?m mesh to explore the contribution rate of AM fungi and rhizobia to intercropping advantage, quantify nitrogen transfer and carbon allocation by nitrogen and carbon isotope labeling in soybean/maize intercropping system. The results were shown as follows: 1. In soybean/maize intercropping experiment with different nutrient and inoculation treatments, different cultivation modes significantly influenced the growth of soybean and maize, and oybean/maize intercropping promoted maize growth. The biomass of the intercropped maize increased by 17% compared to that of monoculture maize at co-inoculation treatment and HPHN condition. Similarly, root nitrogen content had 40% increases in the intercropped maize compared to the monoculture maize at co-inoculation treatments. This suggests that promotion of maize growth in the intercropping systems is achieved by improvement of nutrient uptake.2. In soybean/maize intercropping experiment with different nutrient and inoculation treatments, co-inoculation with rhizobia and AM fungi could significantly promote the growth of soybean and maize. Under LPHN conditions, the biomass of the intercropped maize at single AMF inoculation treatment increased by 152%, and that of the monoculture maize increased by 127% compared to no inoculation treatment. Similarly, the biomass of the intercropped soybean increased by 120%, and that of monoculture soybean increased by 100% under co-inoculation and HPLN conditions. 3. In the isotope tracer experiments, different cultivation modes had significantly influenced growth and nutrient uptake of soybean and maize. Root nitrogen and phosphorus content of the intercropped maize increased by 28% and 22% compared to those of monoculture maize, but root nitrogen and phosphorus content of intercropped soybean decreased by 18% and 21% compared to monoculture soybean at co-inoculation with rhizobia and AM fungi treatment, respectively. Different inoculation treatments also significantly affected the growth of soybean and maize. Compared to no inoculation treatment, the biomass, and shoot N and P content of the intercropping maize increased by 34%, 53% and 51%, those of monoculture maize increased by 23%, 32% and 53%, those of intercropped soybean increased by 59%, 214%, 35%, and those of monoculture soybean increaseed by 75%, 208%, 25% at co-inoculation treatment, respectively. These results were consistent with those of the first experiments. 4. The results from isotope tracer methods showed that 13 C content of soybean at co-inoculation treatment was significantly higher than single inoculation with AM fungi. The 13 C contents in the shoots and roots of soybean at co-inoculation treatment increased by 298% and 203% compared with the single inoculation treatment, respectively. In the 15 N isotope labeling method, nitrogen transfer from labeled soybean to maize increased significantly in soybean/maize intercropping compared with monoculture system at co-inoculation treatment. The percentages of transferred N that occupies the plant N uptake were 87%, 142% higher in intercropped soybean and maize than those of monoculture, respectively, which was also 37% higher at co-inoculation treatment than that of single inoculation. In addition, 55-70% of soybean total nitrogen was derived from symbiotic nitrogen fixation. In summary, the intercropped maize gained more nitrogen nutrient from intercropping system. Therefore, maize benefited more from intercropping system than soybean, whereas soybean is the main contributor to intercropping system under co-inoculation condition.