| Sugarcane (Saccharum officinarum L.) is a major crop for sugar and energy in many areas of the world as well as China. Therefore, sugarcane production is important for increasing farmers’ income and ensuring the sustainable development of sugar industry. However, the cost of sugarcane production is too high at present. The present study used three sugarcane cultivars, i.e. B8, ROC22 and GT21, to be intercropped with soybean. The split block design was employed using cropping pattern i.e. monoculture and intercropping for main plot and sugarcane cultivar for subplots under urea fertilization 150 kg/ha to compare the differences in cane yield and quality, economic benefit, physiological and biochemical parameters in nitrogen metabolism process, diversity of functional endogenous bacteria and nitrogen-fixing bacteria in plant sugarcane and ratoon crop with different treatments, to further explore the effect of soybean intercropping on sugarcane. The main results are as follows.1. Intercropping with soybean significantly affected on the tillering, millable stalks, stalk diameter, and yields of cane and sugar while affected stalk quality very little. Comparison to monoculture treatment, the stalk diameter, millable stalks, cane yield and sugar yield in the treatment intercropped with soybean in ratoon crop were increased by 5.1%-8.7%, 7.9%-31.0%,9.0%-40.5% and 5.6%-39.5%, respectively. Consequently, the incomes from cane and soybeans were increased by ￥59 100-79 300 Yuan/ha and ￥58 300-77 200 Yuan/ha, respectively. The treatment intercropping with soybean improved the cane yield and economic benefit by a wide margin in cultivars GT21 and ROC22, and improved more in GT21 than those in ROC22, while B8 showed high and stable yield in different cropping patterns.2. The contents of chlorophyll, soluble protein, nitrate nitrogen, ammonium nitrogen, and the activities of nitrate reductase and glutamine synthetase showed substantially different in different sugarcane varieties and crops, and they were higher in plant crop than in ratoon crop. The effect of intercropping with soybean on chlorophyll content in sugarcane leaves was not significant, but the chlorophyll a/b ratio was improved in the symbiosis duration of sugarcane and soybean. Affected by the intercropped soybean, the soluble protein content in sugarcane leaves was lower at earlier growth stage but increased later, while nitrate and ammonium nitrogen contents showed increasing first and then decrease, but the effects intercropped soybean on the soluble protein and nitrate contents in sugarcane leaves were not significantly different for whole growth duration of sugarcane. The nitrate reductase activity in leaves of sugarcane intercropped with soybean showed decrease first and then increased slightly, but that in ROC22 showed no obvious change. Intercropping soybean treatment tended to improve the glutamine synthetase activity in sugarcane leaves, especially in the symbiotic duration and ratoon crop of sugarcane.3. The effects of intercropping soybean on the contents of nitrogen, phosphorus and potassium in leaves and stalks of sugarcane were different depending on growth stages, varieties and organs of sugarcane. The intercropping soybean treatment most obviously improved the potassium nutrition in sugarcane, but showed decreased nitrogen and phosphorus contents in leaves and stalks of sugarcane during intercropping period (seedling and tillering stages), and recovered and improved after harvest of the intercropped soybean (elongating stage).4. Spread plate colony counting method was used to estimate the numbers of viable bacteria and rhizobia in roots and stalks of sugarcane, and the most probable number counting method was used to estimate the numbers of viable nitrogen-fixing bacteria in roots and stalks of sugarcane. The results showed that the endogenous bacteria, rhizobia and nitrogen-fixing bacteria levels in roots and stalks of sugarcane varied in different growth stages, planting pattens, crops and varieties of sugarcane. During intercropping period, intercropping with soybean improved the bacteria and rhizobia quantities in sugarcane roots, especially at the early stage (seedling stage of sugarcane), and the endogenous nitrogen-fixing bacteria changed very little compared with monoculture. Intercropping with soybean also improved the quantities of endogenous becteria, rhizobia and nitrogen-fixing bacteria in stalks in different degrees, and the increases of endophytic bacteria and rhizobia were statistically significant in the stalks of ROC22, and the increase of nitrogen-fixing bacteria was the highest in the stalks of ROC22.5. PCR-DGGE technologies were used to analyze the community structure and diversity of endogenous bacteria and nitrogen-fixing bacteria in roots and stalks of sugarcane. The results showed that the community structure of endogenous bacteria and nitrogen-fixing bacteria in roots of sugarcane variated more in plant crop than in ratoon crop, and their diversities were larger at tillering and elongating stages than at seedling stage in plant crop while changed very little in different growth stages of ratoon crop. The diversities of ndogenous bacteria and nitrogen-fixing bacteria were larger in the treatment of intercropping with soybean than in the treatment of monoculture. The similarity of endogenous bacteria and nitrogen-fixing bacteria was low between the root of soybean and the roots and stalks of sugarcane. For the treatment of intercropping with soybean, the community structure of endogenous bacteria and nitrogen-fixing bacteria in roots of sugarcane vared depending on different sugarcane varieties, crops and and organs. The variation of the community structure of endogenous bacteria was smallest in the roots of B8 and the stem of ROC22 at seedling stage of plant crop, and that of nitrogen-fixing bacteria was smallest in roots of B8 in plant crop and stalk of B8 in ratoon crop. The diversity indexes of endogenous bacteria and nitrogen-fixing bacteria in roots of sugarcane were different depending on sugarcane varieties and crops, but not significantly different for those in stalks between different treatments. However, for the relative difference between treatments in plant and ratoon crops, the treatment of intercropping with soybean increased the diversity indexes of endogenous bacteria and nitrogen-fixing bacteria most in ROC22.In conclusion, intercropping with soybean improved the functional diversity of microbes, nutrient accumulation, and yields of cane and sugar in sugarcane, which will be helpful in reducing nitrogen fertilization and improving the economic benift in sugarcane production. |
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