| Agroforestry has a long history in the development of human society. In recent years, owing to the limited resources and the deteriorating ecological environment, agroforestry which aim to obtain synthetic benefit of ecology, economy and society is being taken seriously by various countries around the world. Meanwhile, extensive research and promotion has been carried out. Allelopathy is one of the limiting factors in the development of agroforestry. Litter decomposition is an important pathyway of materials circulation and energy transformation in agroforestry system; meanwhile, it is a principal pathway for allelochemicals releasing into soil and making effect on neighbors and itself. How to reduce the negative effects of allelopathy and maintain the circulation of materials is a key point to improve the benefit of agroforestry.Cinnamomum septentrionale is a common ornamental tree with highly economic and ecological value. In South China, especially in plain and foothill region of Sichuan Province, C. septentrionale has always been included in some traditional agroforestry system such as garden-style agroforestry and planting tree beside the farmland. With the implementation of the Changjiang river headwater’s ecology barrier construction the restoration and reconstruction of degraded ecosystem, C. septentrionale has became an optimized tree species in artificial agroforestry. However, previous studies have shown that this species also has a strong allelopathic potential. C. septentrionale leaf litters always fall off in spring, and may harm the growth of spring crops. Therefore, C. septentrionale leaf litter was chosen as donor material and two nitrogen-fixing plants (soybean, kidney bean), several non-nitrogen-fixing plants (corn, pepper et al.) were chosen as taget plants. The response of target plants in growth, physiology and reproduction and the relieving effects of fertilizing were studied systematacially, and the releasing of allelochemicals and nutrients and the related ecological process were analyzed as well. The results of this study were expected to scientifically guide the practice of C. septentrionale and crops agroforestry, and lay the foundation for the research of allelopathic mechanism. The results were listed as follows:(1) Owing to the decomposiiton of C. septentrionale leaf litter, the growth of soybean, kidney bean, corn and pepper were inhibited in varying degrees, as shown in morphological indexes such as plant height, base diameter, fresh weight, leaf area and so on. This inhibition increased with the increase amount of leaf litter addition, but decreased as time went by. In the four crops, the fixednitrogen ones own advantages when faced allelopathic stress caused by leaf litter. Soybean (RI=-0.186) showed better tolerance than kidney bean (RI=-0.478), while corn and pepper (RI=-0.186) were sensitive to the leaf litter. However, the addition of leaf litter significantly increased the nodulation amount and nodulation weight of soybean and kidney bean seedling. Even when treated by the lowest leaf litter treatments the RI for nodulation amount were 0.368 and 0.614, the RI for nodulation weight were 0.225 and 0.878. The flowering phenology of kidney bean was put off a few days and its total flower amount decreased to 70%-80% of the control. When treated by the highest amount of leaf litter, kidney bean increased its investment to corolla and stamens to maintain its reproduction; however, limited photosynthetic product led to less (52% of the control) and smaller seeds (76% of the control), and finally led to the decrease investment to reproduction (77%). The highert leaf litter treatment decreased the yield of soybean, kidney bean and corn to 54%,43% and 65% of the controls respectively. The lowest leaf litter treatment had negligible effect on the yields of kidney bean and corn, but decreased 32% yield of soybean. The crude fat content of soybean was increased by leaf litter treatments.(2) Disturbance of three strongly interrelated physiological processes, photosynthetic physiology, resistance physiology and nutrient physiology, are the physiological mechanisms which lead to the growth inhibition of the target plants. The addition of leaf litter decreased photosynthetic pigments contents in the leaves of the four crops, and it dcreased the net photosynthetic rates of them as well. Meanwhile, in soybean, kidney bean and pepper, leaf litter also disturbed their utilization efficiency and ecology adaptation of light and CO2. The addition of leaf litter significantly increase H2O2 content in seedling stage of soybean and kidney bean,103% higher than the control, and disturbed activities of three antioxidant enzyme (POD, SOD, CAT) at different stages. Moreover, it increseased soluble sugar content and decreased soluble protein content of the two crops to some extent.The addition of leaf litter could also decrease N, Mg, Ca contents and increase K content in the leaves of soybean and kidney bean. Especially at the seedling stage, it can inhibit 59.90% of N content and 28.46% of Mg content. But it increased the content of K.(3) Fertilizing is an important measure for relieving allelopathic effects of C septentrionale leaf litter. Using the common fertilizing amount in agricultural production could significantly relieve the inhibition of leaf litter on the growth and photosynthesis of corn and pepper, but could not completely eliminate the inhibiton. The most serious inhibition was observed in the highest leaf litter treatments, while the most significant promotion was abserved in these treatments after fertilizing. The RI of corn leaf area increased from -0.831 to -0.467, and the RI of pepper increased from -0.825 to -0.516 (base fertilizing) and -0.343 (topdressing). After topdressing, the growth of pepper quikly recoverred from the standstill, and its growth caught up with the level of base fertilizing treatment. The effect of topdressing is a little superior to base fertilizing.(4) C. septentrionale leaf litter contains abundant active substances, including about 0.75% volatile oil, about 20 mg/g alcohol-soluble phenols and about 50 mg/g flavonoids. The contents of these materials decreased during decomposition. The allelopathic potential of leaf litter decreased as its inhibition of seed germination and seedling growth of Brassica chinensis decreased. During decomposition of leaf litter, terpenoids decomposed easier than steroids and other materials with bigger molecule. Phenols and flavonoids decreased fastest in 10 days of decompoaition. In 100 days of decomposition, the releasing of elements N、K、P was not obvious. The essential oil (terpenoids and derivatives) of C. septentrionale leaf litter can effect the growth and physiological property of the target plant Impatiens balsamina when dissolved in water, but the disturbance is not very serious. In practice, terpenoids may act by cooperation with other materials or long-term effect.(5) The allelopathic effect of leaf litter during decomposition is not a easily releasing-action process, it is a process in which soil, leaf litter and target plant interact with each other complicatedly. In 100 days of mixing with leaf litter, siol nitrate nitrogen concentration decreased significantly (to 61.23% of the control at 100 d), available phosphorus content fluctuated in this process, and ammonium nitrogen content and available potassium content varied non-significantly. The amounts of culturable bacteria, fungi and actinomycetes increased at first and decreased afterwards. In addition, the target plants can promote the releasing of potential allelochemials in leaf litter. Sixty days after sowing (or decomposition), the decrease of volatile (particularly the materials with low boiling point) in leaf litter was more obvious in the treatments planted in soybean, kidney bean and peanut than the treatment with no target plants.In conclusion, volatile materials, phenols and flavonoids are the materials basis of allelopathy of C. septentrionale leaf litter, and they released into the soil gradually.In this process, soil nutrient, soil microorganism and crops may disturb the effect of its allelopathic effect. Finally, by disturb the nutrient absorbtion, photosynthetic physiology and resistance physiology, the growth of target plants were suppressed significantly. In the foure crops, the fixednitrogen ones showed better tolerance than others, but the symbiotic relationship between legumes and rhizobium was disturbed by leaf litter at the same time. Corn and pepper are more sesentive to C. septentrionale leaf litter, but the inhibiton can be relieved by fertilizing. |
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