Research On The Allelopathic Effect Of Decomposing Camphor Tree Leaf Litter

Compound planting patterns with various species and multilayered structure, in which the matter and energy are adequately and circularly used, are able to alleviate the increasing pressure we face in environmental pollution and land resource shortage, and thus promote ecological and economical benefit simultaneously. However, whether the co-existing species share the habitat harmoniously in the compound system determines the benefit we can obtain. Allelopathy, a phenomenon of chemical ecology that one species (commomly plants and microbes) affects another ones nearby by releasing specific secondary metabolites, plays an important role in that relation. Camphor tree (Cinnamomum camphora) is a precious native species in Southern China, and has been widely spread to Japan, India and Malaysia. It is commonly planted in countrysides as scattered tree, in streets and gardens as landscape tree, and also planted as timber tree. Camphor tree contains various terpenoids (the second largest alleochemical family) in almost all organs, and the extracts and decomposing products of these organs have been found phototoxic to plants, microbes, insects and even animals. Residue decomposition is one of the main pathways for a plant to generate allelopathy, via this pathway camphor tree is likely to affect interplanting pasture, crops and ornamental plants because its leaf litter amount is large and concentrate. Therefore, research on allelopathic effect of camphor tree will provide theoretical and technological guides to the management of Cinnamomum plantations and agroforestry systems containing Cinnamomum species and crops or ornamental plants.A pot experiment was conducted to investigate the effects of decomposing leaf litter (an important pathway that generates allelopathy) of camphor tree on vegetative growth, reproductive growth, photo synthetic traits, resistance physiology and nutritional status in the selected receiver plants, hot pepper (Capsicum annuum), balsamine (Impatiens balsamina) and morning glory (Pharbitis nil), as well as on soil biochemical properties, e.g., the contents of nutrients, amounts of microbes, activities of enzymes in soil. Also, potential allelochemicals in the leaf litter were identified and verified based on gas chromatography- mass spectrometry (GC-MS) and bioassays. Finally, the alleviation effect and alleviation mechanism of nitrogen application on the effects of decomposing camphor tree leaf litter were studied. Main results were shown as follows,1. Fierce suppressive effects of decomposing camphor tree leaf litter were observed on leaf expansion, root merisis, height, basal diameter and biomass accumulation of hot pepper, balsamine and morning glory, especially when the decomposition duration reached 60 to 70 days, despite that the leaf litter application rate ranged from 25 to 100 g per 10 kg soil. For each receptor budding and flowering was retarded, and the amount of buds and flowers were reduced obviously by exposure to the leaf litter. Specially, for hot pepper the yield reduced or even no fruits emerged in specific leaf litter treatment, and for morning glory the corolla diameter at the 11th to 15th node decreased significantly treated with 50 g leaf litter. It was confirmed that changes of soil physical structure and permeability or/and nitrogen competition caused by high C/N ratio in the leaf litter could not explain the effects stated previously well, while allelochemicals released from the leaf litter during the decomposition process are more likely to be key factors, though maybe indirect factors.2. The decomposing leaf litter of camphor tree elevated the stomatal conductance (Gs), intercellular CO2 concentration (Cj) and transpiration rate (Tr) in the three receiver plants, during a specific time interval at least. These results led to a significant decrease of water use efficiency (Wue) and CO2 use ability. The receiver plants may also suffer pressure caused by photoinhibition because of variations in the ratio of chlorophyll a to chlorophyll b (Chla/b), ratio of chlorophylls to carotenoids (Chi/Car) and the passive response of the cotyledon of hot pepper to illumination. Response of photosynthesis of balsamine to CO2 was also studied. As a result, the carboxylation efficiency (CE) and photorespiration rate (Rp) were decreased, while the CO2 compensation point (CCP) and CO2 saturation point (Csp) were both increased by the decomposing leaf litter. However, none of the three receiver plants was significantly impacted by the decomposing leaf litter in respect of the net photosynthetic rate (Pn). This may be attributed to the accompanied decrease of respiration rate. It was still concluded that the total amount of the photosynthetic products was limited at a very low level for the sharp decrease of the leaf area.3. More reactive oxygen species (ROS) were induced since the receiver plant hot pepper germinated, and this effect existed at least 74 days. Higher concentrations of ROS caused membrane peroxidation at early stage of hot pepper growth (before two-euphylla phase), and might interfere with the synthesis of proteins or/and attack soluble proteins directly within a much longer time. In case of damage from ROS, superoxide diamutase (SOD) was promoted remarkably by hot pepper, but not for another two antioxidant enzymes, i.e., catalase (CAT) and peroxidase (POD), therefore, the increase of H2O2 attributed to the disproportionation of superoxide radical (O2-") could not be fully removed all the time.4. Nitrogen nutrition in hot pepper, including total nitrogen (TN), nitrate nitrogen (NO3--N) and soluble protein (SP) contents, decreased sharply after exposure to the leaf litter, probably a critical reason that explains the limited growth and development of the receiver plants. No significant variation was determined in total carbon (TC) content, confirming the slight changes of Pn exposed to different doses of leaf litter. The increase of total phosphorus (TP) content might result from the up-regulation of phosphorus participated resistance molecules, e.g., resistance genes. This was a possible adaptive response to the allelopathic effect of decomposing camphor tree leaf litter.5. Soil nitrate nitrogen (NO3--N) content decreased considerably within 41 to 64 days since the leaf litter treatments were implemented, and soil total nitrogen (TN) and extracting nitrogen (EN) contents decreased at the 48thd of decomposition (EN was determined only at the 48th d), while much slighter fluctuations were found in soil ammonium nitrogen (NH/-N) and available phosphorus (AP) contents. Additionally, the decomposing leaf litter of camphor tree is capable of promoting soil peroxidase and catalase activities and increasing soil bacteria, soil fungi and total microbe amounts (microbial biomass N (MBN) was elevated generally), while diversity of soil algae was inhibited apparently. These changes in soil biochemical properties may unbeneficial to the available processes of nutrients, especially to the nitrification.6. Fourty-eight secondary metabolites were identified from the leaf litter of camphor tree using GC-MS, among them 34 chemicals belong to terpenoid with a total relative content of 78.05%, confirming the solid allelopathic basis of the leaf litter. Afer comparison, six terpenes including β-Caryophyllene, β-Elemene, etc., were the most allelopathic, followed by six terpenes including Nerolidol, a-Pinene, etc., and followed by three terpenes, i.e.,β-Pinene,1,8-Cineole and Camphor. These potential allelochemicals may affect receiver plants after slowly transformation or/and generate indirectly effects via soil biochemical environment.7. Nitrogen (Urea) application was able to alleviate the allelopathic effect of decomposing leaf litter on vegetative and reproductive growth of hot pepper by reversing various unsuitable physiological status, i.e., chlorophyll reduction, water and CO2 use efficiency decrease, H2O2 accumulation and nitrogen nutrition shortage. Changes of soil microbe community and the following degradation and transformation of allelochemicals released from the leaf litter, supplementation of available N to soil N pool directly, and enhancement of peroxide scavenging ability of soil may be three important aspects facilitating the alleviation effect of nitrogen application.On the basis of the results stated above, the leaf litter of camphor tree is likely to release terpenoids to soil via decomposition. These allelochemicals, especially their degradation or/and transformation products may target on roots of the receiver plant directly on one hand, leading to the suppression of photosynthesis, respiration and nutrients absorption; on the other hand, they may target on soil microbe community, soil enzyme activity, etc., interfering the available process of soil nutrients (especially nitrification), and affect the receiver plant indirectly. Thus, excessive Cinnamomum leaf litter should be removed, otherwise application of adequate amount of available N is recommended in compound systems containing Cinnamomum species and intercropping plants.