Ecological Mechanism Of Crop Diversification On Continuous Tobacco Cropping Obstacles

Tobacco is an intolerance continuous cropping crop. Continuous cropping obstacles restrict sustainable development of tobacco production, which is the most important issues in agricultural production that need to be soluted urgenty. In this study, the theme is on allelopathic autotoxicity and rhizosphere microbe flora, for this purpose, it starts with the cause of tobacco continuous cropping obstacles, and includes the following experiments:physiochemical response of tobacco continuous cropping and its allelopathic autotoxicity, micro-ecological character of the rhizosphere soil, verification of allelopathic autotoxicity effects. Furthermore, ecological mechanism of crop diversification on continuous tobacco cropping obstacles was explored. This study reveals fully the mechanism of tobacco continuous cropping obstacles and provides a theoretical basis and technical support for further studying to reduce obstacles as well as for achieving effective control of the obstacles as soon as possible.The results were mainly summarized as below.1. Pot experiment was conducted to study effects of continuous cropping soil on the grow status and protective enzymes of tobacco crop plant in the soils mediated by replanted flue-cured tobacco and burley tobacco in cropping sequence for 0,3, and 10 years.Growth of flue-cured tobacco and burley tobacco was significantly inhibited under the condition of continuous cropping, such as shortened plant height and decreased leaf area. The defence response of tobacco was also triggered, indicating that the activities of superoxidate dismutase (SOD) and peroxidase (POD) in the leaves were enhanced, while reverse was true in the case of catalase (CAT), and the malondialdehyde (MDA) content in the leaves of tobacco was increased. The inhibitory effects of continuous cropping obstacles were enhanced as the continuous cropping years increased. This implied that the crop was subjected to stressful environment and changes in its resistant physiology under the continuous cropping treatment.2. To study relationship among autotoxicity, microorganism and continuous cropping obstacle of tobacco, the experiment that the tobacco was planted in the sterilized and non-sterilized soil under flue-cured and burley tobacco, was conducted. The results showed that the sterilized soil under continuous cropping tobacco appeared to reduce the inhibitory effect on the growth of tobacco compared with non-sterilized soil. After sterilization treatment, the detrimental biotic factors were eliminated to some extent, but the inhibitory effects on plant height and leaf area were still enhanced as the continuous cropping years increased. This implied that the detrimental factor on the crop still existed in the sterilized soil under continuous cropping treatment, which appeared to be enriched as the continuous cropping years. Further analysis showed that whether the soil was sterilized or not, the diversity level of bacterial community in the rhizospheric soil was decreased as the continuously cropping years increased, and the similarity indexes of bacterial community were higher in both non-sterilized soil and sterilized soil at the same period. The result suggested that the enrichment effect of secretion residual from the crop had epigamic stimulation to bacterial community with continuous cropping years increasing.The analysis of the microorganism related to continuous cropping obstacle of tobacco showed that the microorganism in the soil under the replanted tobacco after sterilized treatment shifted toward the harmful species, which could lead to decrease in the microorganism related to soil nutrition cycle, reverse was true in pathogens with continuous cropping years increasing. Though the same results happened in the non-sterilized treatment, the soil without sterilized treatment was more abundance in the species of microorganism. This implied that pathogens would become a dominant population in the soil ecological system, the propagation of most probiotics were inhibited, the original balance of soil microbial communities has been disrupted and the soil environment towards the direction of worsening as the prolonging of the continuous cropping years.3. The experiment, which adding additional substances into the soil under tobacco planted in the pot, indicated that the same effect of continuous cropping obstacle, such as the dwarfing of tobacco, reducing in leaf area and lowering of photosynthesis, happened on the tobacco treated with the additional substances and the effect enhanced significantly as additional substances concentration increased. The analysis of the micro-system of tobacco rhizosphere suggested that enzyme activity in related with soil nutrition cycle decreased significantly as the additional substances increased. The T-RFLP analysis of the bacterial community of the rhizosphere soil showed that the bacterial community and the diversity decreased which were consistent with the continuous cropping of tobacco, at the same time, many T-RFs related with the continuous cropping obstacle were determined in soil and eight T-RFs were found in the soil of continuous cropping also. These results showed that the additional substances could make the microorganism in the soil under tobacco evolve toward orientation of microflora in the continuous cropping soil and its evolution speed was related to the concentration of additional substance or the cropping years correspondingly. According to the above, it can be concluted that the substances exudated from the tobacco result in changed microorganism in the soil, and this in turn leads to the continuous cropping obstacle.4. To study ecological mechanism of crop diversification on continuous tobacco cropping obstacle, some plant patterns of crop diversification cultivation such as (flue-cured tobacco-rice-flue-cured tobacco(KR-K), flue-cured tobacco-corn-flue-cured tobacco(KC-K), burley tobacco-rice-burley tobacco (ER-K), burley tobacco-corn-burley tobacco (EC-K) and CK(first crop control) were conducted. The result showed that replanted tobacco in sequential cropping leaded to continuous cropping obstacles, the concrete manifestation was poor agronomic traits including decreasing plant height, stem girth, effective leaflet, and leaf area, hindering photosynthesis, and reducing yields. Chemical component of flue-cured tobacco of replanted tobacco in sequential cropping became uncoordinated compared with first crop control. However tobacco-rice→tobacco show lower barriers than tobacco-corn→tobacco pattern. And changed species inter-annual would reduce obstacle effects partly. In addition, diversity of bacterial flora was decreased after replanting tobacco in sequential cropping, but the Shannon-Weiner index of bacterial flora in the rhizosphere soil of ER-K and EC-K which changed species inter-annual were decreased less than KR-K and KC-K. The similarity coefficient of microbial flora in rhizosphere soil of KR-K, ER-K taking rotation rice was increased with control.The further analysis of microbial groups related to continuous cropping obstacle of tobacco showed that, compared to the CK, the plant patterns of crop diversity cultivation could lead to slight increase in microbial species related to soil nutrient cycle but decrease in the number, while the number and species of pathogen appeared upward trend in some degree. And compared to the soil cropping tobacco sequence for 3 years, the crop diversity cultivation contributed to the significant increase in microbial diversity, and even the number and species of microorganism related to soil nutrient cycle. Compared to plant patterns of KR-K, ER-K, there is a slight decrease in the number and species of microorganism related to soil nutrient cycle under the patterns of KC-K, EK-K, while no significant difference could be found in pathogens and probiotics.Compared to the plant patterns which did not changed species inter-annual (KR-K and KC-K), the plant patterns which changed species inter-annual (ER-K or EC-K) would result in the significant enhancement of the diversity of microorganism community in the rhizosphere soil under tobacco, a slight rise in species and amount of some microorganism related to soil nutrition cycle and the decreasing of pathogens in some parts. There were same changes in diversity, species, amount and pathogens but more significant in comparison with continuous cropping of tobacco for 3 years. However, compared to CK, the microorganism related to soil nutrition cycle under the ER-K or EC-K patterns increased slightly in species but decreased in number, and there was a certain upward trend of pathogens in species and amount. At the same time, compared to patterns of ER-K, there was a slight decrease in species and number of the microorganism related to soil nutrition cycle under the EC-K, and greater number of pathogens but small number of probiotics on the contrary.In summary, continuous cropping of tobacco caused the accumulation of autotoxic substances in soil, and these substances could contribute to a selective promotion of soil microorganism, particularly to pathogens. This in turn led to destroyed microbe community balance, and deteriorated micro-ecological conditions in rhizosphere soil. Through the multi-dimensional measures such as crop diversification, it could regulate the population characteristics of rhizosphere microorganism directionally and recoery a normal rhizosphere ecosystems, will eventually overcome the problems of continuous cropping obstacle.