| The ecological environment is fragile and the economic conditions in the loess region of the northern Wei River backward,yet the agroforestry systems in the region play important role in improving the natural and economic conditions,therefore it is urgent to study the ecological processes of walnut-wheat,walnut-soybean and walnut-rape agroforestry systems in the region.In this study,the types and contents of allelochemicals secreted by walnut roots were analyzed by GC-MS;the ectomycorrhiza fungi of walnut were classified by microscope observation and 18 rs DNA determination;the allelopathic intensity of allelochemicals secreted by walnut roots was identified by bioassay.In order to provide scientific basis and theoretical foundation for the practice of optimizing the management mode of agroforestry system in Weibei loess area,root detector was used to monitor the growth dynamics of roots in different agroforestry systems,and the biomass of crops in agroforestry systems was measured.The main results are as follows:(1)The organic components in root exudates of monoculture and intercropping walnut trees mainly include alkanes,alkenes,alcohols,organic acids,esters,ketones,amides,phenols,quinones,aldehydes and benzene rings.Some of them are considered to be allelochemicals,and walnut monoculture,walnut wheat,walnut soybean and walnut rape agroforestry systems have a greater impact on the components of root exudates of walnut trees,with 35 organic components secreted by walnut roots alone,In walnut wheat agroforestry system,43 kinds of organic components were secreted by walnut roots,50 kinds of organic components were secreted by walnut soybean agroforestry system,and 38 kinds of organic components were secreted by walnut-oil rape agroforestry system.(2)The root exudates of walnut had different inhibitory effects on seed germination rate,seedling height and root length of wheat,soybean and oil rape.Walnut root exudates in walnut-wheat intercropping had the highest inhibition rate on wheat germination rate(62.04%),while walnut root exudates in monocultures had the lowest inhibition rate on soybean germination rate(19.1%).Walnut root exudates had the most significant inhibition rate on oil rape seedling height(51.54%),and walnut root exudates had the least inhibition rate on soybean seedling height(28.5%).The root exudates of walnut in walnut-oil rape intercropping system had the highest root length inhibition rate(42.71%)on oil rape,while the root exudates of walnut in walnut-soybean intercropping system had the lowest root length inhibition rate(38.5%)on soybean.(3)Walnut root exudates had different inhibitory abilities on SOD,POD,CAT activities and MDA content in leaves of wheat,soybean and oil rape.SOD activity inhibition rate was as follows: walnut monoculture(oil rape)> walnut-oil rape(oil rape)>walnut-soybean(soybean)> walnut monoculture(soybean)> walnut-wheat(wheat)>walnut monoculture(wheat).POD activity inhibition rate was as follows: walnut-oil rape(oil rape)> walnut monoculture(oil rape)> walnut monoculture(soybean)> walnut-wheat(wheat)> walnut-soybean(soybean)> walnut monoculture(wheat).The inhibition rate of CAT activity was as follows: walnut-wheat(wheat)> walnut monoculture(soybean)>walnut monoculture(oil rape)> walnut monoculture(wheat)> walnut-soybean(soybean)>walnut-oil rape(oil rape).The inhibition rate of MDA content was as follows: walnut-wheat(wheat)> walnut monoculture(soybean)>walnut monoculture(wheat)> walnut-soybean(soybean)> walnut monoculture(oil rape)> walnut-oil rape(oil rape).(4)There were 25 genera of ectomycorrhizal fungi in walnut monoculture,walnut-wheat,walnut-soybean and walnut-oil rape intercropping modes,among which the fungi of Tarzetta,Hymenoscyphus and Pachyphloeus accounted for the absolute proportion.The relative abundance,frequency and diversity of walnut ectomycorrhizal fungi in walnut-soybean agroforestry system were significantly better than those in walnut-monoculture,walnut-wheat and walnut-oil rape intercropping systems.(5)Allelopathy in agroforestry system can affect the growth dynamics and photosynthesis of plant roots.The root length density of walnut was as follows:walnut-soybean > walnut monoculture > walnut-wheat > walnut-oil rape.The root length density of crops was as follows: wheat intercropping > wheat monoculture,soybean intercropping > soybean monoculture,oil rape intercropping < oil rape monoculture.The average root diameter of walnut was as follows: walnut-soybean > walnut-wheat >walnut-oil rape.The average diameter of fine roots of crops was as follows: wheat intercropping < wheat monoculture,soybean intercropping > soybean monoculture,oil rape intercropping < oil rape monoculture;The net photosynthetic rate was as follows: wheat intercropping < wheat monoculture,soybean intercropping > soybean monoculture,oil rape intercropping < oil rape monoculture;The biomass was as follows: wheat intercropping <wheat monoculture,soybean intercropping > soybean monoculture,oil rape intercropping <oil rape monoculture.To sum up,the results showed that walnut-soybean agroforestry pattern could affect the type and content of walnut root exudates,and reduce the adverse effects of allelochemicals on soybean growth and development.Meanwhile,walnut-soybean agroforestry pattern could promote walnut root to form ectomycorrhiza and thus promote walnut root growth.Compared with walnut-wheat and walnut-oil rape agroforestry patterns,walnut-soybean agroforestry pattern in loess plateau of northwest China has greater economic and ecological benefits. |
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