Research On Interspecific Relationships Between Caragana And Alfalfa In An Artificial Compound System Of Desert Steppe

The eastablishment of shrub-grass is one of the main vegetation restoration patterns in arid and semiarid region of northwest. Shrub-grass land of Caragana-alfalfa appeared in large area in the process of preventing degraded grassland in Ningxia Yanchi. The interspecific relationships between Caragana and alfalfa had a marked impact on productivity and stability of shrub-grass compound system. In this article, based on questions of the interspecific relationships between Caragana and alfalfa and formation mechanism in an artificial compound system of desert steppe in Ningxia Yanchi, interspecific mechanisms between Caragana and alfalfa were revealed by allelopathy and resource competition, in order that plants could survive stably and make full use of resources. Field investigation of Caragana-alfalfa artificial compound system was carried out first to study neighbourhood effects between Caragana and alfalfa. Seed germination bioassay of aqueous extract from different organs of Caragana and alfalfa was conducted, and allelochemicals of aqueous extract from different organs of Caragana and alfalfa and soil around Caragana roots were isolated and identified to analyze and discuss allelopathic effects between Caragana and alfalfa. Activated carbon was used for pot experiment at different moisture level to separate allelopathy from resource competition, and characteristics of allelopathy and resource competition at different moisture level were explored.. Theoretical basis and guidance would be offered by the study for the recovery of degraded grassland and shrub-grass compound planting in desert steppe. The main conclusions are as follows:(1) Alfalfa had negative effects on crown size, plant height, branch number, basal stem and aboveground biomass of Caragana in the artificial compound system of desert steppe in Ningxia Yanchi. And these effects were related to distance, the effects on Caragana became more significant with the closer distance to alfalfa. Competition between Caragana and alfalfa weakened the capacity of Caragana in accumulation of organic matters, in the meantime, phosphorus content of Caragana increased relatively. Caragana decreased the density of neighbouring alfalfa, low density of alfalfa mitigated intraspecific competition within themselves, and microclimate of Caragana understory was of benefit to improve quality of alfalfa, thus the neighbouring alfalfa accumulated more carbon, nitrogen and phosphorus.(2) Aqueous extracts from different organs of Caragana had allelopathic effects on the alfalfa seed germination, and the aqueous extracts improved seed germination at low concentration, whereas inhibited at high concentration. Aqueous extracts from root and stem of Caragana had promotion effect on the growth of radicle and inhibition effect on the growth of hypocotyl of alfalfa. Aqueous extracts from leaf of Caragana had promotion effect on the growth of radicle and hypocotyl of alfalfa at low concentration, whereas inhibition effect at high concentration. Aqueous extracts from different organs of alfalfa had inhibition effect on seed germination of Caragana, the inhibition effect increased with the increasing concentration of aqueous extracts. The inhibitory potency of aqueous extracts from root was stronger than that of stem and leaf at the same concentration.(3) Potential allelochemicals produced by Caragana and alfalfa were different, and so were the allelopathic effects. Potential allelochemicals from different organs differed from each other, and so did the allelopathic effects of different organs from the same species. In trems of compound types in different organs of Caragana,20 kinds of compounds were separated in aqueous extracts from root, and 19 kinds from leaf,15 kinds from stem. The content of alcohol and organic acidic compounds in root and stem of Caragana was higher relatively, while the content of alcohol and hydrocarbon compounds in leaf of Caragana was higher relatively.44 kinds of compounds were separated in aqueous extracts from root of alfalfa,42 kinds from stem and leaf. The content of alcohol, organic acid and phenolic compounds in root of alfalfa was higher relatively. The content of alcohol, ester and nitrogen compounds in stem and leaf of alfalfa was higher relatively Potential allelochemicals had obvious differences between soil and plants, but had the same classes. Aldehydes, ethers, esters, alkanes, phenols, organic acid and nitrogen compounds appeared both in soil and plants. Content of nitrogen compounds was higher relatively in soil. Conclusions from pot experiments of Caragana and alfalfa were that allelopathy and resource competition coexisted when Caragana and alfalfa were planted together. Resource competition predominated at low moisture level, while Allelopathy played an important role at high moisture level, and affect parts of plant activities, which leaded to niche separation among differents plants, and reduced their resource competition. As a high water consumption type of plants, alfalfa benefited a lot from allelopathy when lacking of moisture.