| Current nursery substrates for raising seedlings in agriculture, forestry and flower production are mostly derived from peat which is one of the important natural resources. Peat acts as a carbon sink and can release CO2, CH4and other greenhouse gases during its utilization as resource and due to the change of its storage condition. Massive mining of peat may destroy wetland. It is necessary to develop new materials for nursery substrates to replace peat, from the viewpoint of resource preservation and environmental protection. Organic solid wastes can be transformed to vermicompost through earthworm digestion. Vermicompost is characterized by its porosity, permeability of air and water and water holding capacity. The larger surface area of vermicompost can adsorb nutrients required by plants and provide habitats for microbes. Therefore, vermicompost-based substrate can provide seedlings the ideal growth conditions and is becoming a promising substitute for peat. However, information and investigations on this aspect have been in scarce yet.In this study, earthworm was used to digest cow manure, then vermicompost was obtained. Based on the analysis of the basic properties of vermicompost, different formulas of vermicompost-based substrates were designed. With capsicum seedling as material, the effects of the substrates on seedling growth were investigated and the mechanisms pertaining to vermicompost-formulated substrate strengthening capsicum seedling were analyzed. Results obtained in this study were summarized as follows.Under scanning electron microscope, vermicompost showed granular and bedded structure and had higher microporosity. The vermicompost contained the gelatinous substance(s) after air-drying which helped vermicompost maintain good structure. N-butyl alcohol could washed out the substance(s) and destroyed aggregate structure of the vermicompost, while water had no influence on the substance(s) which indicated that the ideal structure of vermicompost-formulated substrate was not affected by frequent watering during seedling growth. Far-infrared spectrum analysis showed that after earthworm digestion, lignin, cellulose, hemicellulose, sugar and other carbohydrates in cow manure were gradually decomposed and converted to humus substances, while water soluble organic silicon compounds changed to inorganic silicon oxides. An unknown substance in cow manure (Substance I, with retention time35.12-35.27min. detected by HPLC) increased after earthworm digestion. The number of fungi, bacteria and actinomycetes in vermicompost was higher than that in cow manure. After earthworm digestion, hydrogen peroxidase activity increased. The bacterial communities diversity and structure changes significantly between waste organic solid and vermicompost. The latter bacterial community diversity and structure is gradually stabilizing with the extension of digestion time. The bacterial community structure of vermicompost from the same bait is in a certain degree of convergence. Large amount of GA3and IAA was detected in vermicompost, which could stimulate seedling growth, while decline of urease activity in vermicompost helped to delay nitrogen release during seedling growth. Compared with current commercial substrate (CK, peat-based, with total N, P and K of2.5-5.0%), the bulk density, total porosity and aerial porosity and air/water ratio of vermicompost were higher, while water-holding porosity, water-holding and retention capacity was lower. The acid buffer capacity of vermicompost was similar to that of CK, but the buffer capacity of alkalinity and salt was higher than that of CK. High temperature sterilization and grinding, especially the former, decreased the capacity of acid and salt but increased the alkaline capacity.The pH value of vermicompost was lower than that of CK but EC value, total N, available N, total P, available P and organic matter content of vermicompost was2-3,3-4,2,1.6-2.0,5-7and1.5-2.5times higher than those of CK, respectively. During seedling growth, pH value of vermicompost-formulated substrate increased, while EC and available P and K decreased.The growth index values of the seedlings grown in vermicompost-based substrate, including survival rate, plant broadening, plant height, stem diameter, number of leafs, fresh weight of leaf and stem, root surface area, root diameter, root volume and fresh weight of root, were higher than those of the seedlings grown in CK. However, such promoting effects mainly appeared in middle and late stages, rather than early stage, of seedling development, due to the gradual enhancement of nutrient absorption and environmental resistance by the vermicompost.The substrate derived from grinded vermicompost was more advantageous in promoting the increase of survival rate, stem diameter, plant height, plant broadening, number of leafs, root length, root surface area, root volume, root number, root vigor, G value and seedling index, but caused the decline in overall planting rate, chlorophyll content and root diameter. The substrate formulated by mixing grinded vermicompost and vermiculite with the ratio of8-16:1(v:v) was beneficial to the strengthening of the seedlings, due to the increase of bulk density, water holding capacity and lump or granular structure. When vermicompost was mixed with vermiculite (4:1, v:v), grinding increased bulk density, total porosity, water-holding porosity, water content and lump or granular structure of the substrate.Addition of vermiculite helped the substrate enhance plant broadening, number of leafs, root length, root surface area, root volume and root tips of the seedlings. During seedling growth, pH value in the substrate (4vermicompost:1vermiculite, v:v) was higher, but total N, available N, total P, available P, water soluble K, organic matter content and EC was lower. Although addition of vermiculite was beneficial to seedling growth, the suitable ratio of vermicompost to vermiculite was4:1(v:v), considering the cost factor in preparing substrate.After high temperature sterilization, the vermicompost-formulated substrate, compared with fresh one, showed better promoting effect on seedling growth. Substance Ⅰ (retention time:35.12-35.27min. in HPLC) was desatured under high temperature sterilization and more polar substances Ⅱ-Ⅴ (with retention time3.09,11.35,17.55and19.07min., respectively) were derived. These substances were more water soluble and easier to be absorbed by seedlings. Meanwhile, high temperature sterilization caused the increase of GA3and IAA (esp. the former) in the substrate.Moderate addition of urea to the substrate was helpful in enhancing seedling growth. The seedling grown in the substrate added with urea with the rate of1.0kg.m-3had higher leaf chlorophyll content, plant height, number of leafs, fresh weight of seedlings, number of root tips, root surface area, root/shoot ratio, G value and seedling index, compared with those in the substrate with0.5kg.m-3urea added. Over-rate urea added to substrate was toxic to the seedlings and caused decline of survival rate.Addition of urea reduced pH and increased EC value of the substrate during seedling growth. Higher available N level was maintained in the substrate with1.0kg.m-3urea added than in the substrate with0.5kg.m-3urea added. Addition of P and K to the substrate also urea reduced pH and increased EC value, and had promoting effect on survival rate, plant broadening, fresh weight of seedling plant, root diameter, root volume and G value. But P and K addition had no apparent effect on leaf chlorophyll content, plant height, stem diameter, leaf number, root length, root surface area and root fresh weight.In summary, vermicompost-formulated substrate could promote the development of root, leaf and stem of capsicum seedling and was beneficial to the raising of sound seedlings. Such promoting effect displayed more apparently during the middle and late growth stages of the seedlings. The promoting effect was comprehensive and resulted from many aspects pertaining to the properties of the vermicompost, including high porosity, ideal structure, high buffer capacity to acid, alkaline and slat, high nutrient content, slow release of nutrients, abundant microbial flora, high content of active AA, beneficial enzymes and plant hormones. The gelatinous substance(s) in the vermicompost was not affected by frequent watering and helped the substrate maintain good structure during seedling growth.In application, the optimum formula of the substrate for raising capsicum seedling was:8vermicompost:1vermiculite (v:v)+0.5-1.0kg.m-3urea+10.0kg.m-3calcium superphosphate+1.0kg.m-3potassium sulfate. After collection from earthworm bed, high temperature and aerobic composting was recommended to kill the pest, to speed up the formation of active organic substance and to maintain beneficial microbes. After composting, the sieved vermicompost (without air-drying) can be directly used to formulate the substrate, to save the production cost, as well as to preserve the beneficial effect of the vermicompost. |
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