Effects of plant-derived organic materials and humification driving forces on soil microbial community diversity in orchards

【Objectives】As soil fertility in apple orchards is declined continuously and livestock waste organic manure sources are seriously inadequate, application of plant-derived organic manure is becoming one of important measures to improve orchard soil conditions. Effects of plant-derived organic matter and driving forces on soil microbial functional diversity in pot planted apple rootstock seedlings were studied by using the BIOLOG microplate technique. The functions of glucose, urea and earthworm in the plant-derived organic matter transformation process to soil carbon pool were explored, which could provide a reference for revealing evolution mechanism of orchard soil quality.【Methods】2a Malus baccata (L.) Borkh seedlings were planted in apple orchard soils (0-20 cm) mixed with chopped apple branches, corn stalks and orchard weeds. The urea, glucose and earthworms were added into the pots. The soil microbial community diversities were analyzed by the BIOLOG technique. Soil extracts of different treatments were cultured in BIOLOG ecological test plates. The diversity indexes were calculated using microplate optical density values at 96 h. Substrate richness (S) is the number of different substrates used by the community (counting all positive OD readings). Shannons diversity index is the ratio of the corrected absorbance value of each well to the sum of absorbance value of all wells. Substrate evenness (E) is a measure of the uniformity of activities across all substrates. Simpsons Dominance indicates the dominance of certain species. The carbon substrate utilization differences of microbe were obtained by principal components analysis (PCA) of six carbon sources (carbohydrates, amino acids, carboxylic acids, polymers, aromatic and amines) in the BIOLOG microplates.【Results】The results show that the average well color development(AWCD) values are significantly affected by organic material types, small molecular organic compounds and numbers of earthworms. The AWCD values of the corn straw+ glucose+ 12 earthworms（T4）and the orchard weed with glucose（T9）are obviously higher than those of other treatments in the first 24 hour culture, and the activities of microbial communities are also higher and carbon sources are used earlier. The AWCD values show an exponential growth in the 24-96 hour culture, and then become flat after 120 hours. The treatments of the corn straw+ glucose+ 12 earthworms（T4）, the apple branches+ glucose+ 6 earthworm （T2）and the orchard weed with glucose（T9） have the steepest slopes and the highest microbial activities, and secondly for the corn straw+ urea+ 6 earthworm（T6）and the apple branches with urea（T1） treatments. The small molecular organic compounds have significant effects on the microbial community substrate richness（S） and Simpsons dominance （Ds）, and the substrate richness （S） is the highest under the apple branches+ glucose+ 6 earthworm（T2）, while the Simpsons dominance（Ds） is the highest under the corn straw+ glucose+12 earthworms（T4）. The Shannon index（H） and Substrate evenness （E）between different treatments are not significantly different. It is found that the differentiations of soil microbial communities are mainly caused by carbohydrate and polymers carbon. 【Conclusions】Compared with the straw and weeds treatments, the microbial diversity of the branches treatments is high. The glucose provides carbon sources which can be used rapidly by microbe, and the microbial functional diversity is significantly increased. The effect of earthworm activity on the microbial functional diversity is less compared with the glucose. The urea has little effects on the microbial functional diversity, while the combination of urea and glucose could improve the microbial diversity.