| Manchurian walnut (Juglans mandshurica) and larch (Larix gmelinii) are main afforestation species with higher ecological and economic values in northeastern China. Afforestation practice suggests that the productivity of manchurian walnut and larch mixed plantation is much higher than pure plantation, showing a good promotion effect. The reason is root exudates and soil microorganisms which play an indispensable role in increasing productivity. Root exudates can regulate plant nutrition and produce allelochemicals which affect the growth of neighboring plants and also may produce the poison effect that influence themself growth. Soil microorganisms also play an important role in plant growth, it is a basic ingredient of plant life activity. Beneficial microbes can increase the utilization rate of nutrients, promote absorbing nutrients, improve soil structure and enhance the resistance disease, while harmful microbes are bad for the health of plant growth. First of all, our study analysis the influence of four kinds of larch root exudates on the root activity, mycorrhizal infection rate and soil microbial community structure of manchurian walnut in the indoor experiment, and then we detected the dynamic change of root exudates and PLFAs using high performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) in the field stand ages of manchurian walnut pure plantation, larch pure plantation and the mixed plantation. We attempt to know them how to change with plant growth in different months and discuss root exudates how to affect the change of soil microorganisms. Our results suggested that:In the indoor experiment, most of adding larch rhizosphere concentration root exudates treatments had an adverse effect on the growth of manchurian walnut, reduced the root activity and mycorrhizal infection rate, only2,4-2hydroxy benzoic acid treatment had an profitable effect on the growth. There were definite positive correlations between the biomass and PLFAs, meaned that more soil microorganisms contributed to the growth of manchurian walnut. In the field investigation, we found that the mixed plantation had the largest soil microbial. The sequence from the highest to the lowest is bacterial, fungi, actinomycetes and protozoa in soil microflora. Soil microbial quantities were roughly high in spring, summer was low and fall come back in manchurian walnut pure plantation and the mixed plantation, while soil microbial quantity was higher in May and June peak, and then as time gone on, gradually declined, the lowest in September in larch pure plantation. The content of root exudates changed over time and had an obvious difference in different plantations. The total of root exudates was the least in the mixed plantation compared to pure plantations. The content with different kinds of exudates was also extremely significant.2,4-2hydroxy benzoic acid,7-hydroxyl coumarin, abietic acid and phloroglucinol were relatively high, other exudates were low. Cinnamic acid was positively correlated with bacteria, while abietic acid was negatively correlated with bacteria; abietic acid was positively correlated with fungi, while stearic acid and cinnamic acid were negatively correlated with fungi in the three plantations.We preliminary obtained the dynamic change rules of root exudates and soil microorganisms in manchurian walnut pure plantation, larch pure plantation and the mixed plantation. It provides a guidance for future walnut-larch mixed plantation reasonable operating, at the same time, it can make people understand the relationship between allelochemicals and soil microorganisms. |
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