Rhizosphere soil nutrients and bacterial community diversity of four broad-leaved trees planted under Chinese fir stands with different stocking density levels
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Objective Rhizosphere soil nutrients and bacterial diversity of four broad-leaved tree species underplanted in Chinese fir plantation with different stand density levels were analyzed to reveal characteristics of the rhizosphere soil environment and selection of suitable underplanted tree species. Methods Chinese fir plantation with three density levels (900, 1,200, and 1,875 stems ha –1 , respectively) were selected and underplanted with Michelia macclurei , Schima superba , Phoebe zhennan , and Tsoongiodendron odorum . The rhizosphere soil nutrients and bacterial community of the broad-leaved tree species were determined after 4 years. Results Significant differences in rhizosphere nutrient content were detected among different tree density levels, where the contents of total K, available K and available P in 900 stems ha –1 stands were significantly higher than the other stocking density levels. There were also significant differences in the contents of total C, total N, total K, available K and available P in the rhizosphere soils of the four trees species, while there were no significant differences in pH and total P. Rhizosphere soil nutrient contents were higher under S. superba and M. macclurei than under P. zhennan and T. odorum . The rhizosphere soil nutrient contents and bacterial diversity decreased with the increase of stand density, and the bacterial diversity showed significant differences in the rhizosphere soils of P. zhennan , T. odorum and S. superba when underplanted in different stand densities. The bacterial diversity was positively correlated with the available P content of rhizosphere soils, suggesting that soil available P content plays an important role in shaping the structure of bacterial community. Conclusion The nutrient contents and bacterial diversity of rhizosphere soils of underplated broad-leaved species decreased with increasing stand density of Chinese fir plantation. Rhizosphere soils of M. macclurei and S. superba were rich in nutrient contents and bacterial diversity. Thus, low density of Chinese fir plantation (900 stems ha –1 ) underplanted with M. macclurei and S. superba is suitable for the establishment of mixed forest, which will facilitate better tree growth and maintaining soil fertility to realize sustainable management of forests.Keywords:
Diversity index
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Soil nutrients
The present experiment was conducted to investigate the effect of adding S and/or Ni on the concentration of Ni and Mil in the rhizosphere and metal content in plant parts of wheat seedlings by using a rhizobox system.The results indicate slight differences in pH values of rhizoshpere soil compared to that of bulk soil. The increase in solubility of Ni and Mn was most pronounced in the rhizosphere soil compared to that of the bulk soil. The concentrations of Ni and Mn in plant parts were more related to the contents in the rhizosphere soil compared to that of the bulk soil.It was concluded that the distribution of the metals across the rhizosphere is more important for the evaluation of the availability of heavy metals than their solubility in the bulk soil.
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The rhizosphere has a controlling role in the flow of water and nutrients from soil to plant roots; however, its hydraulic properties are not well understood. As roots grow, they change the pore size distribution of the surrounding soil. Roots release polymeric substances such as mucilage into their rhizosphere. Microorganisms living in the rhizosphere feed on these organic materials and release other polymeric substances into the rhizosphere. The presence of these organic materials might affect the water retention properties and the hydraulic conductivity of the rhizosphere soil during drying and rewetting. We used neutron radiography to monitor the dynamics of water distribution in the rhizosphere of lupin (Lupinus albus L.) plants during a period of drying and rewetting. The rhizosphere was shown to have a higher water content than the bulk soil during the drying period but a lower one during the subsequent rewetting. We evaluated the wettability of the bulk soil and the rhizosphere soil by measuring the contact angle of water in the soil. We found significantly higher contact angles for the rhizosphere soil than the bulk soil after drying, which indicates slight water repellency in the rhizosphere. This explains the lower soil water content in the rhizosphere than the bulk soil after rewetting. Our results suggest that the water holding capacity of the rhizosphere is dynamic and might shift toward higher or lower values than those of the surrounding bulk soil, not affected by roots, depending on the history of drying and rewetting cycles.
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The dynamic property of soil nutrients,soil microbes,soil enzyme and pH in ramie rhizosphere soil and non-rhizosphere soil collected from producing areas of 6 ramie varieties was studied.And the relationship between them was discussed.The results showed that the content of available N,available K,organic matter,total K and water,the numbers of soil bacteria,actinomyces and fungi,the activities of urease and acid phosphatase in rhizosphere soils were all significantly higher than those of non-rhizosphere.Meanwhile,there was significant difference in pH between rhizosphere soil and non-rhizosphere soil.The correlations among soil nutrients,soil microbes,soil enzyme and pH in rhizosphere soil were more significant than that in non-rhizosphere soil.It was hypothesized that the ramie root brought about the differences between the rhizosphere soil and non-rhizosphere soil.
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The rhizobox system offers quantitative information concerning the pH changes and the range of the rhizosphere. Changes in pH values around the roots (barley and soybean) were investigated using a rhizobox. The results indicated that the pH values increased near the roots in soils with a low pH of the bulk soil (about 5). Changes of pH were as much as 2 units, compared to the bulk soil. Conversely, the rhizosphere pH decreased by about 2 units in soils with a bulk soil pH of 7. A slight decrease in the rhizosphere pH was observed in soils with a bulk soil pH of 8.4. The ability of soybean roots to influence the pH was larger than that of barley. No effect of N -serve on the pH of the rhizosphere and bulk soil was observed when ammonium sulphate was applied. These results suggest that the extent to which the rhizosphere pH can differ from that of the bulk soil depends more on the plant species and initial bulk soil pH than upon N-fertilizer source or N-level.
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Summary Soil solution was obtained from potted rhizosphere or non‐rhizosphere soils by water displacement or soil centrifugation. The pH of the displaced solutions was lower than that of bulk soils when solutions were obtained from non‐rhizosphere soil, although it increased as plants grew. This increase probably reflected true changes in rhizosphere pH, generated by the uptake by plants of N0 3 ‐N. In contrast, the pH of soil centrifugates was usually close to that of the bulk soils, implying that buffering by colloids had occurred during sampling. Concentrations of elements in solutions from non‐rhizosphere soil were similar for both methods when soils were incubated at ambient pCO 2 . However, when non‐rhizosphere soils were incubated at elevated pCO 2 , displacement solutions had lower pH values, and much larger concentrations of elements, compared to soil centrifugates. Comparison of mass flow of elements versus actual plant uptake showed that Ca and Mg accumulated, while K, Zn and Cd were depleted from the rhizosphere. Displacement solutions showed this accumulation or depletion of the elements more clearly than soil centrifugates. These differences were attributed to the fact that, at constant soil moisture, the rhizosphere developed mainly in larger pores, which were sampled by displacement. With centrifugation, a mixture of pore sizes was sampled, so that rhizosphere solution was only obtained when all of the soil had become rhizosphere. Soil centrifugates obtained after 22 days of growth also contained higher concentrations of organic carbon than displacement solutions, indicating contamination due to the disruption of roots and/or micro‐organisms. We conclude that water displacement is suitable for sampling solution from light to medium textured rhizosphere or non‐rhizosphere soils and that soil centrifugation is only of limited suitability.
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