| This study was conducted with the primary aim of revealing characteristics of soil microbialproperties and nutrient conditions in the rhizosphere of different poplar genetypes and treespecies compositions. The pH values, available N, microbial biomass, enzyme activities,microbial diversities as well as net N mineralization and nitrification rates were investigated inrhizosphere and bulk soils of three poplar clones and six treatments of tree compositions. Threepoplar clones including Nanlin895(a hybrid of I-69(Populus deltoides Bartr. cv.‘Lux’)×I-45(P.euramericana (Dode) Guinier cv.‘I-45/51’),75(a hybrid of I-72(P×euramericana (Dode)Guinier cv.‘San Matino’ ex I-72/58)×I-63(P. deltoides Bartr. cv.‘Harvard’)) and NL80351(ahybrid of I-69×I-63), were selected to be used in this study. These poplars grew in eastern lowerreaches of the Yangtze River where near Ma-an-shan City, Anhui Province, in southeast China.At the same time, laboratory stimulus experiments also carried out of six tree speciescompositions including PAW（poplar×alder×willow）, PA (poplar×alder), PW (poplar×willow), P(poplar), A (alder) and W (willow). For the field study, three different sampling times (earlierNovember2010, later March2011and middle July2011) were selected to explore seasonalvariation of soil microbial properties and nutrient availability in rhizosphere soils of differentpoplar clones. The key findings are as follows:（1）Net N mineralization rate and inorganic N concentration in the rhizosphere soils weresignificantly higher than bulk soil among all six different tree coexistence treatments, whileamong different trees coexistence treatments, the highest N mineralization rate and inorganic Nconcentration were observed in treatment PA which significantly higher than other treatments.Among all six treatments, with the increasing of N mineralization rate in rhizosphere soil,concentration of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) were alsoproportionally increased.（2）Soil microbial biomass C (MBC) and microbial biomass N (MBN) varied significantlyin the rhizosphere soils among different tree species compositions. The content MBC and MBNin rhizosphere of PAW, PA, and PW were significantly higher than P. Significant difference ofmicrobial C/N ratios was observed in rhizosphere soils of treatments tested. The highest wasobserved in PA, followed by PAW, P, W and PW, with the lowest observed in A. Soil urease andL-asparaginase activities in the rhizosphere of PAW, PA, and PW were significantly higher than P.Significantly positive or negative correlation between tested enzyme activity and microbialbiomass were observed in soils evaluated.（3）Soil microbial bacterial and fungal diversity varied significantly in the rhizosphere soilsamong different tree species compositions. In PA microbial diversity was significantly higherthan in P, in the contrest in PAW and PW. Microbial diversity in rhizosphere soil wassignificantly higher than bulk soils in tree species compostions tested. There were significant positive correlations between L-asparaginase, protease enzyme activities, and soil microbialbiomass in the rhizosphere soils.（4）For the same sampling time, a significant difference in all measured chemicalproperties between rhizosphere and bulk soils of all three poplar colones were observed, exceptfor soils sampled in the middle of July2011. Generally, the pH value in the rhizosphere soils aresignificantly lower than in the bulk soils, on the anther hand the concentrations of NO3-N,NH4+-N and inorganic N were significantly higher in the rhizosphere than in bulk soils.Concentrations of NO3-N and NH4+-N in the rhizosphere showed significant variations amongdifferent sampling times, but clonal variation in these concentrations was not significant amongthe rhizosphere soils of the three tested poplar clones. There were significant differences inpotential net nitrogen mineralization and nitrification rate between the rhizosphere soils and bulksoils tested. The potential net nitrogen mineralization rate in NL80351was more than150%greater than the values of clones75, Nanlin-895and bulk soil at three sampling times. However,nitrogen nitrification rate was warkedly lower in the rhizosphere soils compared to bulk soils. Nosignificant seasonal variations in nitrogen nitrification rate was founded for each poplar clones aswell as for bulk soil, whereas a significant difference in net nitrogen mineralization rate amongthe three different sampling times for clones Nanlin-895and NL80351were observed.（5）The trends of microbial biomass among three poplar clones at all sampling times weresimilar, which was Nanlin-895>75> NL80351and rhizosphere soil> bulk soil. MBN and MBCof all three poplar clones were significantly higher in winter than summer or spring. Seasonalvariation had extremely significant influence on microbial biomass and soil enzyme activities ofall poplar rhizosphere soil. There was a significant difference in microbial C/N ratio in bothrhizosphere and bulk soils for all three poplar clones among different sampling times. Thehighest C/N ratio appeared in later October2010, while the lowest was observed in middle July2011, where the C/N ratios for three poplar clones were below5.0. Significant variations in thecontents of MBC and MBN as well as the C/N ratio were observed between rhizosphere and bulksoil for all tested poplar clones. In the rhizosphere soil, significant differences in tested enzymes(means of three poplar clones) were observed among three sampling times, and the greatestenzyme activities always appeared in the middle July2011. However, seasonal dynamics of theenzyme activities varied among the poplar clones.（6）Soil microbial bacteria and fungi diversities varied significantly in the rhizosphere soilsamong different three poplar clones in the same sampling time. In the early November of2010,Nanlin-895was observed significantly higher than in75and NL80351. In the later march of2011, Soil microbial bacteria and fungi diversities in the rhizosphere of Nanlin-895and NL80351were significantly higher than in75clones. Significant difference in microbial diversity was notobserved between Nanlin-895and NL80351. For the seasonal aspects, rhizosphere soil microbialdiversity of all three poplar clones was significantly higher in winter than in summer or spring.(7)RSMIs were estimated from various sampling positions of rhizosphere soils. The RSMIvalue varied from0.23to0.66for the rhizosphere soil sampled within0-2mm to the root mats,0.27-0.65within the0-4mm, and0.39-0.59within the0-10mm, respectively. However, a significant difference in RSMI was observed among the five tree species for all samplingpositions of rhizosphere soils. All the highest RSMI occurred in Nanlin-895, while the lowestwas found in alder. A significant difference in microbial biomass and enzymatic activities wasobserved among the three sampling positions. However little difference in microbial biomass andtested enzymes was detected between the0-2mm and2-4mm distances to the root mats. Resultsfrom this study suggest that defining the soil at0-4mm distance from the root mats asrhizosphere soil should be more reliable for this in situ rhizobox design and experiment.Microbial properties of rhizosphere soil are significantly affected by trees species. MBC, pH,MBN/TN, MBC/MBN and MBC/SOC could be important properties for appraising rhizospheresoil quality of seasonal flooded lands. The RSMI calculated by integrating microbial propertiessuggested that two poplar clones (Nanlin-895and75) and a willow clone (Salix×jiangsuensiscv. J799) could be used for afforestation at seasonal flooded lands of Yangtse River. |
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