Structure And Functions Of Microbial Community In The Rhizosphere Of Mixed Poplar And Alder Plantation

Long-term and continuous monoculture is the vital causation leading to site degradation in poplar plantation management.Nitrogen supply is generally supposed to be the key factor restricting poplar growth.Compared to pure plantation,mixed plantation with different tree species could shift the composition and functions of soil microbial community through diversified litter return and root excretion,and then promote soil nutrition transformation and improve nutrient supply.Therefore,mixing with appropriate tree species,especially with nitrogen-fixation tree species,when the plantation was established,should be an effective way to improve N supply,and to alleviate site degradation to some extent.Previous studies showed that the effects of tree mixing could be detected only after long growth period.It is not clear whether such effects exist after relatively short term after mixed plantation establishment.Rhizosphere is the entrance for the trees to assimilate nutrients from soil,and is the adjacent soil zone affected directly from root growth and metabolism.The properties of rhizosphere closely relate with tree growth.It could act as effective and sensitive indexes to evaluate the nutrition supply in mixed plantation.In this paper,a comparing study was conducted in a 5-year old mixed plantation with poplar and alder and a pure poplar plantation to analyze the regulating effects of mixing with N-fixation tree species on the structure and functions of microbial community in the rhizosphere,and to evaluate the feedback to nutrient transformation and supply.The results showed that soil nutrient condition,enzyme activities,microbial community structure and function presented a regular variation during different growing season in the investigation year.Microbial metabolism activity was the highest in leaf-expanding period,and the observed species richness and diversity indexes of soil bacteria and fungi were higher in mixed plantation than in pure poplar plantation during this period.On the contrary,pure poplar plantation showed more significant rhizosphere effects of soil nutrient and decomposition enzyme activities than mixed plantation.The rhizosphere effect of total organic carbon,nitrogen and phosphorus contents were 34.2%,53.7% and 25.5%,respectively,in pure plantation,and were 20.7%,40.0% and 7.0%,respectively,in mixed plantation.Similarly,the activity of ?-glucosidase,arylamidase,and acid and alkaline phosphatase were 1.24,1.23,1.21,and 1.43 times,respectively,higher in rhizosphere than in bulk soil for mixed plantation,while for pure plantation,such corresponding enzyme activities were 1.65,2.75,1.36 and 1.79 times,respectively,higher in rhizosphere than in bulk soil.The regulating effects of mixed plantation were much significant during the fast-growing season.The nitrogen content was 15.9% higher in the rhizosphere of mixed plantation than in pure plantation,and the rhizosphere effect of soil enzyme activities and community metabolism functions were also significantly higher in mixed plantation during this period.In defoliation season,the observed species richness and metabolic activity of soil microorganisms were greatly decreased when compared to the previous seasons.The species of bacteria and fungi showed a 65.3% and 38.3% decrease,respectively,comparing to leaf-expanding period.The contents of soil total nutrients increased slightly,while the contents of available nutrients and enzyme activities were decreased significantly.No significant difference was found for the rhizosphere effect of nutrient between mixed and pure plantations.However,significant positive rhizosphere effects were still detected for the activities of beta-glucosidase,arylamidase,and acid and alkaline phosphatase in mixed plantation,showing an increase of 60.1%,37.7%,29.1%,and 22.2%,respectively,in rhizosphere than in bulk soil.As a conclusion,mixing alder into poplar plantation could significantly increase the diversity and utilization efficiency for C resource of soil microorganism in rhizosphere,and,therefore,accelerate nutrients transformation and improve soil N supply.The results could provide available information for the sustainable management on establishment of mixed poplar plantation.