Seasonal and clonal variations of microbial biomass and processes in the rhizosphere of poplar plantations

• Magnitudes of rhizosphere effects vary with sampling periods and poplar genotypes.
• Rhizosphere NO3−-N regulates seasonal dynamics of microbial biomass and processes.
• Clonal variations in microbial biomass and tested enzyme activities were significant.
• Poplar clones obviously affected rhizosphere net N mineralization and nitrification rates.

Plant nutrient acquisition and dynamics are closely coupled with microbial activities and rhizosphere processes. This study was conducted using an in situ rhizobox approach and incubation experiments, with the primary goal to elucidate rhizosphere effects of different poplar clones at different sampling seasons on soil microbial processes and biomass content. Evaluations included pH, inorganic N (NO3−-N and NH4+-N), microbial biomass carbon (MBC) and nitrogen (MBN), enzyme activities related to N cycling, as well as net N mineralization and nitrification rates in rhizosphere soils of three poplar clones (Nanlin-895, NL-80351 and 75) at three periods of the year. The magnitude of rhizosphere effects varied with time and poplar genotypes. Significant variations in microbial biomass content and enzyme activities were observed among the tested three poplar clones; and microbial biomass content was positively correlated with soil NO3−-N concentration. The obtained results suggested that genotypes (clones or cultivars) had profound impacts on the rhizosphere microbial communities; and NO3−-N concentration rather than total inorganic N was the primary determinant of seasonal dynamics of microbial biomass and processes in the rhizosphere soils of poplar trees in a seasonally flooded soil.