Linking the rhizosphere effects of 12 woody species on soil microbial activities with soil and root nitrogen status

The rhizosphere effect depicts the changes in soil physical, chemical, and biological properties due to altered microbial activity in response to root activity, which has important implications for soil carbon and nutrient cycling. However, how the plant-induced changes in rhizosphere soil microbial activity correlate with root and soil nutrient status remains uncertain. Here we targeted 12 woody species of distinct growth forms, i.e., tree vs. shrub, in an urban park on a university campus in Beijing, China. We measured root carbon and nitrogen concentrations and soil properties of paired rhizosphere and bulk soils for each plant. We investigated how the rhizosphere effects on microbial properties, as indicated by microbial biomass and potential activity of soil extracellular hydrolases, varied among the 12 woody species and between trees and shrubs, as well as how they were regulated by bulk soil and root nitrogen status. In general, across the 12 species, root nitrogen content, which is a potential indicator of plant life span and nutrient acquisition strategy, was positively correlated with the rhizosphere effect on potential enzyme activity, while the inverse was true for root carbon: nitrogen ratio, which further verified the role of root functional traits in controlling soil nutrient cycling through the rhizosphere effect. Moreover, soil organic carbon, total nitrogen, and available nitrogen concentrations were all found to be negatively correlated with the rhizosphere effect on microbial properties, which highlighted the stimulation of rhizosphere microbial activity for nitrogen acquisition (the nitrogen-mining hypothesis). Trees and shrubs exhibited no significant differences in their rhizosphere effects, and there was a potential difference in the role of rhizosphere effect in regulating soil nitrogen cycling between legumes and non-legumes. Overall, our study highlights the role of plant nutrient acquisition strategy and soil nutrient status in regulating the rhizosphere effect of woody species on microbial-driven soil processes, which is essential for a better understanding of mechanisms by which plants sustain their nutrient supply.