Greater variations of rhizosphere effects within mycorrhizal group than between mycorrhizal group in a temperate forest

The presence of living roots can markedly change soil properties, microbial activities, and biogeochemical cycling in the rhizosphere compared to bulk soil. Such rhizosphere effect has been increasingly recognized to play a crucial role in carbon (C) and nitrogen (N) cycling in terrestrial ecosystems. Here, we investigated the rhizosphere effects of 12 tree species associated with two contrasting mycorrhizal types (arbuscular mycorrhizal, AM vs. ectomycorrhizal, ECM) and co-occurring in a temperate secondary forest in Northeast China. The adhering soil method was adopted to sample paired rhizosphere and bulk soils of field-grown trees in the peak growing season. Generally, we found positive rhizosphere effects on soil properties (ammonium and nitrate content, total C and N content, and C:N ratio), microbial abundances (bacteria, fungi, and actinomycetes), enzyme activities (C- and N-degradation enzymes), and C mineralization rate (Cmin), but minor rhizosphere effects on soil pH, phosphorus content and phosphatase activity, net N mineralization rate (Nmin), and specific Cmin and Nmin (per unit soil C or N). Because of large intraspecific and particularly interspecific variations, less than half of the rhizosphere effects were statistically significant (P < 0.05), and none of the rhizosphere effects differed significantly between the two mycorrhizal types (AM vs. ECM). Moreover, most soil variables varied greatly within and across species, and thus did not differ significantly between the two mycorrhizal types (AM vs. ECM). However, soil pH, nitrate content, microbial abundance, and C-degradation enzyme activity were notably lower under ECM species than under AM species. Collectively, these results show that rhizosphere effects varied greatly among the six species within both mycorrhizal groups, and such variations within mycorrhizal group were even larger than those between mycorrhizal group. Future work should include a larger number of tree species to increase statistical power, and combine phylogenic history and functional traits of plant species with the type and traits of mycorrhizal fungi to better understand how plant-microbe interactions influence biogeochemical cycling in the rhizosphere.