Nutrient availability influences rhizosphere effect on soil microbial properties

Background/Question/Methods

Rhizosphere soils receive a substantial amount of belowground allocation of plant photosynthetic carbon (C).  These organic C compounds are easily available energy resources for microbial growth and activity.  Consequently, we expect rhizosphere soils to support larger microbial populations and higher decomposing activity compared to root-free (bulk) soils, an idea referred to as the rhizosphere effect.  Soil nutrient availability in forest ecosystems has the potential to influence belowground C allocation, and consequently the rhizosphere effect.  We tested the microbial response in the rhizosphere to elevated (nitrogen) N, (Phosphorus) P, and N+P in a fertilization study in northern hardwood forests, NH.  Rhizosphere and bulk soils of two tree species, red maple and yellow birch, were sampled in fertilized and control plots of three forest stands.  We analyzed microbial respiration, N mineralization, and fungi:bacteria ratio, and calculated the rhizosphere effect for these parameters as the relative difference between rhizosphere and bulk soils: (rhizosphere-bulk)/bulk. 

Results/Conclusions

Rhizosphere effect was shown in our study with higher microbial abundance, respiration, and N mineralization in rhizosphere compared to bulk soils.  The response to N addition depended on tree species: elevated N decreased the rhizosphere effect on bacteria abundance, microbial respiration and N mineralization with red maple, but not with yellow birch.   Phosphorus did not influence the rhizosphere effect.  Our results indicate that high N availability diminishes the difference between rhizosphere and bulk soil for microbial properties.  In red maple, responses by microbial respiration in the rhizosphere suggests that N addition either reduced root C supply or increased microbial carbon use efficiency.  The lack of effect in yellow birch indicates that C allocation at the ecosystem level depends on species-specific responses to fertility.  This study suggests that elevated availability of N but not P affects forest ecosystem C and nutrient fluxes via plant-soil interactions in rhizosphere.