Contrasting effects of N and P on rhizosphere processes in two northern hardwood species

Rhizosphere soils receive a substantial input of plant carbon (rhizosphere C flux, RCF), and therefore support larger microbial populations and higher decomposing activity compared to bulk soils. The relative difference between rhizosphere and bulk soils (rhizosphere effect) may be sensitive to the effects of soil nutrient availability on plant belowground C allocation and RCF. However, it is not known whether rhizosphere effects are similarly responsive to availability of different nutrients, such as N or P, or to a combination of N and P. We tested the effects of elevated nitrogen (N) and phosphorus (P), alone and in combination, on microbial activity and abundance in bulk and rhizosphere soils associated with two tree species, red maple and yellow birch, in young, possibly N-limited northern hardwood forests in a long-term fertilization study in New Hampshire, USA. We found that N addition increased microbial respiration, soluble inorganic N, and potential net N mineralization and nitrification, and decreased fungal abundance and fungal:bacterial ratios in red maple soils, but decreased microbial respiration, fungal abundance and the F:B ratio in yellow birch soils. Positive rhizosphere effects were observed for microbial respiration, potential net N mineralization, and microbial abundance for both tree species. Nitrogen addition suppressed rhizosphere effects on microbial activity and abundance in red maple soils but did not alter rhizosphere effects in yellow birch soils. Our results show strong effects of N and some interactions with P on soil processes, microbial abundance and rhizosphere effects in these forests. Nitrogen effects strongly depended on tree species, suggesting the importance of differential responses to nutrients between arbuscular and ectomycorrhizal associations, and indicating the importance of belowground processes to species-specific effects on C and mineral nutrient transformations.