Does eutrophication affect interactions of arbuscular mycorrhizal fungi with tropical dry forest tree seedlings?

Background/Question/Methods

Atmospheric nutrient deposition affects vegetation composition and performance in various ecosystems.  Deposition-mediated increases in soil nutrients might also affect plant investment in nutritional symbionts such as arbuscular mycorrhizal fungi (AMF).  Here we present results from an experiment studying the effects of nitrogen (N) and phosphorous (P) addition on AMF densities in tropical dry forests tree seedlings in India.  We asked: (a) How does N and P addition affect plant investment in AMF?  And (b) Are AMF responses to increased nutrient availability different in N-fixing and non-N fixing host plants, given their different nutrient acquisition strategies and requirements?  We conducted a factorial experiment with seedlings of 13 dominant dry forest tree species, 6 of which were N-fixers, subjected to N and P addition (as urea and single super phosphate respectively).  Six months after nutrient addition, the seedlings were harvested and AMF densities measured.  Inferences on AMF responses to nutrient addition were based on log response ratios and associated 95% confidence intervals (CIs), calculated using a meta-analytical framework.

Results/Conclusions

At a functional group level, there were no consistent patterns of AMF responses to nutrient addition, with CIs spanning zero in all cases.  However, on average, AMF densities in N-fixers increased by 19.5% and 30.5% under N and N+P addition respectively.  In non-N-fixers, densities declined by 6.5% and 10.5% respectively.  P treatment responses were weaker in both functional groups.  AMF density responses were also highly host species specific, with contrasting responses in species of the same genera.  These results suggest that AMF responses to nutrient addition cannot be generalized for plant functional groups based on N-fixation.  Species-level differences may be due to different basal AMF densities or nutrient acquisition strategies.  It could also be that any one nutrient does not drive host investment in AMF, given the multiple roles of AMF symbionts of plants.