Mycorrhizal guilds decline similarly with depth in two northern hardwood forests

Background/Question/Methods

Plants depend on mycorrhizas for mineral nutrient uptake.  In the northeast, mycorrhizal hosts largely fall into two categories, those that form arbuscular mycorrhizas and those that form ectomycorrhizas.  Evidence from global plant distributions as well as laboratory experiments has implicated the arbuscular mycorrhizas in the mining of phosphorous from otherwise inaccessible mineral pools.  In contrast, ectomycorrhizal hosts often dominate in nitrogen-limited environments, which suggests that they may be more specialized in nitrogen acquisition.  Thus we might expect change in the fungal community at the guild level as we descend from the soil surface to the mineral soil, reflecting changes in available nutrient conditions.  To investigate whether such a change was observable we collected soils from two northern hardwood stands in central New Hampshire.  A total of 80 cores were taken, 40 collected at 30-50 cm depth in the mineral soil and 40 from the top 10 cm including the forest floor.  The length of roots in each host category was assessed as well as the colonization of the roots by each guild of fungi through microscopic observation. 

Results/Conclusions

We found no difference in the ratio of arbuscular mycorrhizal root colonization to ectomycorrhizal colonization across depth (p=0.45).  The ratio is near 1:1 in all cases examined.  Both the arbuscular mycorrhiza and the ectomycorrhizas were found to colonize roots more completely in the shallow soil (p<0.001 in both cases).  Each guild was also found to have a nearly double the length of colonized roots in the shallow soil compared to the deep, though the ratio of colonized lengths also did not change across depths (p=0.7).  The stands did not differ either in the proportion of roots colonized by each guild (p=0.90), nor in the length of roots colonized by each guild (p=0.99).   That these measures of colonization should be so consistent between the two guilds and the two sites was unexpected. Comparison to work in other sites could provide more information regarding the controls on these two mycorrhizal systems in mixed-host stands