What's P got to do with it? Mycorrhizal and biochemical response to P and lime additions in acidic hardwood forests

Background/Question/Methods

Like many hardwood forests of the Northeastern United States, Ohio forests have received significant acid deposition since the industrial revolution, potentially reducing the soil pH. With pH<5, net primary productivity should be essentially P limited in these systems; however, previous studies have not demonstrated such limitation. We hypothesize that shifts in the structure and function of microbial communities ameliorate P limitation by accessing P that is otherwise unavailable for direct plant uptake. We tested this hypothesis by manipulating soil pH and P availability in acidic hardwood forests of Northeastern Ohio (NO) and Southeastern Ohio (SO). In 2009, three treatments were initiated: elevated pH, elevated P, and both elevated pH and P (x-treatment). To quantify treatment responses after one year, we examined ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) colonization and communities. ECM roots were categorized by morphotype and sequenced with fungal ITS primers to identify the fungal symbiont for each ECM morphotype. AM roots were lyophilized and ground in liquid nitrogen prior to DNA extraction and T-RFLP analysis of the AM fungal community. Additionally, we measured the activity of soil extracellular enzymes associated with P, N, and C cycling.

Results/Conclusions

A total of 27,064 ECM root tips were counted and a total of 764 morphotype groups were determined. Plots from SO had nearly three times more ECM root tips than the NO sites; however, the number of ECM root tips and morphotypes per plot varied greatly and there were no significant differences in the number of ECM roots or morphotypes among the treatments. The AM root biomass was similar across the NO and SO sites, but there was a significant treatment effect, with elevated P plots having twice the AM root biomass on a dry weight basis compared to the control. Interestingly, AM root biomass did not respond to elevated pH or the x-treatment. All enzyme activities were lower than expected and did not vary among the treatments, likely because the late summer 2010 was exceptionally dry. Enzyme activity did differ between the regions with NO generally having greater activity than SO. However, this difference may further reflect the effect of water content on enzyme activity as the SO sites had a significantly lower water content. These results demonstrate some consistent treatment responses across regions and suggest that AM and ECM respond differently to the treatments.