Varied nitrogen deposition effects on decomposition are driven by site-specific fungal communities in temperate forests

Background/Question/Methods

Nitrogen (N) deposition impacts soil carbon turnover and storage by altering the rates at which soil microbes decompose soil organic matter (SOM). However, impacts have been shown to vary across temperate forests, and multiple mechanisms have been suggested to control these responses including alterations of enzyme activity, microbial communities, and physical changes in soil structure.  Recent findings suggest that forests varying in tree and microbial community composition may respond differently to N deposition. Temperate forest trees that associate with arbuscular mycorrhizal (AM) fungi have cellulose-rich litter, exist in high N soils, and depend on free-living microbes to mobilize N.  In contrast, ectomycorrhizal (ECM) trees produce litter high in lignin, exist in low N soils, and depend on their fungal symbionts to access N.  Thus, the objectives of this research were to investigate how differences in organic matter chemistry and microbial community composition between ECM- and AM-dominated forests mediate decomposition responses to experimental N addition.  We quantified the impacts of N addition on leaf and SOM decomposition in forest stands dominated by either AM- or ECM-associated trees in southern Indiana.  Using ingrowth treatments that isolated fungal communities, we examined how N addition impacted extracellular enzyme production by AM, ECM, and saprotrophic fungi.

Results/Conclusions

After 2 years of N fertilization, we found that N addition increased litter decomposition in AM plots (p=0.054), but not in ECM plots (p=0.703).  Additionally, cellobiohydrolyase (cellulose-degrading) activity significantly increased in response to fertilization in AM (p=0.092) but not ECM plots (p=0.91). These results suggest that N deposition enhances microbial decomposition of labile cellulose in AM sites, possibly by alleviating microbial demand for N.  Thus, it appears that the mycorrhizal association of temperate forest trees controls the magnitude and direction of N deposition impacts on soil C storage. At the mechanistic level, we found that N addition tended to induce declines in the phenol oxidase activities of both ECM and saprotroph fungi in ECM plots, though these effects were not significantly different (p>0.1).  In AM plots, N deposition responses were dominated by changes in the physical structure of the soil with decreases in the abundance of small macroaggregates (p=0.006), likely driven by declines in the production of glomalin by AM fungi. This suggests that declines in SOM decomposition in ECM plots are driven by direct shifts in microbial activity whereas the increase in SOM decomposition in AM plots is driven by a reduction in the physical protection of SOM.