Long-term nitrogen enrichment affects the structure of soil fungal communities

Nitrogen (N) deposition from fossil fuel burning has the potential to affect ecosystem processes such as the decomposition and storage of soil organic matter. The Harvard Forest Chronic Nitrogen Addition experiment was established in 1989 to test the effects of long-term N fertilization on ecosystem processes in a northeastern US mixed-hardwood forest. Three plots receive one of three treatments: ambient N deposition (control), 50 kg N ha ^-1 yr ^-1 (low N), or 150 kg N ha ^-1 yr ^-1 (high N). Researchers at this site have observed an accumulation of soil C in the N fertilized plots and a decrease in fungal biomass, ligninolytic enzyme activity, and rates of litter decay. Soil fungi, especially Basidiomycetes, are the primary decomposers of lignin in these communities. We hypothesized that decreased decomposition rates in N fertilized plots may be due to a decrease in diversity and changes in the structure of the fungal community. We performed a marker gene study of fungal communities in the organic soil horizon using 454 high-throughput sequencing of three separate loci: the internal transcribed spacer units 1 and 2 of fungal ribosomal DNA and the D2-D3 region of Basidiomycete 28s ribosomal DNA.

The dominant fungal genus in all treatments was the Russulas, with one OTU being dominant (9.71% in control, 29.84% in low N, 37.91% in high N). This OTU significantly increased in relative abundance by 28.2% from control to high N (P < 0.05). The ACE richness estimator indicated higher species richness in the high N community, though there were no significant differences in Shannon diversity estimates. Fungal communities in high N soils had different community structure than control and low N soils as predicted with OTU based beta-diversity metrics. The Basidiomycete community was structured by both low and high levels of N as predicted by phylogenetic beta-diversity metrics. Taxa biplots against OTU based beta-diversity estimates showed the Amanitaceae associating with the control treatment, the Thelephoraceae and Tricholomataceae associating with the low N treatment, and the Sclerodermataceae and Mortierellaceae associating with the high N treatment. Long-term N additions have altered fungal community structure, which may contribute to the declines in decomposition rates observed under N additions.