Nitrogen fertilization alters microbial community composition and reduces the soil organic carbon pool in an old-growth subalpine forest

Background/Question/Methods

Anthropogenically driven increases in carbon (C) and nitrogen (N) have decoupled C and N biogeochemical cycles leading to severe environmental degradation such as climate change and eutrophication. The soil microbial community mediates many biogeochemical processes, and changes in community composition or function, such as those associated with climate change, likely influence ecosystem-level properties such as C storage. We hypothesized long term N fertilization of a subalpine forest has altered soil C by shifting the composition of the microbial community and the proportion and composition of organic matter within the total, dissolved and microbial biomass pools. We expected that N fertilization would result in a shift from fungal to bacterial dominance in subalpine forest soils. Because bacteria have a lower biomass C:N ratio than fungi, and microbial biomass has been shown to be a major component of soil organic matter, we expected this microbial community shift would result in lower soil organic C (SOC), lower dissolved organic C (DOC), and lower microbial biomass C (MBC) with N fertilization. 

Results/Conclusions

This work was conducted at the Loch Vale watershed in Rocky Mountain National Park, a subalpine ecosystem with an ongoing N fertilization experiment (15 kg NH₄⁺, NO₃^- /yr, 16 years to date). O horizon soils were collected in July and September 2012, homogenized, sieved to 8 mm and extracted with water and 1% chloroform. Microbial community composition was examined with qPCR. Overall we found that with fertilization: 1) bacteria made up a greater proportion of the microbial community in July, but not in September; 2) the SOC was overall lower by 13%; 3) there was no effect on the quantity of the DOC; and 3) MBC was lower by 25% . Given these results, the microbial community composition may not be the primary mechanism responsible for the observed C dynamics following N fertilization. However, N fertilization did result in lower C storage and MBC pools. Future work to improve our understanding of microbial community composition and microbial contribution to SOC dynamics will include phospholipid fatty acid and amino sugar analyses, and to determine if fertilization is effecting DOC chemical composition and quality, rather than quantity. The chemical makeup of DOC may be additive with microbial community composition shifts resulting in the observed lower C storage observed with N fertilization in this subalpine forest.