Responses of soil microbes to N deposition: Implications for ecosystem function

Background/Question/Methods

Nitrogen (N) deposition can affect soil microbial community structure and function via a variety of mechanisms. Direct effects can be caused through increases in the substrate pool for nitrifiers, denitrifiers and for general microbial growth and assimilation. Indirect effects can be mediated through changes in the quality and quantity of plant root exudates and litter inputs as well as shifts in soil pH. Here, we describe results from fertilization experiments, lab incubations and natural nutrient gradients in an attempt to better understand the contributions of both direct and indirect effects of N deposition on soil microbial communities. 

Results/Conclusions

Data from three different ecosystems reveal that archaeal relative abundance is responsive to N fertilization, but that the magnitude and direction of effects vary both within and between ecosystems. For example, the ratio of bacteria:archaea decreased nearly twenty-fold in a long-term tundra fertilization experiment. In a lowland tropical rainforest, fertilization also decreased the relative abundance of archaea, but only when N was applied in combination with phosphorus (P). Interestingly, in a temperate forest soil, N fertilization increased the relative abundance of archaea in the mineral soil while decreasing it in the litter layer. Results from laboratory incubation experiments demonstrate that N amendments cause increases in the overall efficiency of soil microorganisms as well as shifts in the C:N ratios of microbial biomass when added with or without carbon (C). Incubation studies also suggest that changes in the microbial community structure in response to N amendments may be more subtle in the short term than those observed in fertilization experiments. However, when C and N were added in combination, more pronounced shifts in microbial community composition occurred. Analysis of microbial communities over natural gradients in soil nutrients supports that archaea are responsive to variation in N pools and are found in higher relative abundance in soils with lower C:N ratios. Functional gene analyses reveal that these shifts in overall archaeal relative abundance may reflect changes in nitrifier population sizes, but that these relationships are complex. Together our results demonstrate that the responses of soil microbes to N deposition are ecosystem specific, show strong interactions with C and P, and likely reflect both indirect and direct effects.