Nitrogen deposition leads to a weaker carbon sink in a nutrient-limited bog

Background/Question/Methods

Atmospheric nitrogen (N) deposition is increasing owing to fossil fuel burning and agriculture. In nutrient-limited peatland ecosystems, the excess of reactive N has been found to increase vascular plant growth, but decrease Sphagnum growth. Higher vascular plant abundance and higher nutrient content alter decomposability of plant litter. These changes are likely to affect net imbalance of production and decomposition and thus carbon (C) accumulation in peatlands, which store about a third of global soil C. We studied whether the vegetation feedbacks of N deposition lead to stronger or weaker C sink in a nutrient-limited bog. We investigated vegetation and ecosystem CO₂ exchange at Mer Bleue Bog, Canada that has been fertilized with NO₃ and NH₄ (2-8 times ambient annual wet deposition) with or without phosphorus (P) and potassium (K) for 7-12 years. Gross photosynthesis, ecosystem respiration and net CO₂exchange were measured weekly during May-September 2011 using chambers. To isolate microbial respiration, a substrate induced respiration technique was used to determine whether fertilization increased the basal rate of microbial respiration in peat or changed the functional ability of the microbial community. To examine vegetation changes with increasing N influx, vegetation structure in 2011 was compared with that in 2001-2009.

Results/Conclusions

The highest nutrient additions were associated with 40% less net CO₂ uptake than in the control. The two sets of treatments revealed contrasting patterns: in the NPK additions, a diminished C sink potential was due to a 20-30% increase in the ecosystem respiration, while gross photosynthesis rates did not change as increased vascular plant biomass compensated for the loss of Sphagnum mosses. In the highest N only addition, the gross photosynthesis was reduced, but ecosystem respiration was unaltered. Substrate induced microbial respiration was significantly higher in all levels of NPK additions compared to control. A weaker C sink potential could be explained by changes in nutrient availability, higher woody:foliar ratio, moss loss, and enhanced decomposition. Stronger responses to NPK fertilization than to N only fertilization for both shrub biomass production and decomposition suggest that the plant and microbial communities of this bog ecosystem are N-P/K co-limited rather than N limited, likely due to prolonged elevated atmospheric N deposition in the region. The two mechanisms enhancing N induced C sink in forests: increased formation of woody biomass and accumulation of surface litter, seem to reduce C storage capacity in bogs owing to the loss of main peat forming mosses.