Nitrogen pollution impact on bogs depends on N load and plant performance - a modelling analysis

Christian Blodau1, Yuanqiao Wu1
1 University of Muenster

O 6.5 in Biogeochemistry of wetlands

15.07.2014, 12:20-12:40, H17

Nitrogen (N) pollution of peatlands alters their carbon (C) balances, yet long-term effects and controls are poorly understood. We applied the model PEATBOG to analyze impacts of long-term nitrogen fertilization on C cycling in an ombrotrophic bog. Simulations of summer gross ecosystem production (GEP), ecosystem respiration (ER) and net ecosystem exchange (NEE) were evaluated against 8 years of observations and extrapolated for 80 years to identify potential effects of N fertilization and factors influencing model behavior. The model successfully simulated moss decline and raised GEP, ER and NEE on fertilized plots. GEP was systematically overestimated in the model compared to the field data due to high tolerance of Sphagnum to N deposition in the model. Model performance regarding the 8-year response of GEP and NEE to N was improved by introducing a N content threshold shifting the response of photosynthesis capacity to N content in shrubs and graminoids from positive to negative at high N contents. Such changes also eliminated the competitive advantages of vascular species and led to resilience of mosses in the long-term. Regardless of the large changes of C fluxes over the short-term, the simulated GEP, ER and NEE after 80 years depended on whether a graminoid- or shrub-dominated system evolved. When the peatland remained shrub-Sphagnum dominated it shifted to a C source after only 10 years of fertilization at 6.4g N m-2 yr-1, whereas this was not the case when it became graminoid-dominated. The suppression of mosses by N fertilization was mitigated by daily deposition of N at low concentration. An extinction of mosses occurred only in the simulated N deposition with a high concentration and a high load, analogous to fertilization experiments. The results suggested that the observed detrimental effect of N on mosses were most likely due to the toxic effect of N on mosses owed to the internal stress between N uptake and N assimilation. Therefore, N deposition at a high concentration may raise the N content in mosses and induce toxicity more effectively, highlighting the importance of considering N concentration in the discussion of critical load of N.  

The modeling results overall highlight the importance of ecosystem adaptation and reaction of plant functional types to N deposition when predicting the future C balance of N-polluted cool temperate bogs.

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last modified 2014-06-19