Effects of experimental nitrogen deposition on peatland carbon pools and fluxes: a modelling analysis

摘要

Nitrogen (N) pollution of peatlands alters their carbon (C) balances, yetlong-term effects and controls are poorly understood. We applied the modelPEATBOG to explore impacts of long-term nitrogen (N) fertilization on Ccycling in an ombrotrophic bog. Simulations of summer gross ecosystemproduction (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 factorsinfluencing model behaviour. The model successfully simulated moss declineand raised GEP, ER and NEE on fertilized plots. GEP was systematicallyoverestimated in the model compared to the field data due to factors thatcan be related to differences in vegetation distribution (e.g. shrubs vs.graminoid vegetation) and to high tolerance of vascular plants to Ndeposition in the model. Model performance regarding the 8-year response ofGEP and NEE to N input was improved by introducing an N content thresholdshifting the response of photosynthetic capacity (GEP_max) to N contentin shrubs and graminoids from positive to negative at high N contents. Suchchanges also eliminated the competitive advantages of vascular species andled to resilience of mosses in the long-term. Regardless of the largechanges of C fluxes over the short-term, the simulated GEP, ER and NEE after80 years depended on whether a graminoid- or shrub-dominated system evolved.When the peatland remained shrub–Sphagnum-dominated, it shifted to a C source afteronly 10 years of fertilization at 6.4 g N m?2 yr?1, whereas thiswas not the case when it became graminoid-dominated. The modelling resultsthus highlight the importance of ecosystem adaptation and reaction of plantfunctional types to N deposition, when predicting the future C balance ofN-polluted cool temperate bogs.