Abstract
Abstract. Recently a considerable amount of effort has been put into quantifying how interactions of the carbon and nitrogen cycle affect future terrestrial carbon sinks. Dynamic vegetation models, representing the nitrogen cycle with varying degree of complexity, have shown diverging constraints of nitrogen dynamics on future carbon sequestration. In this study, we use LPJ-GUESS, a dynamic vegetation model employing a detailed individual- and patch-based representation of vegetation dynamics, to evaluate how population dynamics and resource competition between plant functional types, combined with nitrogen dynamics, have influenced the terrestrial carbon storage in the past and to investigate how terrestrial carbon and nitrogen dynamics might change in the future (1850 to 2100; one representative "business-as-usual" climate scenario). Single-factor model experiments of CO2 fertilisation and climate change show generally similar directions of the responses of C–N interactions, compared to the C-only version of the model as documented in previous studies using other global models. Under an RCP 8.5 scenario, nitrogen limitation suppresses potential CO2 fertilisation, reducing the cumulative net ecosystem carbon uptake between 1850 and 2100 by 61%, and soil warming-induced increase in nitrogen mineralisation reduces terrestrial carbon loss by 31%. When environmental changes are considered conjointly, carbon sequestration is limited by nitrogen dynamics up to the present. However, during the 21st century, nitrogen dynamics induce a net increase in carbon sequestration, resulting in an overall larger carbon uptake of 17% over the full period. This contrasts with previous results with other global models that have shown an 8 to 37% decrease in carbon uptake relative to modern baseline conditions. Implications for the plausibility of earlier projections of future terrestrial C dynamics based on C-only models are discussed.
Highlights
The nature of future climate change will depend on anthropogenic emissions of CO2, and climate and CO2-mediated feedbacks through carbon (C) cycling in both terrestrial ecosystems and oceans (Friedlingstein et al, 2006; Heimann and Reichstein, 2008; Sitch et al, 2008; Arneth et al, 2010a; Raupach, 2011)
A reduced past-topresent cumulative C sequestration in C–N versions of terrestrial models seems to emerge as a robust pattern that has been found in previous studies (Table 1), even though the relative importance of N–C interactions in LPJ-GUESS is comparatively large compared with other models
The differences imply N limitation of C cycling over this period, but indirect effects vary as the C-only and C–N configurations of the model differ in plant functional types (PFTs) composition and C pool sizes after the spin-up (Appendix Table A2)
Summary
The nature of future climate change will depend on anthropogenic emissions of CO2, and climate and CO2-mediated feedbacks through carbon (C) cycling in both terrestrial ecosystems and oceans (Friedlingstein et al, 2006; Heimann and Reichstein, 2008; Sitch et al, 2008; Arneth et al, 2010a; Raupach, 2011). Considerable attention has focused in recent years on whether and how interactions of the C and nitrogen (N) cycles will affect future terrestrial ecosystem C balance. Until relatively recently these interactions were not considered in models of the global C cycle, in many ecosystems N is regarded as a limiting factor, controlling C uptake in present-day environments Relatively few full-scale multi-factorial ecosystem experiments have been conducted to date, and these have been of limited duration, leaving the effects of interactions among drivers of, and on, future change in climate, atmospheric CO2 and other environmental changes such as atmospheric N deposition unclear (Beier, 2004; Leuzinger et al, 2011)
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