Nutrient limitation reduces land carbon uptake in simulations with a model of combined carbon, nitrogen and phosphorus cycling

[摘要] Terrestrial carbon (C) cycle models applied for climate projections simulatea strong increase in net primary productivity (NPP) due to elevatedatmospheric CO₂ concentration during the 21st century.These models usually neglect the limited availability of nitrogen (N) andphosphorus (P), nutrients that commonly limit plant growth and soil carbonturnover. To investigate how the projected C sequestration is altered whenstoichiometric constraints on C cycling are considered, we incorporated a Pcycle into the land surface model JSBACH (Jena Scheme for Biosphere–Atmosphere Coupling in Hamburg), which already includesrepresentations of coupled C and N cycles.

The model reveals a distinct geographic pattern of P and N limitation. Underthe SRES (Special Report on Emissions Scenarios) A1B scenario, the accumulated land C uptake between 1860 and 2100 is13% (particularly at high latitudes) and 16% (particularly at lowlatitudes) lower in simulations with N and P cycling, respectively, than insimulations without nutrient cycles. The combined effect of both nutrientsreduces land C uptake by 25% compared to simulations without N or P cycling.Nutrient limitation in general may be biased by the model simplicity, butthe ranking of limitations is robust against the parameterization and the inflexibility of stoichiometry.After 2100, increased temperature and high CO₂concentration cause a shift from N to P limitation at high latitudes, whilenutrient limitation in the tropics declines. The increase in P limitation athigh-latitudes is induced by a strong increase in NPP and the low P sorptioncapacity of soils, while a decline in tropical NPP due to high autotrophicrespiration rates alleviates N and P limitations.The quantification of P limitation remains challenging.The poorly constrained processes of soil P sorption and biochemical mineralizationare identified as the main uncertainties in the strength of P limitation.Even so, our findings indicate thatglobal land C uptake in the 21st century is likely overestimatedin models that neglect P and N limitations. In the long term, insufficient Pavailability might become an important constraint on C cycling at highlatitudes. Accordingly, we argue that the P cycle must be included in globalmodels used for C cycle projections.

[学科分类] 地球化学与岩石