Examining soil carbon uncertainty in a global model: response of microbial decomposition to temperature, moisture and nutrient limitation

[摘要] Reliable projections of future climate require land–atmosphere carbon (C)fluxes to be represented realistically in Earth system models (ESMs). There areseveral sources of uncertainty in how carbon is parameterised in thesemodels. First, while interactions between the C, nitrogen (N) and phosphorus(P) cycles have been implemented in some models, these lead to diversechanges in land–atmosphere fluxes. Second, while the first-orderparameterisation of soil organic matter decomposition is similar betweenmodels, formulations of the control of the soil physical state on microbialactivity vary widely. For the first time, we address these sources ofuncertainty simultaneously by implementing three soil moisture and three soiltemperature respiration functions in an ESM that can be runwith three degrees of biogeochemical nutrient limitation (C-only, C and N,and C and N and P). All 27 possible combinations of response functions andbiogeochemical mode are equilibrated before transient historical (1850–2005)simulations are performed. As expected, implementing N and P limitationreduces the land carbon sink, transforming some regional sinks into netsources over the historical period. Meanwhile, regardless of which nutrientmode is used, various combinations of response functions imply a two-folddifference in the net ecosystem accumulation and a four-fold difference inequilibrated total soil C. We further show that regions with initially largerpools are more likely to become carbon sources, especially when nutrientavailability limits the response of primary production to increasingatmospheric CO₂. Simulating changes in soil C content thereforecritically depends on both nutrient limitation and the choice of respirationfunctions.

[学科分类] 地球化学与岩石