Sequestration of anthropogenic carbon (C) in terrestrial systems is a critical factor in reducing future buildup of CO2 in the atmosphere. The magnitude of the terrestrial C sink is likely constrained by nutrients, in particular nitrogen (N). N availability can further be altered by land-use practices which extract N-containing biomass and thus have the potential to intensify N limitation. In addition, the most important N source in natural vegetation, biological N fixation (BNF) may operate as a natural feedback responding to increasing N demands, particularly in early successional or tropical forests. Both the representation of interactive effects between human land-use and subsequent C-N dynamics and direct feedbacks between BNF and N availability have not been considered in global dynamic vegetation models. Here, we address with the Princeton-GFDL land model (LM3V-N) how C-N interactions control the terrestrial C exchange over the past 200 years. LM3V-N includes prognostic equations for BNF and captures succession-specific C-N feedbacks by integrating age/size classes of secondary forests separately. We subjected the model to observed changes in relevant environmental drivers, namely atmospheric CO2 concentrations, patterns of bio-available N deposition, and climate change.
Results/Conclusions
Over the historic period, we obtain a terrestrial net carbon loss of 52 PgC in agreement with estimates from budgeting fossil fuel emissions with ocean and atmosphere C inventories. Our model realization resulted in a ~200 PgC loss due to land-use activity, suggesting a residual terrestrial sink of ~150 PgC. BNF in tropical forests and anthropogenic N deposition in high latitudes are the dominant source of new N in these two broad regions. Our results however suggest that N restrictions reduced carbon uptake by 51 PgC between 1800 and 2000 and hampers current sequestration by 0.45 PgC yr-1. We further find that N requirements and BNF increase in natural forests as a consequence of human land-use practices. The modeled terrestrial C exchange is within the uncertainty range of IPCC estimates for recent decades, and within variations as inferred from ocean uptake and 13C/12C deconvolution. Likewise, the simulated distribution of recent carbon sequestration across latitudes agrees with available data, although uncertainties in estimations of regional carbon sinks are large. Overall, our model captures the historic dynamics of the land carbon inventory, thereby reconciling the net carbon exchange with N supply and the successional dynamics of C-N interactions from human land-use activities.
