Abstract
Abstract. Global biosphere models vary greatly in their projections of future changes of global soil organic carbon (SOC) stocks and aggregated global SOC masses in response to climate change. We estimated the certainty (likelihood) and quantity of increases and decreases on a half-degree grid. We assessed the effect of changes in controlling factors, including net primary productivity (NPP), litter quality, soil acidity, water saturation, depth of permafrost, land use, temperature, and aridity associated with probabilities (Bayesian network) on an embedded, temporally discrete, three-pool decomposition model. In principle, controlling factors were discretized into classes, where each class was associated with a probability and linked to an output variable. This creates a network of links that are ultimately linked to a set of equations for carbon (C) input and output to and from soil C pools. The probability-weighted results show that, globally, climate effects on NPP had the strongest impact on SOC stocks and the certainty of change after 75 years. Actual land use had the greatest effect locally because the assumed certainty of land use change per unit area was small. The probability-weighted contribution of climate to decomposition was greatest in the humid tropics because of greater absolute effects on decomposition fractions at higher temperatures. In contrast, climate effects on decomposition fractions were small in cold regions. Differences in decomposition rates between contemporary and future climate were greatest in arid subtropical regions because of projected strong increases in precipitation. Warming in boreal and arctic regions increased NPP, balancing or outweighing potential losses from thawing of permafrost. Across contrasting NPP scenarios, tropical mountain forests were identified as hotspots of future highly certain C losses. Global soil C mass will increase by 1% with a certainty of 75% if NPP increases due to carbon dioxide fertilization. At a certainty level of 75%, soil C mass will not change if CO2-induced increase of NPP is limited by nutrients.
Highlights
Soil organic carbon (SOC) represents about three-quarters to four-fifths of the terrestrial organic carbon (C) mass (Prentice et al, 2001)
The large variation in expected future changes is due to the balance of, on one hand, different expected increases of C input from net primary productivity (NPP) by CO2 fertilization and higher temperatures and, on the other hand, faster decomposition accelerated by higher temperatures
We compared the soil organic carbon (SOC) stocks for reference conditions to the SOC stocks calculated from Harmonized World Soil Database (HWSD)
Summary
Soil organic carbon (SOC) represents about three-quarters to four-fifths of the terrestrial organic carbon (C) mass (Prentice et al, 2001). Global SOC mass in five general circulation models (GCM) was projected to change by between −46 and +51 Pg (Schaphoff et al, 2006) by the end of the century. Projections differ in where changes occur (Sitch et al, 2008). The large variation in expected future changes is due to the balance of, on one hand, different expected increases of C input from net primary productivity (NPP) by CO2 fertilization and higher temperatures and, on the other hand, faster decomposition accelerated by higher temperatures
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