On seasonal timescales the gains and losses from the terrestrial biosphere are reflected in the cyclic variations in atmospheric CO2 concentrations. Although there is significant interannual variability in the seasonal balance between the gains and losses, it is the trends in this net balance between NPP and carbon losses through decomposition, fires, and disturbances that can affect climate on timescales of decades to centuries. The scientific community currently believes that the terrestrial biosphere has been acting as a net sink for atmospheric carbon for the past few decades (Foley and Ramankutty, Chapter 14, this volume). For example, Pacala et al. (2001) recently estimated the carbon sink of the coterminous Unites States using a combination of inventory and atmospheric concentration inversions. They determined a consistent land- and atmosphere-based carbon sink for the United States in the range of 0.37-0.71 PgC y-1. Similar studies have also shown a net sink in Europe (Janssens et al. 2003). The mechanisms underlying the sinks have been the subject of much recent research. The leading hypothesis in the 1970s and 1980s was that the sinks were mainly a result of more rapid plant growth from elevated CO2 and climate change. This hypothesis has gradually been replaced with a multi-mechanism explanation, including contributions from changes in forest management, agriculture, long-lived products (e.g., wood), aquatic systems, and nitrogen deposition, in addition to CO2 fertilization and changes in plant growth and soil carbon pools resulting from climate change (Schimel et al. 2001).
NPP is sensitive to a broad range of factors, including climate, soil fertility, atmospheric CO2, atmospheric pollutants, and human management. An increase in NPP can lead to a carbon sink, but only if it is not matched by a corresponding increase in carbon losses. In general, processes that promote gradual increases in NPP can lead to carbon sinks, because increases in respiration tend to follow changes in biomass and soil carbon and not NPP directly (Field 1999). For the past 30 years, carbon cycle researchers have hypothesized that gradual increases in NPP in response to the 30 percent rise in atmospheric CO2 since preindustrial times explain much or all of the terrestrial sink inferred from atmospheric studies (Bacastow and Keeling 1973). Experimental studies at the ecosystem level often demonstrate accelerated plant growth in response to elevated atmospheric CO2 (Mooney et al. 1999), but these growth rates only explain a small fraction of the sink required to balance the global atmospheric CO2 budget (Friedlingstein et al. 1995). Other factors that can increase NPP, including warming, greater precipitation and humidity, nitrogen deposition, and changes in plant species composition, may also contribute to terrestrial carbon sinks.
Processes that retard carbon releases can also lead to terrestrial sinks. Evidence indicates that a substantial fraction of the forest sink in temperate forests is a result of changes in land use and land management. In essence, forests, cut in the past, are regrowing, with a growth rate that is faster than the rate of harvesting (Goodale et al. 2002). Some of this change is a result of shifts in land use, especially the abandonment of agriculture over large regions of North America and parts of Europe (Foley and Ramankutty, Chapter 14, this volume). Fire suppression and the thickening (more trees per unit of area) of marginal forests also contribute to increased forest biomass in some areas (Pacala et al. 2001).
Changes in agriculture can also promote terrestrial carbon sinks. Increases in yield, incorporation of crop residue, and areas in perennial crops, as well as a reduction in tillage, can all contribute to carbon sinks (Lal et al. 1998).
Recent terrestrial carbon sinks reflect a number of mechanisms (Pacala et al. 2001). The potential for sinks to persist varies from mechanism to mechanism. Sinks due to CO2 fertilization are likely to persist until NPP is limited by another factor. In some settings, this limitation might occur very soon, and in others it might be far in the future. Sinks caused by forest regrowth saturate as the forests mature (Nabuurs, Chapter 16, this volume). In general, sinks saturate when increases in NPP are outpaced by increases in the sum of decomposition and combustion. The persistence of terrestrial sinks in the future is by no means assured. Many lines of evidence suggest that the prospect of increasing carbon sources in the terrestrial biosphere is a real possibility (Gruber et al., Chapter 3, this volume).
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