Carbon Fluxes in Terrestrial Ecosystems

Oceans, despite their much larger surface area, contribute much less than half of the global NPP. The reason is related to highly nutrient deficiency in surface waters, which limits the photosynthesis process. Oceanic production is mainly concentrated 

The amount of annually decaying organic matter is the subject of speculation. However, some estimates might be done. For instance, in terrestrial ecosystems only, the humus accumulation of carbon in soils is about 70% of the total accumulation of CO2 in the atmosphere. We may presume therefore that the stable long-lived humic compounds acquire some 30% of carbon annually from the dead organs of plants, and the complete renewal of humus in soils extends over period of 0.3-l.Ox 10 years. The variance depends on the moisture and temperature conditions in the region of question. 

The undecomposed litter (4% of the soil carbon) has a turnover time measured in tens of years, and the 22% of the soil carbon in the form of fulvic acids is intermediate with turnover times of hundreds of years. The largest part (74%) of the soil organic carbon (humic acids and humins) also has the longest turnover time (in thousands of years) (Holmen, 1992). 

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