<P> Human activities over the past two centuries have significantly increased the amount of carbon in the atmosphere, mainly in the form of carbon dioxide, both by modifying ecosystems' ability to extract carbon dioxide from the atmosphere and by emitting it directly, e.g., by burning fossil fuels and manufacturing concrete . </P> <P> The terrestrial biosphere includes the organic carbon in all land - living organisms, both alive and dead, as well as carbon stored in soils . About 500 gigatons of carbon are stored above ground in plants and other living organisms, while soil holds approximately 1,500 gigatons of carbon . Most carbon in the terrestrial biosphere is organic carbon, while about a third of soil carbon is stored in inorganic forms, such as calcium carbonate . Organic carbon is a major component of all organisms living on earth . Autotrophs extract it from the air in the form of carbon dioxide, converting it into organic carbon, while heterotrophs receive carbon by consuming other organisms . </P> <P> Because carbon uptake in the terrestrial biosphere is dependent on biotic factors, it follows a diurnal and seasonal cycle . In CO measurements, this feature is apparent in the Keeling curve . It is strongest in the northern hemisphere, because this hemisphere has more land mass than the southern hemisphere and thus more room for ecosystems to absorb and emit carbon . </P> <P> Carbon leaves the terrestrial biosphere in several ways and on different time scales . The combustion or respiration of organic carbon releases it rapidly into the atmosphere . It can also be exported into the ocean through rivers or remain sequestered in soils in the form of inert carbon . Carbon stored in soil can remain there for up to thousands of years before being washed into rivers by erosion or released into the atmosphere through soil respiration . Between 1989 and 2008 soil respiration increased by about 0.1% per year . In 2008, the global total of CO released by soil respiration was roughly 98 billion tonnes, about 10 times more carbon than humans are now putting into the atmosphere each year by burning fossil fuel (this does not represent a net transfer of carbon from soil to atmosphere, as the respiration is largely offset by inputs to soil carbon). There are a few plausible explanations for this trend, but the most likely explanation is that increasing temperatures have increased rates of decomposition of soil organic matter, which has increased the flow of CO . The length of carbon sequestering in soil is dependent on local climatic conditions and thus changes in the course of climate change . </P>

What are the key inorganic reservoirs for carbon and nitrogen