Balancing the carbon budget for the period from 1850 to 1998 yields a global net terrestrial source of 26 ± 60PgC. Hence, a residual terrestrial sink of 110 ± 80PgC is required to reconcile the difference between the net terrestrial source and the larger terrestrial source resulted from land-use change (136 ± 55 PgC). This residual terrestrial uptake is often referred to as the 'missing carbon sink', although it can be attributed to well-known biophysical mechanisms.
Stimulation of photosynthesis at higher CO2 is one of the well-known mechanisms to which the 'missing carbon sink' is normally attributed. Carbon dioxide fertilization effect on plant productivity is not linear:
It is weakening at high atmospheric concentration of CO2. If 7 = 0.35, then NPP increases by 24%, when CO2 increases two times. Thus, the growth of atmospheric concentration of CO2 enhanced plant productivity by less than 10% in comparison to 1850.
Carbon dioxide fertilization produces only an excess amount of organic matter. The mechanism of carbon sequestration associated with CO2 growth is more complicated. Carbon sequestration stems from disbalance between production and decomposition of organic matter - that is, from NEP. NEP approaches to naught when ecosystem approaches to equilibrium. Continuous growth of CO2 maintains ecosystem disequilibrium. Therefore, the rate of carbon sequestration is determined by the rate of atmospheric CO2 growth and the rate of carbon turnover in terrestrial ecosystems.
Production of organic matter is generally limited with nitrogen and some other nutrients. Therefore, NPP is expected to increase with a rapid growth in reactive nitrogen deposition over the last 150 years. Reactive nitrogen is released into the atmosphere in the form of nitrogen oxides (NO*) during fossil fuel and biomass combustion. The annual deposition even in rural areas may amount to 40 kg ha-1. Another source (mainly of ammonia) is animal husbandry and fertilizer use. The nitrogen deposition mainly affects ecosystems close to cities and industrial centers as well as in the vicinity of intensive agricultural enterprises.
Agricultural and forest management practices significantly affect carbon stocks in managed ecosystems. These practices dramatically changed since 1850; they began to be oriented to the sustainable use of natural resources. A forest that is managed in sustainable manner operates as a machine that removes carbon from atmosphere and exports it as forest products. Similarly, alteration of tillage practices leads to protection and increase of soil carbon content.
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