Net Primary Productivity (NPP) is considered as one of the key variables for the modelling of carbon sequestration and release – an important aspect of the Terrestrial Carbon Initiative (TCI). NPP is defined as the difference between the total carbon uptake from the air through photosynthesis and the carbon loss due to respiration by living plants (measured in kg/m2/yr). The NPP distribution is a direct indicator on the productivity of croplands, forests and grasslands and thus an important information base required for improving management strategies for sustainable development of natural resources.
The Terrestrial Carbon Initiative (TCI) is aimed to have a better understanding of the regional and global cycles of carbon, the distribution of sinks and sources, their seasonal and annual dynamics and the interactions of the various ecosystems and between the ecosystems and the atmosphere. The study requires further research and large scale observations where space borne and in situ measurements have to be analyzed in an integrated manner. Main variables to be observed include status and dynamics of land cover, canopy structure of vegetation, plant nutrition status and biogeochemistry, Leaf Area Index, Net Primary Productivity, atmospheric CO2 concentration, meteorological and other data.
This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities. Agenda 21 recommends coordinating regional and subregional research on carbon sequestration.
In 1995 researchers reported in Science that vegetation in the Northern Hemisphere in 1992 and 1993 converted into trees and other plant tissue 3.5 billion tonnes of carbon – more than half the carbon produced by the burning of hydrocarbon fuels worldwide.
A report analyzed the performance of alternative incentive designs and payment levels if US farmers were paid to adopt land uses and management practices that raise soil carbon levels. At payment levels below $10 per metric ton for permanently sequestered carbon, the model predicts landowners would find it cost-effective to adopt changes in rotations and tillage practices, but none of the potential land use changes. At higher payment levels, afforestation dominates sequestration activities, mostly through conversion of pastureland. The model predicted converting cropland to grassland was not competitive up through a $125 carbon price, in part because conversion to afforestation (where feasible) was more profitable – with its higher sequestration rate per acre. A 50-percent cost-share for cropland conversion to forestry or grasslands would increase sequestration at low carbon payment levels but not at high payment levels.