The terrestrial phytomass and soil humus are important reservoirs of carbon within its global biogeochemical cycle and the interactive system of the atmosphere, land, coastal oceanic zone, and open ocean. The mass of carbon in living land plants and soils is greater than in the present-day atmosphere, and land is also the source of anthropogenic emissions from fossil fuel burning and land-use activities. The flows of carbon between the atmosphere and other major reservoirs, driven by inorganic as well as biological processes, have been significantly affected during the past 300 years of human industrial and agricultural activity, with the well established results of accumulation of CO2 in the atmosphere, oceans, and oceanic sediments. The partitioning of the atmospheric CO2 and, in particular, of the anthropogenic emissions to the atmosphere among the major global reservoirs depends on the driver roles of the nitrogen and phosphorus cycles within the coupled C-N-P-S system. The coupling between the biogeochemical cycles of carbon (C), and other biologically essential elements nitrogen (N), phosphorus (P), and sulphur (S), is evidenced by their joint occurrences as the major constituents of organic matter, and by the roles of N and/or P as limiting nutrients that control primary production in the terrestrial and aquatic environments. For the past 300 years, since the year 1700, the global system of the coupled cycles was analysed by a process-based model TOTEM (Terrestrial Ocean atmosphere Ecosystem Model), based on 13 reservoirs in the four major environmental domains: atmosphere (1); land (6); coastal zone (3), and open ocean (3). In every biological transfer process, the individual element cycles are coupled through the average C:N:P:S ratios, such as the Redfield ratios in terrestrial and oceanic primary production, humus formation, and organic matter burial in sediments. The global system in the past 300 years experienced five perturbations that were included as external forcings in the model. One is a forcing due to changes in a global climatic variable, mean global temperature; and four are due to human activities: fossil fuel buming, landuse activities, application of N and P fertilizers to agricultural soils, and disposal of municipal and industrial sewage. On a time scale of the past 300 years, the most important forcings that control the coupled C-N-P-S cycles are fossil fuel burning and changes in land-use activities. However, extrapolation to the future of the present-day uses of chemical fertilizers and sewage disposal to rivers and the coastal zone indicates, in our model analysis, that increases in the N and P input rates from land to the coastal zone may lead to its much stronger role as a CO2 sink through increased primary production. Land-use activities (deforestation, reforestation, shifting agricultural cultivation) impact the carbon cycle via three mechanisms. First, through emissions of CO2 from these activites. Second, through a negative feedback to rising atmospheric CO2 concentrations. And third, through material transfer from land to the oceanic coastal zone by soil erosion, mineral dissolution, and surface water runoff. Landuse activities generally change the reservoir size and residence time of terrestrial organic matter, releasing CO2, and N and S gases to the atmosphere, and remobilizing N, P, and S to the soil-water reservoir. The increased availability of nutrients in soil water, coupled with rising atmospheric CO2 and rising temperature, promotes enhanced primary production and storage of carbon in the organic matter of the land phytomass. Because of the differences between the C:N:P ratios in humus (140:10:1) and living plants (510:4:1), remineralization of N and P in humus can result in 370 moles C becoming sequestered in new biomass, while 6 moles of N are released to continental soil waters, from where they are eventually transported to the atmosphere by denitrification or to the coastal zone by rivers. Our model analysis shows that during most of the 300-year period of human forcings, land was a net source of carbon through emissions to the atmo-