Human activities have altered rates of carbon erosion from soils and carbon deposition in sediments.  We are developing methods to quantify the present-day and historical effects of these changes on the carbon mass balance of the conterminous U.S. land surface.  Because our analysis uses a combination of diverse existing datasets, we devote particular attention to methods for the estimation of uncertainties that are consistent with the statistical character of the source data.

Mechanistic explanations for the downward excursion in d¹⁸O of atmospheric CO₂ observed during the mid-1990s and the generally large interannnual variability characteristic of this isotopologue are lacking. We hypothesize that the excursion and related variations in d¹⁸O of atmospheric CO₂ may be linked to global-scale variations in cloud cover. However, very little is known about the influence of clouds on biosphere-atmosphere CO¹⁸O exchanges. Recent work has demonstrated the influence of boundary layer clouds on canopy photosynthesis through increases in the diffuse radiation fraction and relative humidity, combined with decreases in leaf temperature. In concert, these alterations tend to increase canopy photosynthesis and conductance, which should also increase CO¹⁸O isofluxes. However, photosynthetic CO¹⁸O isofluxes also depend critically on the d¹⁸O of leafwater, and enhanced cloudiness typically decreases the d¹⁸O of leafwater by enhancing relative humidity and water vapor exchange across stomata. Thus, the net impact of differing cloud regimes on biosphere-atmosphere CO¹⁸O exchanges is difficult to predict. Preliminary simulations suggest a large impact of diffuse radiation on canopy photosynthesis by increasing the flux from shade leaves. The impact of this effect on biosphere-atmosphere CO¹⁸O exchanges is diluted somewhat by the lower enrichment in leafwater d¹⁸O on cloudy days with high diffuse radiation fractions. Our results suggest that these effects are very dependent on LAI and photosynthetic pathway (C₃ or C₄).

A process model was used to simulate changes in the carbon fluxes and stocks at a site that was transformed a grassland to a plantation at Qian Yanzhou. The total carbon storage of the zonal vegetation (evergreen broadleaf forest) was simulated and taken as the saturated carbon storage value of that site. The simulated vegetation density and soil organic carbon (SOC) were compared with the observed. The simulates indicate that after 20 years planting of the needle leaf forests (Pinus Massoniana, Cunninghamia lanceolata and Pinus elliottii ect) on ex-grassland, the net carbon storage increase in the plantation was 8.03 kg C/m²，in which the vegetation carbon storage increased 8.5334 kg C/m² and the soil carbon storage decreased 0.518 kg C/m². The total carbon storage of 20 years plantation is 58.6％ of the saturated value. The study also shows that between 0 and 7 years of land use change the soil carbon was decreased and between 7 and 20 years it was predicted to increase slowly.

We build upon a previous approach to predict C₃ and C₄ fractions on the land surface using new higher resolution satellite datasets on vegetation growth form and crop type coverage. The approach relies upon the near-universal restriction of C₄ photosynthesis to the herbaceous growth form and the differing performance of C₃ and C₄ plants in various temperature and radiation regimes. MODIS-derived data provide detailed information on growth form composition (%herbaceous, %woody, and %bare for each grid cell). Precipitation and temperature variations are derived from station data climatologies. Combining these data with MODIS-derived NPP fields from 2001, we predict latitudinal variations in C3 and C₄ photosynthesis for South America. These variations will be discussed in the context of the global carbon cycle and the difficulty they pose for interannual inversion studies using global CO₂ and d¹³C atmospheric data. 

In this study, we developed an integrated modeling system to simulate dynamics of a stable carbon isotope of CO₂, moisture, energy, and momentum between boreal ecosystems and the atmosphere as well as their diffusion processes through the whole convective boundary layer (CBL), using remotely sensed surface parameters to characterize the surface heterogeneity, and the marine boundary layer matrix data to represent the CBL top condition. Model validation and primary results in boreal ecosystems were presented in this paper.

During photosynthesis, atmospheric carbon sequestration goes on at the expense of formation and accumulation of organic substance, and an inverse process (carbon emission in the atmosphere) takes place during decomposition and oxidation of this organic substance. On land, in non-swamp ecosystems, these processes are balanced as a whole both under climax forms and interchange of: 1) periods of active oxidation of organic substance under influence of disturbing factors (more often, fires), and 2) periods of active formation of organic substance in the process of regeneration successions.

The development of agriculture responding to increasing demand for food raises the question of the role of cultivated land in relation to carbon sources and sinks, their spatial patterns and temporal variability.