Carbon sequestration or net ecosystem exchange (NEE) has received a great deal of attention during the past decade. I will provide some general numbers that can serve as a basis for comparing various land types, and discuss some of the factors which influence the calculations. Common units of g C m-2 yr-1 will be used for all comparisons, and positive NEE will denote C movement to the biosphere (sequestration). From a large scale perspective, it has been estimated that North America was a net sink of 40 g C m-2 yr-1 for vegetated land area during 1990-1994. Productive temperate forests can sequester 450 to 600 g C m-2 yr-1, whereas more arid or mature forests may fall in the 100 to 200 g C m-2 yr-1 category. Rangelands (grasslands, shrub steppes, and deserts) appear to generally be weak or moderate sinks, although annual weather patterns strongly control sink/source relations in these biomes. Some of the more productive prairies can average 100 g C m-2 yr-1, whereas hot deserts of the Southwestern U.S. can be sources of about the same magnitude. Cropland can be a sink or source depending on specific management practices. Broad-scale estimates of C losses from European cropland vary from -24 to -90 g C m-2 yr-1 depending on calculation method. Continuous grain cropping has reduced soil organic carbon by 19 to 51% in North America. Applying best management practices may reduce the C source or even create a sink. Recent research demonstrated that a no-till corn/soybean rotation could be a sink for 90 g C m-2 yr-1. Patterns of NEE within the various land types can be defined, but weather, calculation methods, and spatial variation present major barriers to estimates of biome-level NEE.