Long-term regional carbon balance subjected to heavy anthropogenic disturbances has been a concern and challenge for both global change research and regional ecosystem management because of the difficulties in quantification of interplay between climatic changes and managerial disturbance related to agricultural production [Chapin et al., 2002; Murty et al., 2002; Brody, 2003; Ludwig et al., 2005]. In particular, great uncertainties exist about quantitative impacts of large-scale erosion-induced loss in soil organic carbon (SOC) on net ecosystem carbon balance, as reported in a large volume of literature [e.g., Bajracharya et al., 2000; Lal, 2000, 2002b; Houghton, 2003b; Jacinthe et al., 2004; Lal, 2004]. Soil erosion has been known to directly reduce the soil carbon pool by transferring carbon laterally to aquatic systems, by deposing carbon in small lowland areas, and by increasing carbon oxidation and soil efflux [Smith et al., 2005]. Erosion has also been shown to decrease soil nutrient content, degrade soil structure, and cause a reduction in net primary production [Tan et al., 2005]. Heterotrophic respiration of regional ecosystems can also be significantly altered by severe soil erosion, because erosion tends to reduce the substrates of heterotrophic respiration, to change soil texture and structure, and thus to modify soil microbe activities [Parton et al., 1987; Murty et al., 2002]. The net effect of soil erosion on the regional carbon budget depends on many factors including climate and managerial activities [Lal, 2000, 2002a; Houghton, 2003a; Liu et al., 2003].
 Optimal land use planning and management necessitate comprehensive understanding of the coupling between dynamic soil carbon budget and other terrestrial ecosystem processes. Rational land use planning is especially important for China, due to its dense population and heavy pressure brought about by the demands for rapid economic development [Cao et al., 2003; Li et al., 2003; Gao et al., 2004b]. The issue is even more pressing for the crop-pasture transition regions in northern China, because such areas are in general more sensitive to natural and anthropogenic driving forces, thus are often frontiers for combating desertification.
 The crop-pasture transition belt in northern China (CCPB) lies within the geographical ranges between 34.7 and 48.6°N and between 100.8 and 124.8°E, with a total area of 725,527.9 km−2 (Figure 1). The long axis of CCPB runs from the northeast verge of the Tibet Plateau, crossing the Loess Plateau, the Yellow River basin, the Mongolia Plateau, and ends in the Northeast Plain. The region includes 205 counties/cities in 10 provinces, and has a population of approximately 60 million. Ecosystems in the region are under a typical continental monsoon climate with the precipitation gradient approximately perpendicular to the long axis. Annual precipitation decreases from approximately 580 mm in the Southeast to less than 200 mm in the Northwest. Annual mean temperature decreases from 14.0°C in the South to less than −1.0°C in the North (1959–2001). Vegetation patterns largely correlate with precipitation and temperature, with a small portion of coniferous forests in the cold North and mountains, deciduous broadleaf forests in the Southeast and North, large portions of shrubs and grasses on the northwest side of the dry plateaus, and croplands distributed mostly in the middle and the southeast regions. The area has been undergoing severe soil erosion, degradation, and desertification because of the changing climate, inappropriate land use, and overgrazing by livestock. Planning and implementation of ecosystem restoration programs have to be guided by the correct understanding of regional ecosystem processes including soil erosion and quantitative responses of these processes to changes in climate in the past and future.
 Process-based ecosystem models have been developed and used to analyze ecosystem responses to climate and disturbances on regional and global scales. Biogeochemical models simulate carbon, water, and nutrient cycling within ecosystems [Raich et al., 1991; McGuire et al., 1992, 1993; Running, 1994; Schimel et al., 1997; Xiao et al., 1997; Peng and Apps, 1999; Cao and Li, 2000]. On the other hand, biogeography models determine ecosystem structure represented by vegetation distribution [Prentice et al., 1992; Neilson, 1995]. Development of dynamic global vegetation models (DGVM) starting in the late 1990s couples ecosystem processes with ecosystem structure [Steffen et al., 1996; Beerling et al., 1997; Peng, 2000]. Models that explicitly take into consideration erosion-induced lateral soil carbon loss in analyses of regional ecosystem carbon balance are still rare, and soil erosion has been widely studied in agricultural rather than ecological sciences. Recent efforts to bridge ecosystem carbon cycle and soil erosion processes provide examples to analyze the impacts of lateral processes on spatially heterogeneous ecosystems on landscape and regional scales [Reid et al., 1999; West and Wali, 2002; Liu et al., 2003; Van Oost et al., 2005; Izaurralde et al., 2006].
 One difficulty in incorporating erosion into regional ecosystem models is that water erosion is one of the spatial processes that requires quantification of interactions among neighborhood ecosystems due to spatial heterogeneity in resource demands and ecosystem processes, which cause transfer of mass and energy across neighborhood ecosystems [Rupp et al., 2000; Weir et al., 2000; Loreau et al., 2003; Rastetter et al., 2003; Ludwig et al., 2005]. States of a local ecosystem are controlled both by processes within the system (e.g., vertical movement of soil water and nutrients, and plant growth) and by mass and energy flows across neighborhood ecosystems. Not only does a change in any local ecosystem in a heterogeneous regional mosaic affect the processes of the system itself, but also it may bring about a series of consequences in the neighborhood ecosystems including changes in hydrologic cycles, soil erosion, nutrient loss, and net primary productivity, by decreasing or increasing runoff/run-on flow, and by cutting off or connecting the pathway of spatial plant propagation [Band et al., 2001; Tague and Band, 2001; Tchir et al., 2004].
 Simulation models have been shown to be ubiquitously scale-dependent because of the nonlinearity of various ecosystem processes [Levin, 1992; Gao et al., 2001; Rastetter et al., 2003]. Models are usually parameterized with local scales of 101–102 m at research sites, but might have been used on regional or even global scales without considering errors associated with scaling up. Research indicates that cross-scale ecosystem modeling has a high risk of scaling errors with magnitudes comparable to, or even larger than, the values of key ecosystem variables simulated on local scales [Rastetter et al., 2003].
 In this paper, we revised, scaled, and applied a regional ecosystem model to simulate net primary production and soil processes in the CCPB region with existing climate, soil, and vegetation data. The model connects local ecosystem processes of carbon assimilation, plant growth, and nutrient cycling, to lateral processes of erosion-induced soil organic carbon loss. The model was run to determine the responses of the CCPB to climatic changes from 1959 to 2001. Effects of soil erosion and climate shifts in the early 1980s on net ecosystem carbon balance (NECB) were analyzed, and the results discussed in the contexts of future global change and regional ecosystem management. We found that soil erosion did not cause significant decreases in regional net ecosystem carbon balance because of decreases in heterotrophic respiration associated with soil organic carbon removal by erosion and relatively unaltered NPP maintained by fertilization and irrigation of agricultural crop fields.