Abstract: Farmlands are important terrestrial ecosystems with characteristic soil respiration and carbon budget processes. Data from the long-term experiment at Hailun Agricultural Ecology Station of Chinese Academy of Sciences for 2005—2007 were analyzed for the characteristics of crop carbon fixation, soil cumulative CO₂ flux and carbon budget in the black soils of wheat-maize-soybean rotation systems. Results showed that in wheat-maize-soybean rotation system, the amount of carbon fixed by crops decreased in the order of cultivated maize in 2006 > cultivated soybean in 2007 > cultivated wheat in 2005. The averages of fixed carbon by maize, soybean and wheat were 6 513 kg(C)·hm^-2, 4 025 kg(C)·hm^-2 and 3 655 kg(C)·hm^-2, respectively. Soil cumulative CO₂ flux during crop growth seasons was highest for soybean ecosystem average of 4 062 kg(C)·hm^-2, followed by maize ecosystem average of 3 813 kg(C)·hm^-2 and then lowest for wheat ecosystem average of 2 326 kg(C)·hm^-2. Rhizosphere derived CO₂ flux accounted for about 69%~73%, 66%~80% and 46%~63% of soil cumulative CO₂ flux for soybean, maize and wheat ecosystems respectively. This indicated that CO₂ flux derived from crop rhizosphere was crucial for soil CO₂ emission. NEP (net ecosystem productivity) was positive in black soil/crop systems of wheat-maize-soybean rotation, where farmlands served as CO₂ sink from the atmosphere. NEP in different crop systems decreased in the order of maize > wheat > soybean and with averages of 3 215 kg(C)·hm^-2, 1 643 kg(C)·hm^-2 and 512 kg(C)·hm^-2, respectively. Compared with plots without sufficient nutrients, long-term applications of chemical fertilizers (NPK) or organic manure amended chemical fertilizers (NPKOM) increased crop carbon fixation and soil cumulative CO₂ flux in all cropping systems. They were highest for plots with organic manure amended chemical fertilizers (NPKOM). NEP in each cropping system was significantly affected by different fertilization treatments. NEP was highest in plots with NPK fertilizers and lowest in plots with organic manure amended chemical fertilizer (NPKOM). The findings of this study were significant for the proper management of black soils as potential carbon pools in Northeast China.