Abstract: In this investigation, we continually measured the soil respiration in a maize farmland ecosystem from late April to late September 2005. Soil heterotrophic respiration is indirectly evaluated by established linear regression relations between soil respiration rates and dry root weights. Soil respiration fluctuates greatly during the growing season of 2005. The mean soil respiration rate is 3.16 μmol (CO₂)·m^-2·s^-1; with a maximum of 4.77 μmol (CO₂)·m^-2·s^-1 on July 28 and a minimum of 1.31 μmol (CO₂)·m^-2·s^-1 on May 4. The proportions of soil heterotrophic respiration to soil respiration are 45.5%, with a range of 36.4%~56.9%. Assuming that all sequestered C in the grains and straws is not removed from the fields by harvest, net carbon budget of the maize ecosystem is -1 127.0 g (C)·m^-2, with a range of carbon exchange rate of 0.52~ -18.05 g(C)·m^-2·d^-1. Maize ecosystems serve as a minor source of C in early grow stages of maize, and a C-sink from 36 days after planting lasting until harvest.