Cropland is a large source of global N_2O emissions, while forestland emits much less N_2O. Thus, returning cropland to forestland would help reduce global N_2O emissions. Here static chamber-GC technique was used to compare soil N_2O emission fluxes from sloping cropland(CL) and forestland returned from cropland for 15 and 30 years. Soil temperature, moisture, nitrate(NO_3~- -N) and ammonia(NH_4~+ -N)nitrogen content in the soils were also recorded for one year. Results showed that average fluxes of soil N_2O from CL, CL-CK(CL-Check test), FL_(15) and FL_(30) (15 and 30 years old forest)were 25.6, 6.60, 1.20, and 4.35 mug N_2O-N·m~(-2)·h~(-1), respectively, with significantly higher N_2O emissions from CL than from CL-CK, FL_(15) and FL_(30) (P<0.01). The N_2O emissions from FL_(30) were also significantly higher than that from FL_(15) (P<0.01). Annual cumulative N_2O emissions from CL, CL-CK, FL_(15) and FL_(30) were respectively 1.01, 0.400, 0.050, and 0.310 kg N_2O-N·hm~(-2). The potential of N_2O mitigation by nitrogen fertilizer reduction was as high as 0.620 kg N_2O-N·hm~(-2). Compared with CL-CK, FL_(15) and FL_(30) reduced N_2O emissions by 0.350 and 0.090 kg N_2O-N·hm~(-2), respectively, may due to increases in soil carbon to nitrogen ratios and changes in soil properties.