Soil nitrous oxide (N 2 O) emissions are influenced by land use adjustment and management practices. To meet the increasing socioeconomic development and sustainable demands for food supply, forestland conversion to cropland occurs around the world. However, the effects of forestland conversion to cropland as well as of tillage and fertilization practices on soil N 2 O emissions are still not well understood, especially in subtropical regions. Therefore, field experiments were carried out to continuously monitor soil N 2 O emissions after the conversion of forestland to cropland in a subtropical region in Southwest China. One forestland site and four cropland sites were selected: forestland (CK), short-term croplands (tillage with and without fertilization, NC-TF and NC-T), and long-term croplands (tillage with and without fertilization, LC-TF and LC-T). The annual cumulative N 2 O flux was 0.21 kg N ha −1 yr −1 in forestland. After forestland conversion to cropland, the annual cumulative N 2 O flux significantly increased by 76‒491%. In the short-term and long-term croplands, tillage with fertilization induced cumulative soil N 2 O emissions that were 94% and 235% higher than those from tillage without fertilization. Fertilization contributed 63% and 84% to increased N 2 O emissions in the short-term and long-term croplands, respectively. A stepwise regression analysis showed that soil N 2 O emissions from croplands were mainly influenced by soil NO 3 − and NH 4 + availability and WFPS (water-filled pore space). Fertilization led to higher soil NH 4 + and NO 3 − concentrations, which thus resulted in larger N 2 O fluxes. Thus, to reduce soil N 2 O emissions and promote the sustainable development of the eco-environment, we recommend limiting the conversion of forestland to cropland, and meanwhile intensifying the shift from grain to green or applying advanced agricultural management practices as much as possible.