Impact of conversion from forestland to cropland on soil N₂O emission from purplish soil

Land use change is one of the main reasons for changes of greenhouse gases concentration in the atmosphere. However, the impacts of land use change process, such as conversion process from forestland to cropland, on soil N₂O emission is still lack of a systematic study. In this study, forestland was converted into cropland in the hilly area of the Central Sichuan Basin in mid July 2016, and then planted as winter wheat-summer maize rotation from October 26, 2016 to September 15, 2017. Static chamber-gas chromatographic technique were used to compare the characteristics of N₂O emission from croplands, converted from forest land with tillage without fertilizer (CL-T) and with tillage and fertilizer (CL-TF), and forestland (CK). The results showed that during the experimental period, the soil N₂O emission fluxes of CL-T and CL-TF both significantly increased compared to CK (P < 0.01), and there was an obvious emission peak at the beginning of land use conversion from forestland to cropland. The mean N₂O emission fluxes in the wheat season and maize season were 2.52 μg(N)·m^-2·h^-1 and 4.60 μg(N)·m^-2·h^-1 under CK, 3.55 μg(N)·m^-2·h^-1 and 11.63 μg(N)·m^-2·h^-1 under CL-T, 6.26 μg(N)·m^-2·h^-1 and 22.16 μg(N)·m^-2·h^-1 under CL-TF, respectively. N₂O emission peak flux was significantly higher in maize season than in wheat season. The annual accumulative soil N₂O emissions were 0.271 kg(N)·hm^-2, 0.515 kg(N)·hm^-2 and 0.957 kg(N)·hm^-2 under CK, CL-T and CL-TF, respectively. Compared to CK, CL-T and CL-TF increased the annual accumulative soil N₂O emissions by 89.8% and 253.0%, respectively. This indicated that land use conversion from forestland to cropland significantly induced the increase of N₂O emission in purplish soil. Tillage changed soil structure and significantly increased soil inorganic nitrogen content (P < 0.05), then fertilizer application increased soil inorganic nitrogen content sharply by a large quantity, which resulted in pulsive N₂O emission. While soil temperature (5 cm) and soil moisture (WFPS) were not changed significantly (P> 0.05), and planting did not change N₂O emission significantly in short time during the process of land use conversion. It was suggested that the fundamental mechanism of stimulating N₂O emission caused by land use conversion from forestland to cropland might be derived from the increase of mineralization rate of soil organic nitrogen. However, the effect of land use conversion on soil nitrogen transformation and the mechanism of changing of soil N₂O emission need to be further studied.