Investigating the arable land that is the main contributor to global warming between paddy and upland vegetable crops under excessive nitrogen fertilization

Methane (CH4) and nitrous oxide (N2O) have been considered as two of the major greenhouse gases (GHGs) from agricultural land. CH4 is normally emitted from water-logged paddy fields, while N2O is emitted from aerobic upland soils. Both of these arable lands are thought to be major GHG emission sites; however, the type of land that is responsible for the highest contribution to GHG emissions in the same region has not been studied. A two-year field study was conducted to calculate the contributions to global warming of rice paddy fields and red pepper cultivation in upland soil under different nitrogen (N) fertilization levels. Urea was applied as a source of nitrogen at four different levels (0, 50, 100, and 200% of the recommended doses for cultivation), and rice and red pepper were cultivated in paddy and upland fields, respectively, from May to October. CH4 and N2O were measured from both arable lands to quantify GHG fluxes. The CH4 emissions increased in a quadratic response with increasing nitrogen application level in rice paddy fields and upland soil, but rice paddies had significantly higher CH4 emissions than red pepper upland soils. In contrast, N2O emissions showed a linear correlation with increasing N fertilization level in both arable lands; however, upland soil showed significantly higher N2O emissions than rice paddy fields. The rice paddy field had much higher global warming potential (GWP) than upland soil under low levels of N fertilization (below 265 kg N ha−1) according to a carbon dioxide (CO2) equivalent comparison. However, upland soil showed a clearly higher GWP than rice paddy fields under excessive N fertilization. Therefore, we concluded that upland vegetable crops contributed the most to the GHG emissions caused by high N2O fluxes under excessive N fertilizer application.