[Effects of Land-use Conversion on Soil Nitrification and NO & N₂O Emissions in Tropical China Under Different Moisture Conditions]

Rain and heat conditions are abundant in tropical areas, and rubber and tea are widely planted in this region; the nitrification process produces nitrate content, which is not conducive to the maintenance of nitrogen nutrients, and has negative environmental effects (nitrogen oxide emissions). The characteristics of soil nitrification rate and nitrogen oxide emission under different land use patterns remain unclear. An incubation experiment was conducted under the 5 a (T5) and 15 a (T15) tea plantation soils and the nearby typical rubber plantation (XJ) soils in Baisha county of Hainan province under two moisture contents (50% WFPS-L and 80% WFPS-H) for 71 d at 25℃. The results showed that:① after the rubber plantation was converted to a tea plantation, the net nitrification and soil NO and N₂O emissions were significantly reduced under high moisture content. The overall trend was in the order of XJH>T15H>T5H, and the values of soil net nitrification and NO and N₂O emissions were as high as 4.2 mg·(kg·d)^-1, 1.4 mg·kg^-1, and 14.3 mg·kg^-1 in the XJH treatment, respectively. Under the low moisture content, soil NO emissions in tea field soil were significantly reduced relative to those in rubber plantation soil, N₂O emissions had no significant difference among different treatments, and net nitrification had no significant difference between the XJ and T15 treatments. There was a significant positive correlation between NO emissions and net nitrification rate (P<0.01). ② The net nitrification of XJH was higher than that of XJL, but the net nitrification values under different moisture contents in tea field soil was in contrast to that in rubber plantation soil. The NO emissions of XJ and T15 under different moisture contents were consistent with the trend of net nitrification, and the high nitrification promoted NO emissions, whereas NO emissions of T5 were not significantly affected by moisture content. The high moisture content treatment significantly promoted N₂O emissions relative to those under the low moisture content treatment. The results showed that SOM, TN, pH, and moisture content were the key factors affecting soil net nitrification rate, NO, and N₂O emissions. The conversion of the rubber plantation to a tea plantation significantly reduced the net nitrification rate and negative impact on the environment under high moisture content.