Nitrous Oxide Emissions From The Pinus Massoniana Woodland In The Typical Hilly Region Of Southern Subtropical China

Human activity has resulted in the increase of concentration of greenhouse gases inthe atmosphere, which is concerned as a main cause of the global warming andenvironmental changes. Nitrous oxide （N₂O） is one of the potent greenhouse gases, andsoils are the most important sources of N₂O emissions. To our best knowledge, N₂Oemissions from soils are mainly attributed to two soil microbial processes ofnitrification and denitrification. Forests are important part of terrestrial ecosystem,occupying a vital position in the greenhouse gases emissions. Consequently, theresearch on N₂O emissions characteristics of woodland at subtropical hilly region andits controlling factors can predict total N₂O emissions at this region. Furthermore, it alsoplays an important role on formulating N₂O reduction measures. We thereforemonitored N₂O emissions from the woodland soil of a Pinus massoniana using a staticclosed chamber-GC method in a typical hilly red soil region in southern subtropicalChina over a whole year, and analyzed the relationships between N₂O emissions andenvironmental factors. At the same time, a laboratory incubation experiment wasconducted to quantitatively study the impacts of different temperature and soil moistureon N₂O emissions from the woodland soil. The main results were as follows:（1） N₂O fluxes during the observation period showed obviously seasonal variationas high fluxes corresponded well to warmer season in summer and early autumn, andthe low fluxes to cold season in late autumn, winter and spring. The mean daily N₂Oflux was3.89g N/ha/d. Negative fluxes were also observed in winter. The result of the Pearson’s correlation analysis indicated that daily N₂O fluxes showed positive andsignificant correlations with daily minimum air temperature, soil temperature at5cmand soil volumetric water content of0²0cm topsoil. In addition, it showed extremelysignificant correlation with the cumulative precipitation over previous four days and soilmineral nitrogen contents of0²0cm topsoil. In addition, a model for predicting N₂Oemissions from the Pinus massoniana woodland was developed using multiple linearregression analysis.（2） The results for a15-day laboratory incubation showed that under the incubationtemperatures of15℃,25℃and30℃, and at the incubation soil moistures ofWFPS=40⁶0%, N₂O emissions from soils increased with the incubation time describedby a “S-shaped” curve and the cumulative N₂O fluxes for15days were also high, up to242.85mg N₂O-N/kg. When the incubation soil moistures were at WFPS=20³0%, thecumulative N₂O fluxes for15days were very low, ranging from1.92to2.72mgN₂O-N/kg. The nitrification and denitrification were both inhibited at5℃, and theaverage value of the cumulative N₂O fluxes of15days at this temperature was just0.55mg N₂O-N/kg. The average cumulative N₂O fluxes increased with the increases of soiltemperature and moisture, especially when the WFPS was greater than40%. The resultof the nonlinear parameter optimization estimation indicated that the N₂O emission Q10values of soil nitrification and denitrification were1.15and3.0, respectively; theoptimal soil moisture of nitrification-induced N₂O emission was at WFPS=56%; the soilWFPS exponential response coefficient of N₂O emissions from soil denitrification was8.8.