A Study Of Soil N Transformation And N₂O Production For Castanopsis Carlesii Dominated Natural Forest As Affected By Temperature And Moisture

Nitrogen is a restricted nutrient that is biologically necessary,and the process of soil nitrogen transformation is closely related to the supply of available nitrogen in soil.Temperature and moisture are important factors in soil nitrogen transformation,and subtropical forest soil is rich in inorganic nitrogen resources.Under high temperature and rainy climate conditions,there are abundant inorganic nitrogen resources in subtropical forest soil.However,the nitrogen transformation process and nitrogen retention mechanism of soil are still not clear.In this study,short-term(144h)lab incubation experiments were conducted for a surface soil of 0-10cm layer under an typical natural forest vegetation dominated by Castanopsis carlesii tree collected in Jian'ou Wanmulin Natural Reserve in the midsubtropical region of Southeastern China,in which five temperatures levels(5?,15?,25?,35?,45?)and 5 moisture levels(30%WHC,50%WHC,70%WHC,90%WHC,110%WHC)were set up and were used the ¹⁵N paired(¹⁵NH₄¹⁴NO₃and ¹⁴NH₄¹⁵NO₃)and Hart improved the Kirkham analytical method.To study the effect of temperature and moisture on the gross rates of soil nitrogen and N₂O emission fluxes,aiming at revealing the dynamic change law and principle of the content of inorganic nitrogen in different forms in soil in this area,and in-depth understanding of soil nitrogen retention mechanism.The main results are as follows:(1)The increase of temperature significantly promoted the gross mineralization rate and net mineralization rate.The gross NH₄⁺immobilization rate increases at 5-25?,but decreases at 35-45?.The gross nitrification rate increased gradually with the increase of soil temperature and decreased at 45?,but the sensitivity of the gross nitrification rate to temperature was lower than that of the gross mineralization rate.At 5-25?,the gross NO₃^-immobilization rate is higher than the gross nitrification rate,and the net nitrification rate is negative.At 35-45?,the gross NO₃^-immobilization rate is lower than gross nitrification rate,and the net nitrification rate is positive,and NO₃^-accumulates.The above results showed that the contents of nitrogen NH₄⁺and NO₃^-in soil increased with the increase of temperature,but NH₄⁺was the main form.Under the influence of temperature change,the increase of nitrogen availability in soil was attributed to the higher gross mineralization rate and gross nitrification rate than the immobilization rate,indicating that rapid nitrogen mineralization and nitrification were the causes of nitrogen enrichment in humid subtropical forest soil.Therefore,under the condition of global warming,the contents of NH₄⁺and NO₃^-in subtropical forest soils will further increase.(2)With the increase of temperature,soil N₂O emission showed an increasing trend,but not significant.The emission rate of N₂O was 25?>15?>35?>45?>5?.The highest emission rate of N₂O was cultured at 25?.N₂O mainly comes from nitrification pathway.The change of soil physical and chemical properties caused by temperature change may be the main reason for the increase of N₂O emission.(3)The increase of soil moisture promoted the gross mineralization rate and gross NH₄⁺immobilization rate.The sensitivity of gross mineralization rate to soil moisture was higher than the gross NH₄⁺immobilization rate.With the increase of water content,the gross nitrification rate increased but was not significant.When the soil water content increased from 30%WHC to 90%WHC,the gross nitrification rate also increased.When the soil water content was saturated(110%WHC),the gross nitrification rate decreased.Soil moisture was significantly negatively correlated with the net nitrification rate.The gross NO₃^-immobilization rate increased significantly with the increase of soil moisture content.The change of soil moisture would change the net nitrification rate and the gross NO₃^-immobilization rate,and reduce the loss of NO₃^-leaching.The results showed that the main form of soil available N content was NH₄⁺and the increase of soil available N content under higher water content was attributed to the high gross mineralization rate and relatively low the gross NH₄⁺immobilization rate,primary nitrification rate and the gross NO₃^-immobilization rate.Nitrogen enrichment in humid subtropical forest soils can be attributed to rapid nitrogen mineralization under relatively high soil moisture,resulting in the release of more NH₄⁺.(4)The N₂O emission increased with the increase of soil moisture,but it was not significant.In the range of 30-70%WHC,N₂O emission decreased with the increase of water content,and then increased with the increase of water content(90-110%WHC).Under different water conditions,NO₃^-is the reaction substrate for N₂O production,and denitrification is the main source of N₂O production.The emission of N₂O is mainly related to the reaction substrate NH₄⁺and NO₃^-,and C/N is the main factor affecting N₂O.