Effects Of Soil Microbes And Environmental Factors On Soil Nitrogen Cycling In Habitats Of Different Altitudes On Changbai Mountain

Nitrogen is one of the most vital factors to limit plant growth in the forest ecosystems,and it has an important influence on the maintenance of species coexistence and biodiversity.The proportions of different nitrogen forms in soils are different under different conditions.The difference of soil nitrogen forms may be related to environmental factors or community structure and function of key microorganisms in soil nitrogen transformation.Environmental factors can directly affect soil nitrogen transformation,and also can indirectly affect soil nitrogen forms and nitrogen transformation by affecting the community structure and function of key microorganisms in soil nitrogen transformation.However,we still do not know the relative role of environmental factors and key microorganisms of nitrogen transformation in the formation of soil nitrogen forms.Therefore,the differences of different nitrogen forms and nitrogen transformation in broad-leaved Korean pine forest and Betula ermanni forest on Changbai Mountain were studied.We investigated the relative role of environmental factors(temperature and p H)and key microorganisms in the transformation of soil nitrogen form.The main findings are as follows:(1)The content of different nitrogen forms in soils of broad-leaved Korean pine forest and Betula ermanni forest from high to low was as follows: soluble organic nitrogen,ammonium and nitrate.The decreasing order of nitrogen transformation rates was as follows: protease activity,urease activity,and net nitrification rate.Nitrate:ammonium ratio,nitrate:soluble organic nitrogen ratio,urease activity:net nitrification rate ratio,protease activity:net nitrification rate ratio of broad-leaved Korean pine forest soil were significantly higher than Betula ermanni forest soil.However,there were no significant differences in soluble inorganic nitrogen:soluble organic nitrogen ratio,ammonium:soluble organic nitrogen ratio and protease activity:net mineralization rate ratio between broad-leaved Korean pine forest soil and Betula ermanni forest soil.The results showed that the differences of different nitrogen form contents from different habitats are related to the transformation process from ammonium to nitrate.(2)Compare with p H,temperature had a greater effect on nitrogen form and nitrogen transformation in soils of two different habitats.The effect of temperature on nitrogen transformation and the contents of different nitrogen forms in broad-leaved Korean pine forest soil was greater than that in Betula ermanni forest.(3)The variations of the different nitrogen form contents with temperature wheninoculated with soil microorganisms of broad-leaved Korean pine forest were greater than that when inoculated with soil microorganisms of Betula ermanni forest,but the trend of nitrogen transformation parameters with temperature was not obvious.There were no remarkable trends of different nitrogen form contents and nitrogen transformation parameters with the increase of p H when inoculated with soil microorganisms of broad-leaved Korean pine forest and Betula ermanni forest,respectively.(4)At p H 5.5,the responses of different nitrogen form contents and nitrogen transformation parameters(not including net mineralization rate and urease activity)to the increase of temperature were less than those at p H 4.9 when inoculated with soil microorganism of broad-leaved Korean pine forest,but the responses of different nitrogen form contents and nitrogen transformation parameters(not including urease activity)to the increase of temperature were greater than that at p H 4.9 when inoculated with soil microorganism of Betula ermanni forest.(5)At 10 ?,the responses of different nitrogen form contents and nitrogen transformation parameters to the increase of p H were less than those at 20 ? when inoculated with soil microorganism of broad-leaved Korean pine forest and Betula ermanni forest,respectively.The content of different nitrogen forms in soils of broad-leaved Korean pine forest and Betula ermanni forest from high to low was as follows: soluble organic nitrogen,ammonium and nitrate.Soil soluble inorganic nitrogen:soluble organic nitrogen ratio and ammonium:soluble organic nitrogen ratio in broad-leaved Korean pine forest soil have no significant difference when compared with those in Betula ermanni forest soil,but nitrate:ammonium ratio and nitrate:soluble organic nitrogen ratio were significantly higher than those in Betula ermanni forest soil.All these results indicated that the differences of different nitrogen form contents from different habitats may be related to the transformation process from ammonium to nitrate.The soil water content and organic matter of broad-leaved Korean pine forest were significantly higher than those of Betula ermanni forest,and C:N ratio was significantly lower than that of Betula ermanni forest.Nitrate,net nitrification rate,and nitrification potential were positively correlated with soil water content and organic matter,and negatively correlated with C:N ratio.As a result,soil nitrification of Betula ermanni forest was greatly affected,and the nitrate content decreased.