Effects Of Simulated Nitrogen Deposition On Physiology,morphology And Function Of Sphagnum In Wetland,southwestern Hubei Province

The Sphagnum wetland is a special wetland type formed by the dominant plant Sphagnum and the herbaceous plants.The Sphagnum wetland occupy only 1.5%of the earth’s surface area,but its’carbon storage accounts for 10%to 20%of the soil carbon pool in the global terrestrial ecosystem,which plays an important role in the process of soil carbon accumulation.However,in recent decades,because of human activities,which has significantly changed the nitrogen（N）cycle and increased the input of exogenous nitrogen,the Sphagnum faced the risk of nitrogen deposition has also increased.Therefore,the effects of N deposition on related morphological and physiological indexes of Sphagnum may profoundly change the carbon sequestration process of the Sphagnum wetland.N deposition has a significant effect on the physiological,morphological and functional indexes of Sphagnum.However,there are still great controversies in different studies,and there are few reports on the studies of N deposition on the physiological,morphological and functional indexes of Sphagnum in subtropical wetlands.We selected a Sphagnum wetland in southwestern Hubei Province as the study area.Different concentrations of NH₄Cl solution were sprayed in situ to investigate the effects of simulated N deposition on physiological and morphological characteristics of Sphagnum.Four nitrogen concentrations were applied,namely 0 g·m^-2·a^-1（N0）,3 g·m^-2·a^-1（N3）,6 g·m^-2·a^-1（N6）and 12 g·m^-2·a^-1（N12）,with N0 representing the control（CK）.The results showed that:（1）Physiologically,nitrogen deposition has significant effects on the contents of total carbon（C）and total N in Sphagnum.Among the treatments,the contents of total C and total N in Sphagnum under the N3 treatment were the highest,and compared with those of the CK,increased by 3.78%and 88.52%,respectively.However,the change of carbon contents in Sphagnum was not as obvious as the change of nitrogen contents in Sphagnum,but it was basically consistent with the change trend of N contents,and there was a significant correlation between C and N contents（R²=0.57）.N deposition had no obvious effect on chlorophyll and fluorescence activities of Sphagnum.However,N deposition significantly promoted the antioxidant enzyme activities and the contents of osmotic substances in Sphagnum,especially the contents of soluble sugars and peroxidase activity.This indicates that N deposition makes Sphagnum under the environmental stresses,and the enzyme activities and osmotic substances in Sphagnum can reflect the stress resistance of Sphagnum and the relationship between Sphagnum and the environment to a certain extent,thus producing a certain regulatory mechanism when Sphagnum responds to environmental changes.（2）Morphologically,with the increase in N deposition,the height,branch number,mass per plant,capitulum volume,and leaf cell area of Sphagnum tended to initially increase and thereafter decrease.The maximum values were observed in response to N deposition of 3 g·m^-2·a^-1（N3）.When the amount of N deposition is greater than 3 g·m^-2·a^-1,it has obvious inhibiting effect on the morphological indicators of Sphagnum.This study also found that under different N treatments,the area of the base cell was the largest in different parts of the leaf,and the area of the tip cell was the smallest.（3）Functionally,the special cellular structure of Sphagnum and the way of obtaining external nutrients make Sphagnum has strong capacity of carbon fixation and water retention.N deposition promoted the net primary productivity of Sphagnum to a certain extent,and when the amount of N deposition was greater than3 g·m^-2·a^-1,the net primary productivity of Sphagnum will be inhibited.The maximum water retention of Sphagnum was roughly the same as that of the net primary productivity,and the maximum water retention of Sphagnum treated in N12was the lowest.Compared with the control,N deposition had a certain inhibitory effect on the water holding capacity of Sphagnum.The regularity showed that water loss rate of Sphagnum under N3 treatment was the highest,but the differences of water loss rate of Sphagnum under nitrogen deposition was not obvious.Sphagnum was sensitive to N deposition,and there was a certain load value of the effect of N deposition on physiological,morphological and the function of Sphagnum after 2 years treatment,which was approximately 3 g·m^-2·a^-1.When the amount of N deposition is greater than 3 g·m^-2·a^-1,the effects on morphological indicators of Sphagnum are detrimental and the stress to Sphagnum will increase significantly.The results of this study indicated that the current natural atmospheric N deposition is beneficial to the growth of Sphagnum in the wetland in southwestern Hubei Province.However,continual or doubled N deposition might be harmful to the growth of Sphagnum.