Effects Of Nitrogen Addition On The CO₂ Emissions In The Reed（Phragmites Australis） Wetland Of The Yellow River Delta,China

Human activities have increased atmospheric nitrogen（N）deposition.The processes of carbon cycling in wetlands have been affected by increasing N input derived from anthropogenic activities.The N deposition could be classified into two forms as inorganic（e.g.nitrate and ammonium）and organic N,while the proportions of these two N forms in atmospheric N deposition have great spatial variation.Many studies concentrated on the relationships between the amount of N deposition and CO₂ emissions.However,how N forms affect CO₂ emissions is still poorly understood.In this study,the two-years’field experiment（2016²017）and incubation experiment were conducted to investigate the effects of N forms on CO₂ emissions in the reed（Phragmites australis）wetlands of the Yellow River Delta,China.（1）The results of two-years’field experiments showed that three types of N additions stimulated CO₂ emissions（P<0.05）.Compared with the control treatment,nitrate,ammonium and organic N additions increased CO₂ emissions by 23.24%,32.92%and49.02%,respectively.Nitrate and ammonium additions produced similar and positive effects on CO₂ emissions,whereas organic N addition had larger and positive effects on CO₂ emissions than those of inorganic N addition（P<0.05）.This may be attributed to the following reasons.First,three types of N addition tended to increase soilβ-glucosidase activity and Q₁₀0 value of CO₂ emissions,but only organic N addition significantly enhanced soilβ-glucosidase activity and Q₁₀0 value（P<0.05）.Second,inorganic N addition had neutral and negative effects on the decomposition of soil organic carbon（SOC）,whereas organic N addition increased SOC decomposition compared with control treatment（P<0.05）.（2）An incubation experiment was carried out to determine the interactive effect of soil moisture and N addition on SOC decomposition.The results showed that,N addition produced neutral effect on SOC decomposition under 200%WHC.Nitrate addition had no effect on SOC decomposition,whereas ammonium and organic N addition mainly promoted SOC decomposition under 75%WHC¹00%WHC（P<0.05）.Compared with control treatment,the cumulative SOC decomposition was increased ranging from 10%to14%under ammonium addition and ranging from 12%to 20%under organic N addition.Nitrogen addition mainly suppressed SOC decomposition under 50%WHC（P<0.05）.The interactive effect of N addition and soil moisture decreasing from 200%WHC to100%WHC or from 200%WHC to 75%WHC on CO₂ production was mainly synergistic,whereas the interactive effect on CO₂ production was additive when soil moisture further decreased from 200%WHC to 50%WHC.In general,following the decreasing of soil moisture concentration,the interactive effect of soil moisture and N addition was larger than the sum of their single effects on SOC decomposition.This will further weaken the stability of SOC pool in the reed wetlands.（3）An incubation experiment was carried out to distinguish the effects of organic and inorganic N additions on SOC decomposition.The results showed that,nitrate and ammonium additions suppressed SOC decomposition from 8.84%to 25.63%in the soil layer of 0²0 cm（P<0.05）,and had no effect on SOC decomposition in the soil layer of 20⁶0 cm.Nitrate addition increased SOC decomposition from 23.24%to 32.75%in the soil layer of 60⁸0 cm（P<0.05）,and had no effect on SOC decomposition in the soil layer of 80¹00 cm.Ammonium accelerated SOC decomposition from 46.42%to 109.7%in the soil layer of 60¹00 cm（P<0.05）.Organic N addition decreased SOC decomposition from 11.33%to 25.93%in the soil layer of 0²0 cm（P<0.05）,had no effect on SOC decomposition in the soil layer of 20⁴0 cm,and increased SOC decomposition from 26.77%to 231.64%in the soil layer of 40¹00 cm（P<0.05）.In general,nitrate and ammonium additions had no effect on SOC decomposition in the soil layer of 0¹00 cm,and organic N addition accelerated SOC decomposition in the soil layer of 0¹00 cm（P<0.05）.Thus,organic N addition will reduce the stability of SOC pool in the reed wetlands of the Yellow River Delta,China.（4）Three forms of N addition promoted CO₂ emissions from reed wetlands soil,and the effect of organic N input was significantly higher than those of inorganic N input.Moreover,the interactive effect of N addition and soil moisture on CO₂ production was mainly synergistic,and this will further weaken the stability of SOC pool in the reed wetlands.