Effects Of Soil Warming And Nitrogen Addition On Fine Root Production And Ecophysiological Traits Of Chinese Fir (Cunninghamia Lanceolata) Seedlings

There have been a large number of studies on the separate responses of fine roots to warming and nitrogen deposition,but with contradictory results.Fine root production plays a critical role in ecosystem carbon,nutrient and water cycling,yet how it responds to the interactive warming and nitrogen addition is not well understood.In the present study,we aimed to examine the interactive effects of soil warming and nitrogen addition on fine root of 1-year-old Chinese fir(Cunninghamia lanceolata)seedlings in subtropical China.A mesocosm experiment,with a factorial design of soil warming(ambient,+5℃)and nitrogen addition(ambient,ambient+40 Kg·hm-2·a-1,ambient+80 kg·hm-2·a-1),was carried out in the Chenda State-owned Forest Farm in Sanming City,Fujian Province,China.We measured changes in fine root production(indicated by fine root length production)after two year of soil warming and nitrogen addition using minirhizotron method.Ingrowth core method were also applied to measure fine root growth,morphology(specific root length and specific root surface area,root tissue density),stoichiometry(C,N,P,C/N,N/P),and metabolic characteristics(specific root respiration rate,soluble sugar,starch,nonstructural carbohydrates)The results showed that:(1)Soil warming had significant effect on total fine root length production,while nitrogen and soil warming×nitrogen had no effect on total fine root length production.There were seasonal patterns of fine root length production in different treatments.In the first year,fine root production was significantly increased by soil warming and soil warming×nitrogen addition in spring,but was significantly decreased by soil warming in summer.While in the second year,soil warming increased fine root production in winter,and soil warming and nitrogen addition increased fine root production in autumn.Soil warming significantly increased fine root production at soil depths of 10-20 cm,20-30 cm and 30-40 cm,indicating that fine roots will grew deeper under soil warming.Compared with the 1-2 mm roots,the 0-1 mm roots were more flexible to change in production;soil warming increased 0-1 mm fine root production only.(2)Fine root growth measured by ingrowth core was decreased by soil warming,while soil warming and nitrogen addition had no significant effect on fine root growth at the first time of sampling.Fine root growth was decreased by nitrogen addition only in the first year,and was increased by nitrogen addition only at the third time of sampling.The 0-1 mm root growth was effected by soil warming×nitrogen addition at all the four samplings.(3)Fine root morphological traits were only changed slightly by soil warming,nitrogen addition and soil warming×nitrogen,in all.(4)Soil warming had significantly decreased fine root C concentration and increased fine root N concentration.The C:N and P concentration were significant decreased by soil warming and nitrogen addition.(5)Specific root respiration rate(SRR)was not significantly affected by soil warming,nitrogen addition and soil warming × nitrogen,indicating that fine root respiration had not acclimated to soil warming and nitrogen addition.Root soluble sugar was decreased by soil warming only in winter,which might show that fine roots under soil warming were more active than those under the control during the dormant season.In conclusion,soil warming increased soil temperature and soil nitrogen availability,while decreased soil moisture,as a result,fine root production was promoted.Although nitrogen addition increased soil nitrogen availability,nitrogen addition had no significant effect on fine root production.With the soil warming,fine root N enriched and fine root C、P decreased,indicating that the nutrient were out of balance and fine root increment decreased accordingly.With the soil warming,fine root respiration did not acclimate to warming and sugar significantly decreased in winter.Thus,soil warming might cause fine root metabolic imbalance,therefore soil warming increased the fine root mortality and decreased fine root increment.