Response Of Decomposition In Different Diameter Fine Roots Of Pinus Tabuliaeformis Plantation To N Addition

Increasing nitrogen(N)deposition significantly affects global forest ecosystems.To maintain forest ecosystems health,it is necessary to understand the responses of forest ecosystems function,including material recycling and energy flowing,to nitrogen(N)deposition.The continuous increase of nitrogen deposition has already had an important impact on the process of the decomposition of fine root.Fine root decomposition is a key part of the nutrient cycling process of forest ecosystems,which is valuable for revealing the response mechanism of ecosystem energy cycle to the environmental changes.However,it is unclear about how the increasing soil available nitrogen,caused by raising N deposition,influences the fine root decomposition in different diameter roots,restricting the understanding of nutrient cycling process and mechanism and the developing of ecological model in forest ecosystems.We simulated N addition experiment to examine the effects of four N addition levels(N0(0),N3(3),N6(6)and N9(9 g N·m-2 a-1))on decomposition and nutrient release of different diameter fine root in Pinus tabulaeformis plantation using litterbag method.The results showed that:(1)The fine root decomposition process of fine roots with different diameters could be divided into rapid(0-120 d)and slow(120-300 d)decomposition stages.The annual decomposition coefficient of 0-0.4 mm diameter(mainly the first two order roots)were 0.148-0.205 kg·kg-1·a-1,0.4-1 mm diameter(mainly second and third order roots)were 0.236-0.260 kg·kg-1·a-1,and 1-2 mm diameter(mainly the third and above order roots)were 0.301-0.350 kg·kg-1·a-1,indicating that finer root had lower decomposition rates.N addition significantly reduced the decomposition rate of roots with 0-0.4 mm diameter.N addition treatments,however,had no significant influence on those in the fine root with 0.4-1,1-2 mm diameter.(2)N addition changed the element and the chemical composition content of the fine root,and N addition had different effects on different elements and components.Fine root decomposition in the process of C,N and P elements and the content of soluble sugar and starch were calculated,and lignin content were rising wave type,and the trend of change had not been changed by the exogenous N input,but N addition to the extent of the change had different degrees of influence because of the differences of fine root diameter level and N fertilizer level.(3)N treatments increased the C,N and lignin remaining percentages in the fine root with 0-0.4 mm diameter and 0.4-1 mm diameter,but had no significant influence on those with 1-2 mm diameter,and had no significant influence on P and soluble sugar,starch remaining percentages with all the three diameter classes.The transfer models for C,N and P in all the fine root were direct releasing,except that the transfer models for N showed a riching-releasing model.The nutrient release rates of fine roots with 0-0.4 mm diameter were higher than those of fine roots with 0.4-1 and 1-2 mm diameter.(4)C,N,P,lignin,C/N and lignin/N are common chemical predictor of fine roots decomposition rate.Under different N treatment,decomposition of fine root remaining percentages of the 300 days had significant negative correlation relationship with C,C/N and lignin/N,but had significantly positive correlation relationship with N,P,lignin.During the study period,C,N and P retention rate had positive correlation relationship with the residual rate of dry matter in the process of fine root decomposition,and the correlation reached extremely significant,lignin,lignin/N,lignin/P had negative correlation relationship with the fine root retention rate.Overall,N treatment did not change the direction of the relationship between the fine root nutrient retention rate and residual rate of dry matter,but the correlation intensity had changed.(5)During the 300-day study period,enzyme activity of different diameter classes firstly increased and then decreased.N treatment significantly affected cellobiase activity in different diameter classes fine root,low nitrogen treatment increased the acid phosphatase activity,but high nitrogen inhibited the acid phosphatase activity.C and N content of 0-0.4mm fine root had significant positive correlation with enzyme activity,P content of different diameter classes fine roots had significant positive correlation with enzyme activity,enzyme activity had significant positive correlation with fine root decomposition rate under different N addition levels.In a word,N addition affected forest ecosystems fine root decomposition and nutrient release process,and the material cycle of forest ecosystems mainly by influencing the chemical composition of fine roots with 0-0.4,0.4-1 mm diameters.