Effects Of Simulated Nitrogen Deposition On Fine Roots Decomposition Of Main Plant Species In Temperate Typical Forests And Grasslands

Nitrogen(N)deposition is having a significant impact on global forest and grassland ecosystems,as well as the decomposition of fine roots,which is the main pathway for nutrient return to the soil.Studying the effect of N sedimentation on fine root decomposition is helpful to clarify the response mechanism of ecosystem material circulation and energy flow to the continuous increase of nitrogen deposition.However,in the context of the continuous increase of atmospheric N deposition,it is still unclear how the increase in soil available N affects the fine roots.Therefore,this study takes temperate fine roots of forests and grasslands as the research object.The fine roots of forest tree species are studied by root order method,and the fine roots of grassland are studied by the diameter method.Based on N fertilizer and control treatment.The effect of N deposition on fine root decomposition of temperate forests and grasslands was studied by litterbag method.The results showed that:Significant differences in the decay rate between different root orders of four tree species(P<0.01).The fine root decay rate of the four tree species is generally shown as higher order[3+4]roots larger than lower order[1+2]roots.The decomposition constant k values of the high-order roots and low-order roots of Pinus koraiensis are 0.56 and 0.5,respectively.The k values of fine roots of Larix gmelinii are 0.59 and 0.51.The k values of fine roots of Fraxinus mandschurica are 0.62 and 0.58.The k values of fine roots of Betula platyphylla are 0.69 and 0.56.Under the background of long-term simulated nitrogen deposition,Soil available nitrogen increase significantly affects fine root decomposition rate.Using the asymptotic decay model to simulate the decomposition process of fine roots,it was found that N addition promoted the initial stage of decomposition of high-order[3+4]roots,and there was a significant difference(P<0.01)except for Fraxinus mandschurica.The decay constant ka values of fine roots of Pinus koraiensis under N fertilizer and control treatment were 0.65 and 0.56,respectively.The ka values of the fine roots of Larix gmelinii were 0.69 and 0.59.The ka values of the fine roots of Betula platyphylla were 0.78 and 0.69.However,the N addition had no significant effect on the initial stage of the low-order[1+2]root decomposition(P=0.29).In the late stage of decay,when the decay constant ka value approaches zero,the mass remaining rate(A)of the lower order[1+2]root and the higher order[3+4]root generally shows that the nitrogen fertilizer is larger than the control treatment,and has significant Difference(P<0.05).The results showed that nitrogen deposition promoted the decomposition rate of fine roots in the forest and inhibited the decomposition rate in the later stage.The responses of the five microbial enzymes that degrade either cellulose or lignin provide an important mechanism explaining decreased decay rates caused by N enrichment during the later stages of decomposition.After 2 years,addition of N generally stimulated the activity of microbial cellulose(P-1,4-glucosidase,cellobiohydrolase,and endocellulase),but decreased the activity of ligninolytic enzyme in this experiment Therefore,the change of enzyme activity in the process of fine root decomposition is an important mechanism to explain the decrease of decomposition rate in the late stage of fine root decomposition caused by nitrogen addition.However,based on the study of fine root decomposition of four dominant species in the grassland,nitrogen addition generally promoted the rate of fine root decomposition.The decay constant k values of Allium tenuissimum under N fertilizer and control treatment were 1.219 and 0.932,respectively;The k values of fine roots of Stipa grandis are 0.966 and 0.892.The k values of fine roots of Chenopodium aristatum are 0.575 and 0.457.The k values of fine roots of Leymus chinensis are 0.568 and 0.404.