Effect Of Nitrogen And Phosphorus Addition On Fine Root Functional Traits Of Pinus Massoniana And Chinese Fir

Fine roots are important organs for trees to obtain nutrients and water from the soil.They are usually defined according to their diameter(such as ≤2 mm),but this definition ignores the heterogeneity of fine roots and functional differences between different root levels.Based on the root order method,dividing fine roots into absorbing roots and transporting roots can help to study tree strategies for soil resource utilization.Fine root functional traits include anatomical,morphological,chemical,and physiological aspects,which are a balance between maximizing nutrient resources for tree species and minimizing the cost of constructing fine root tissues.Therefore,studying the relationship between fine root functional traits and environmental factors will enhance the understanding of tree species’ adaptation strategies to environmental changes.Nitrogen(N)and phosphorus(P)are essential elements for plant growth,which constrain plant growth and terrestrial ecosystem productivity.Due to the difference of N and P transformation in soil and the different ways of nutrient utilization of mycorrhizal species(arbuscular mycorrhizal AM and Ectomycorrhiza ECM),under the background of atmospheric N deposition,how N and P addition affects the functional properties of fine roots of trees needs to be further studied.This study selected the main afforestation tree species in southern China,Pinus massoniana(ECM type)and Chinese fir(AM)artificial forest(built in 2008),and designed field experiments with CK(pure water)and 4 levels of N and P addition,with 4 replicates for each treatment.Among them,N was added with ammonium chloride(NH4Cl)at four levels:N1(25 kgN·hm-2·a-1),N2(50 kgN·hm-2·a-1),N3(100 kgN·hm-2·a-1),and N4(200 kgN·hm-2·a-1).P was added with sodium dihydrogen phosphate(NaH2PO4),and the four levels were P1(25 kgP·hm-2·a-1),P2(50 kgP·hm-2·a-1),P3(75 kgP·hm-2·a-1),and P4(100 kgP·hm-2·a-1).Measure the biomass of fine roots using the soil drilling method,and collect complete fine roots for grading,including specific surface area(SRA),specific root length(SRL),tissue density(RTD),and mycorrhizal infection rate.Main research findings:(1)Compared with the control(CK),with the increase of nitrogen(N)addition,the fine root biomass of masson pine plantations in the non growing season(January)and growing season(August)first significantly increased,and then significantly decreased at high N(N3-N4).The specific root length(SRL)and specific surface area(SRA)of non growing season 2nd and 3rd grade roots and growing season 3rd grade roots all decreased,with a more significant downward trend in non growing season.The infection rate of root mycorrhizal fungi at all levels significantly decreased,and the downward trend was most significant at the middle N.The carbon(C)content in fine roots significantly decreased except for an increase at the second level roots.The fine root biomass of Chinese fir plantations in non growing and growing seasons increases with the increase of N addition,and the increasing trend is more significant in the growing season.During the growth season,the SRL and SRA of the 2nd and 3rd grade roots significantly increased,with the most significant increase trend at low and medium N levels.The 1st grade roots significantly increased at low and medium N levels,while they decreased significantly at high N levels(N3-N4).There was no significant change during non growth seasons;The infection rate of various levels of mycorrhizal fungi significantly decreased at low N and medium N,and showed a rebound trend with the increase of N content;The C content of fine roots at all levels significantly decreased.Each N treatment had almost no effect on root diameter and root tissue density(RTD).(2)Compared with the control(CK),except for a slight increase in low phosphorus(P1)during the growth season,the fine root biomass of masson pine plantations decreased with the increase of phosphorus(P)addition during the non growth and growth seasons.The SRL and SRA of roots at all levels decreased,with a more significant downward trend during the growing season.The infection rate of various levels of mycorrhizal fungi significantly decreased at low and medium P(P2)levels,and showed an upward trend with the increase of P content.The C content of fine roots at all levels decreased.All P treatments promoted an increase in fine root biomass of Chinese fir plantations.The SRL and SRA of roots at all levels increased significantly during the growing season,while there was no significant change during the non growing season.The infection rate of various levels of mycorrhizal fungi significantly decreased with the addition of low and medium P,and showed a trend of recovery with the increase of P content.The C content of fine roots at all levels significantly decreased.Each P treatment had almost no effect on root diameter and RTD.(3)After the addition of N and P,the changes in biomass and functional traits of fine roots of Chinese fir were more significant than those of Pinus massoniana,indicating that Chinese fir mainly relies on do it yourself to obtain nutrients,while the response to mycorrhizal infection rate of Pinus massoniana fine roots is more obvious,indicating that Pinus massoniana mainly relies on the cooperation and outsourcing of fine roots and mycorrhizal fungi to obtain nutrients.