Effects Of Nitrogen Addition On Plant Roots And Soil Chemistry In Two Subtropical Forests

Since the industrial revolution,human's activities product a large number of reactive nitrogen?N?and emission it to atmosphere.Increasing reactive N concentration in atmosphere caused N deposition problem globally.Over the past 50 years,numerous studies were conducted and aiming to reveal the effect of N deposition on terrestrial and marine ecosystems.Among those,a lot of studies indicated that N deposition changed soil nutrient availability and decreased soil pH in forest ecosystems.Those changes have a direct effect on the interactive interface between plant and soil——root.Root,especially fine root,has an important role in biogeochemistry.In order to further reveal the effect of N deposition on underground carbon?C?and other elements cycling in subtropical forests.Castanopsis platyacantha and Pleioblastus amarus roots were chosen as study objects.Using root order research frame and function group research frame,combining with minirhizotron technology,the response of C.platyacantha root's lifespan,morphology,chemistry and decomposition to different N addition gradient(CK,LN,HN,i.e.+0,+50,+150 kg N·hm^-2·a^-1,respectively)and the response of P.amarus root's morphology,chemistry to different N addition levels(CK,LN,HN,SN,i.e.+0,+50,+150,+300 kg N·hm^-2·a^-1,respectively)were studied.The main results were as follows:?1?The C.platyacantha root's?diameter<1 mm?average lifespan in 0-15 cm,15-30 cm,and 30-45 cm soil layers were 186,113,130 d,respectively.Root's median lifespan were 151,87,87 d and turnover rate were 2.42,4.20,4.20 a^-1 in three soil layer,respectively.The turnover rate in LN and HN both are 3.17 a^-1.The results of log-rank test indicated that N addition had no significant effect on root lifespan in all three soil layers.?2?The C.platyacantha fine root biomass?diameter<1 mm?in 0-15 cm,15-30 cm and 30-45 cm soil layer were 65.0,40.8 and 22.5 g·m^-3,respectively.The C.platyacantha fine root biomass in CK,LN,HN were 128.3,125.9,143.9 g·m^-3 in 0-45cm soil layer,respectively.Compared with CK,SN decreased P.amarus root's biomass by 34%.The C.platyacantha fine root diameter were 0.26,0.31,0.43,0.59,and 0.90 mm by order 1-5.With the increase of order,fine root diameter,root tissue density?RTD?increased and specific root length?SRL?,specific root surface?SRA?decreased.N addition had no significant effect on C.platyacantha root diameter,RTD,SRL,and SRA.N addition significantly affect P.amarus root's SRA and SRL.SN treatment decreased branch ratio between order?1+2?and order 3 by 49%.?3?With the increase of order,C.platyacantha root's N,P,and Mg concentration decreased and K,C/P increased.N treatments increased fine root N content and decreased Mg and C/N.However,N addition did not significantly affect the content of C,P,K,Na,Al,Mn,and Fe.N addition changed the liner relationship between C.platyacantha root morphology and root Mg,N concentration.The results of two-way ANOVA indicated that N treatments increased P.amarus root Na concentration,C/N and C/P,and decreased P,N,K,and Mn concentration.Root order significantly affect P.amarus root C,N,P,K,Mn concentration.?4?The C.platyacantha absorb root group?order 1-3?and transport root group?order 4-5?had significantly difference in substrate quality.Absorb root group had higher content on N,P,K,Mg,and Acid-unhydrolyzable fraction than transport root group,while transport root group had higher content on Ca,Mn,and acid-hydrolyzable fraction than absorb root group.After 1.3 a decomposition process,the mass remain rate in absorb root group and transport root group were 72%and 63%and decomposition coefficients?k?were 0.25±0.02 and 0.33±0.03 in CK,respectively.The mass remain rate in LN and HN were 77%and 74%in absorb root group and were 60%and 67%in transport root group.The results of repeated measurement ANOVA indicated that N treatments significantly affected Ca and Mn dynamics and root function group significantly affected mass remain rate,C,N,P,Mn dynamics.As for N,absorb root group showed a releas process since the early stages,while transport root group accumulated N at the early stages and then released.As for Mn,transport root group showed a releas process since the early stages,while absorb transport root group accumulated Mn at the early stages and then released.The reslts of repeated measure ANOVAs indicated that N addition had a significantly effect on Ca and Mn dynamics and root group had a significant effect on mass remain rate and C,N,P,Mn dynamics.?5?Six years of N addition not significantly affect soil TOC,EDOC,IOC and POC in P.amarus forest.SN treatment enhanced soil NO₃^--N concentration.Soil pH in CK,LN,HN,SN were 4.71,4.62,4.71,4.40,respectively.Compared with CK,SN treatment increased exchangeable Al³⁺content by 70%?6?N addition not significantly affect soil TOC,ROC,EDOC and POC in the C.platyacantha forest.Root isolation decreased soil TOC,TN and C carbon activity index in humus layer.N addition increased soil NO₃^--N concentration in 0-20 cm soil layer,and root isolation further enhanced this content.N addition decreased soil pH and root isolation further decreased this content.The above results indicated that,as for C.platyacantha forest,N addition not significantly affect soil carbon fraction,fine root biomass,root lifespan,and root morphology,while N addition changed the relationship between fine root N,Mg content and morphology,and enhanced C.platyacantha's Mg requirement.Root isolation had an important influence soil C and N content,that imply root play an important role on soil C or N response to N addition.C.platyacantha absorb root group litter and transport root group litter had different initial nutrient content,that induced absorb root group had a slower decomposition rate than transport root group and diffent dynamics of N and Mn in early stages.However,N addition did not affect C.platyacantha decomposition rate in the early process.As for P.amarus forest,N treatments not significantly affect soil carbon fraction,however,SN increased soil NO₃^--N concentration,induced soil acidify and Al³⁺activation.Thus,N addition influences root biomass,morphology and the concentration of N,P,and K.