Influencing Mechanisms Of Vegetation Restoration On Key Processes Of Nitrogen Cycle And The Relationship Between Soil Carbon And Nitrogen In Hilly Area Of Central Hunan Province

Nitrogen(N)is an essential element for the functioning of terrestrial ecosystems and is often the primary limiting resource for the productivity of many terrestrial ecosystems.Ecosystem N cycle is the center of biogeochemical cycle of various elements.An assessment of N dynamics in ecosystem is of great significance for predicting carbon(C)sequestration capacity under global climate change.Existing systematic knowledge on ecosystem N biological cycle in different restoration stages is none.The effects of vegetation restoration on soil N contents,availability,mineralization in subtropical regions are inconsistent.Meanwhile,the effect of soil N accumulation and mineralization on SOC pool accumulation and its stability along vegetation restoration are poorly understood.By using the method of "space-time substitution",four distinct plant communities along a restoration gradient from ER(4-5 years scrub-grassland),MRI(10-12 years shrub),MRII(40-50 years coniferous and broadleaved mixed forest),LR(80-90 years evergreen broadleaved forest)were set up as the fixed plots.Samples were collected.We compared the process of ecosystem N biological cycle,soil N accumulation and N availability.Incubation experiment was conducted in the laboratory to explore soil N mineralization and its temperature sensitivity under different restoration stages.The relationships between soil N accumulation,mineralization and SOC accumulation and its stability were analyzed.The objective was(ⅰ)to reveal the driven mechanisms of vegetation restoration on the process of ecosystem N cycle and C-N coupled relationship;(ⅱ)to provide scientific basis for sustainable nutrient management,predict future C,N cycles and resilience global climate change;(ⅲ)It plays an extremely important role in achieving the ’double carbon’target.The main findings were as follows:(1)Storage of N in ecosystem,vegetation layer,litter layer,and soil layer increased significantly with vegetation restoration.N storage within ecosystem showed a consistent vertical distribution pattern:0-40 cm soil layer>vegetation layer>litter layer.The contribution of vegetation N storage to ecosystem N storage increased with restoration,while the contribution of soil N decreased.The results implied that soil N storage accounted for the most of the ecosystem N,but transformed from soil to the vegetation layer during vegetation restoration.The N biocycle characteristics of ecosystem varied significantly with vegetation restoration.The annual N retention in MRI was the highest(64.34 kg/ha),followed by LR(56.45 kg/ha),and that in ER was the lowest(15.89 kg/ha).The annual N absorption and return generally increased with vegetation restoration,with LR ecosystem having the highest,followed by MRI,and ER having the lowest.The annual N absorption increased from 42.50 kg/ha of ER to 132.80 kg/ha of LR,and the annual return increased from 23.80 kg/ha of ER to 76.35 kg/ha of LR.The highest absorption and utilization coefficient were in MRI(0.028,0.55),followed by LR(0.018,0.42),and MRII(0.015,0.37)was the lowest.The order of N cycling coefficient was:MRII(0.71)>ER(0.63)>LR(0.58)>MRI(0.45).The circulation period was highest in MRII(11.94 a),followed by LR(10.18 a),and the lowest was in ER(1.11 a).(2)Total N(TN),soluble organic N(SON),microbial biomass N(MBN),and ammonium(NH4+)contents increased remarkably with vegetation restoration,while nitrate N(NO3-)decreased from ER to MRI and then increased.IN/SON and NO3-/NH4+ratios were<1 and decreased with vegetation restoration,while the MBN/IN ratio increased.The results implied that soil N availability reduced with vegetation restoration and N cycling remained closed,although it has been restored to the broadleaved forest stage,which was conducive to soil N sequestration.Substantial connection with soil N contents and vegetation biomass,SOC,soil C/N,microbial biomass C,urease activity and soil pH,indicating that soil N accumulation with vegetation restoration were mainly due to the increase of biomass,the improvement of soil physic-chemical properties and microbial activities.The close correlation between plant diversity index and soil N availability indicated that soil N availability reduces largely due to increasing plant N demands induced by the improvement in plant species diversity.Soil N,especially SON,was significantly positively correlated with ecosystem N biological cycle.The comprehensive index of soil N accumulation and N biological cycle developed synchronously in ER and MRI,and gradually changed into the lagging development of N biological cycle with vegetation restoration.Soil N availability was significantly negatively correlated with ecosystem N biological cycle.(3)Vegetation restoration remarkably increased soil net N ammonification rate(NAR),net N nitrification rate(NNR),and net N mineralization rate(NMR).NAR increased rapidly in the early restoration stage,which increased by 214.45-233.17%from ER to MRI.NAR increased slowly in the middle and late restoration stages,increasing by 20.43-31.32%from MRI to MRII,and 20.43-31.32%from MRII to LR,respectively.The results revealed that the enhancement of N ammonification dominantly occurred in the early stage of vegetation restoration.NNR increased slowly from ER to MRI by 64.18-72.03%,from MRI to MRII by 34.88-60.21%,and increased rapidly from MRII to LR by 181.65-184.23%,indicating that the increase of N nitrification mainly occurred in the late stage of restoration.NMR increased rapidly in the early and late stages of restoration,from ER to MRⅠ increased by 102.01-112.24%,from MRⅡ to LR increased by 124.65-134.62%,and slowly increased in the middle stage,from MRⅠ to MRⅡ increased by 25.95-44.15%.Among different restoration stages,soil initial NH4+,0.05-2 mm soil sand percentage(Sand),N content in litter(LN),litter biomass(LB)and microbial biomass phosphorus(MBP)were the important factors affecting the process of N mineralization.LB and MBP were the direct regulators of NAR,and LB,LN,Sand,MBP were the direct regulators of NNR and NMR.There were positive correlations between NAR,NNR,NMR and annual N retention,N return,N absorption,circulation period,indicating that the enhancement of soil N ammonification,nitrification and mineralization were beneficial to accelerate the process of ecosystem N biological cycle.The process of soil N mineralization was synchronized with the process of N biological cycle in ER,and then gradually transformed into the lagging development of N biological cycle with vegetation restoration.(4)A rise of incubation temperature could promote net N ammonification(Na),NAR,net N mineralization(Nm),NMR,could promote net N mineralization(Nm),NMR first and then inhibited latter.Temperature had a significant interaction with vegetation restoration and soil layers.The microbial activity of soil N ammonification was the highest at 30-40℃,and the microbial activity of soil N nitrification was the highest at 10-25℃.The Q10 of N ammonification(NAR-Q10),nitrification(NNR-Q10),and mineralization(NMR-Q10)ranged from 1.89 to 2.15,2.18 to 3.28,1.92 to 2.12,respectively,at the 0-10 cm and 10-20 cm soil layer.NAR-Q10,NNR-Q10,and NMR-Q10 showed general trends of decline with vegetation restoration.The results revealed that vegetation restoration would be helpful to reduce the sensitivity of N ammonification,nitrification,and mineralization to warming,to improve the stability of N mineralization.NNR-Q10 was much higher than NAR-Q10,indicating that the sensitivity of soil N nitrification process to warming was much higher than that of ammonification process.The sensitivity of soil N mineralization to temperature rise depends on the sensitivity of N nitrification.Soil initial inorganic N(IN)content was the main factor reducing NAR-Q10,NNR-Q10,and NMR-Q10.NAR-Q10 and NMR-Q10 were directly inhibited by initial IN content,microbial biomass and urease activity,NNR-Q10 was directly restrained by initial IN content and microbial biomass.(5)Soil TN and SOC contents increased simultaneously with vegetation restoration.Soil N forms were also correlated with SOC pool and stability,except for the significant correlation between SOC and TN content.SON and NO3-contents explained the changes of active,slow,and resistant SOC of 63.2%and 5.8%,respectively.The results indicated that SON and NO3-accumulation accelerated the accumulation and stability of SOC pool.In ER,there were weak correlation between soil N forms and the accumulation and stability of SOC fractions.In MRI and MRII,SON was positively correlated with SOC pool accumulation and stability.In LR,the accumulation and stability of SOC pool increased with the increment of NH4+.MBN/IN increased,IN/SON,and NO3-/NH4+decreased with vegetation restoration,which significantly increased Ca,Cs,Cr and Cr/SOC,suggesting that the decline of soil N availability promoted the accumulation and stability of SOC pool.NAR and NNR significantly positively affected Ca,Cs,Cr and Cr/SOC,with 50.5%and 7.40%interpretation,respectively.The results implied that the enhancement of soil N mineralization improved the accumulation and stability of SOC pool during vegetation restoration.In LR,the increment of NNR significantly promoted the accumulation and stability of SOC pool.To sum up,throughout the whole vegetation restoration process,the accumulation and stability of SOC pool were promoted by soil N accumulation,mineralization,and N availability increment.The effects of soil N accumulation,availability and mineralization on SOC accumulation and stability were different in different vegetation restoration stages.Reasonable management measures promoting vegetation restoration is conducive to improving ecosystem N storage,N utilization,N return,and is of great significance to the restoration of soil fertility,and the increase in ecosystem productivity.Besides that,vegetation restoration is in favor of the perennially N accumulation,retention and fixation,improving soil N supply capacity,reducing the sensitivity of soil N ammonification,nitrification and mineralization to warming,reducing nitrate loss,enhancing soil N sink capacity,and promoting SOC accumulation and stability.