Study On The Change Characteristics Of Soil Nitrogen And Ecosystem Nitrogen Transformation With Stand Age Of Chinese-fir Plantations

Nitrogen(N)is one of the important elements for plant growth,and even affects the forest productivity.The maintenance of soil N is important for sustainable management of forests,especially in plantations.Both tree N demands and N return are significant different in the process of stand development,thus soil N changed accordingly.Therefore,understanding how the dynamics of soil N status shift with stand age,studying the changes of N stocks allocations and N cycle characteristics with stand age,are significant for sustainable management of plantations.Chinese-fir(Cunninghamia lanceolata[Lamb.]Hook)is an important fast-growing tree species in China.The area of Chinese-fir plantations accounts for 18.01%of the total plantations,and take the first place in China.We selected a chronosequence of Chinese-fir plantations with stands aged 3,16,25,32,and>80 years,resulting in totally 20 sample plots(4 repeated plots in each stand).Both organic and mineral soil sample in each plot were collected.Soil total N(TN),dissolved organic N(DON),microbial biomass N(MBN)and mineral N(i.e.,ammonium(NH₄⁺)and nitrate(NO₃^-)contents in each plot were measured,and compared with the annual N accumulation by trees,annual litter N return and microbial N transformation rates.Among them,microbial N transformation rates were quantified by 15N isotope dilution.We aim to investigate the dynamics of soil N status with stand development and its main drivers,and put forward some suggestions for nutrient management for sustainable management of Chinese-fir plantations.The main fundings in this study are as follows:(1)Stronger changes with stand age were observed in all N forms in the organic horizon compared to the mineral soil.TN and DON in the organic horizon increased steadily with stand age.MBN increased from 206 mg kg-1 to 327 mg kg-1 in the first 25 years and stabilized thereafter.NH₄⁺ content increased sharply from 3 to 16 years and then stabilized,while NO₃^-increased linearly with stand age.The soil N dynamics were co-regulated by N accumulation and litter N input via tree growth and microbial N transformations.Rapid tree biomass accumulation in younger stands greatly depleted soil N,while the older stands with larger litter return and slower growth enabled N re-accumulation in the topsoil.The close positive correlation between total organic N and gross mineralization rate,as well as between NO₃^-and gross nitrification rate indicated that the mineralization of soil organic matter and the nitrification of released NH₄⁺ were the two primary microbial processes controlling the available N supply.NO₃^-:NH₄⁺ ratio in the organic horizon in older stands(32-and>80 years)was greater than 1.0,and the MBN:(NH₄⁺+NO₃^-)ratio was greatly reduced with stand age.This indicates that N availability increase and N cycling is accelerated with stands age.We recommend that the harvest periods should be prolonged so that the stands maintain soil fertility but avoid a slowing growth rate and an increased risk of NO₃^-leaching in the process of plantation management.(2)Tree N stocks increased with stand age.Forest floor litter N stocks increased initially and then decreased with stand age,suggesting that proper extending the harvest periods would help to return more nutrients to soil through litterfall.With increasing age,the percentage contributions of foliage to the total tree N stocks decreased,but always higher than that of stem and branch.This showed that the wood harvesting was not the main component of N losses of Chinese-fir plantations.And we recommend that remained the harvest residues in the site,thus N stocks depleted due to uptake by trees can be returned,in order to maximize the retention of nutrients at the plantations.In addition,the increase of soil N contents in the organic horizon caused by the litter return might be the main reason for the increasing of runoff N stocks with stand age.Although deposition of atmospheric N might to some extent compensate for soil N depletion,we recommend proper maintain certain coverage of understory vegetation in Chinese-fir plantations,to alleviate soil nutrient leaching and reduce the N outputs.Across all stands(except>80 years),soil N outputs were higher than inputs(i.e,annual N uptake were higher than annual litter N return)because of tree uptake,especially in pre-mature stands(25 years).In younger stands,rapidly-growing trees uplift N from deep soil,thus belongs to N-acquiring ecosystems.In older stands,the relative growth rate is decreased,with decreased tree biomass accumulation,low N demands and increased litter N return.This indicated that ecosystem N cycle gradually enhanced.A framework was concluded through comprehensive analyses of N bioavailability mechanism and N transformation characters with stand age in Chinese-fir plantations.In young plantations,high tree relative growth rate with rapidly biomass growth,and very low N return by litterfall resulted in decreased of soil N stocks.At this growth stage,plantation belongs to N-acquiring soil ecosystem with an closed soil N cycle.With stand age increasing,tree relative growth rate decreased,N demand decreased resulting in decreased soil N depleted,and high N return via litterfall increased which become the major N source,soil N stocks increased and recovering,thus ecosystem N cycle gradually enhanced.