| Foliar litter chemical composition reflects the initial quality,degradability and litter degradation level,influenced by climate factors,degradation level and litter initial quality caused by species.Due to the limitation of low temperature,the soil development of subalpine coniferous forests is often hindered,and the decomposition of litter plays an important role in maintaining forest productivity.At the same time,gap regeneration is generally occurred and the micro-environment differences formed by the re-distribution of light,temperature and moisture inside forest gaps may significantly alter litter decomposition process,but it is still unclear Therefore,exploring litter chemical composition dynamics and its response to forest gap is vital to deeply understand the mechanism of litter decomposition and subalpine ecological processes.In this way,the following scientific questions are:(1)What are the chemical composition dynamics during foliar litter decomposition in subalpine forests?(2)Does the forest gap regeneration in subalpine forests affect the chemical composition dynamics in the decomposing litter?(3)Does the seasonal snow coverage influence the chemical composition dynamics in the decomposing litter?Therefore,in order to understand the effects of forest gap regeneration on chemical composition dynamics in decomposing litter,the litter bag method was conducted to monitor the foliar litter of six dominant species in an alpine forest in Sichuan,China from 2012 to 2015.Litter mass loss,proximate carbon fractions and chemical components at the end of snow formation period,snow coverage period,snowmelt period and growing season in each year were measured to explore the dynamics of chemical changes during litter decomposition and the effects of forest gap on it.The results are as followed(1)After three years of decomposition,the litter remaining mass of the six dominant tree species was 33-50%,and the mass loss mainly occurred in the first year of decomposition,accounting for over 50%of the total mass loss,indicating that fresh litter experienced a rapid mass loss at the beginning of decomposition.Forest gap improved litter the mass loss in first two years but did not significantly affect mass loss in the third year,which means that the influence of forest gap on litter mass loss principally occurred in the early stage of decomposition.At the same time,forest gap increased the percentage of mass loss during winter from the total three years' mass loss.(2)The initial water-soluble extractables content of 6 litters was 25-43%,the initial dissolved carbon content was 24-148 mg/g,the initial dissolved nitrogen content was 0.07-0.25 mg/g,and the initial dissolved phosphorus content was 0.24-0.58 mg/g.During the decomposition,water-soluble extractables,dissolved carbon and dissolved phosphorus are rapidly lost in the initial stage of decomposition with the water-soluble extractables content decreased to 9%-12%after three years.Dissolved nitrogen accumulates during the snow formation period and then immediately reduced.It is indicated that water-soluble extractables containing dissolved carbon and dissolved phosphorus all had rapid leaching losses at the initial stage of decomposition,meanwhile,the accumulation of dissolved nitrogen may be related to microbial mortality caused by decreasing temperature during snow formation period.The ratio of DC/DN and DC/DP increased during winters and decreased during growing seasons,while DN/DP rose to the highest level in the second decomposition year.Compared with closed canopy,the contents of water-soluble extractables,dissolved carbon and dissolved phosphorus at the gap center were lower during the first decomposition year,and the content of dissolved nitrogen was higer during the second year.(3)The initial non-polar extractables content in 6 litters was 11-33%,and the initial total phenolic concentration was 11-32 mg/g.Non-polar extractables and total phenol were quickly lost in the initial stage of decomposition.After three years of decomposition,the content of non-polar extractables and total phenol decreased to 4-12%and 0.9-1.9 mg/g,and total phenol concentration at gap center was higher than that under closed canopy.(4)The initial content of acid-soluble carbohydrates 6 litters was 27-32%,the hemicellulose content was 8-21%,and the cellulose content was 11-17%.During the decomposition,hemicellulose and cellulose content increased slightly in the first two years while acid-soluble carbohydrates fluctuated in the first two years,but cellulose and acid-soluble carbohydrates content started to decline in the third year.After three years of decomposition,litter acid-soluble carbohydrates and cellulose contents at the gap center were higher than those at the closed canopy(5)The initial content of acid-unhydrolyzable residue in 6 litters was 22-51%,the initial content of lignin was 14-37&,and the content of condensed tannin was 0.2-22 mg/g.During the decomposition,condensed tannins quickly lost in the first year,while the lignin and acid-unhydrolyzable residue slowly accumulated.Birch litter with the highest initial content of acid-unhydrolyzable residue showed immediate loss of acid-unhydrolyzable residue and lignin after natural senescence fallen,declined to 50%and 51%remaining of the initial litter after three years During the first and second year of litter decomposition,acid-unhydrolyzable residue and lignin contents at the gap center were higher than those at the closed canopy(6)The content of initial functional carbon in acid-unhydrolyzable residue in fir and birch litters was:alkyl carbon(37.4%,38.8%),O-alkyl carbon(13.5%,16%),aromatic carbon(13.2%,13.6)%),phenolic carbon(11%,9.4%),carboxyl carbon(10.4%,10.3%).During the litter decomposition,the litter acid-unhydrolyzable residue of fir accumulated with the contents of alkyl carbon,methoxyl carbon,O-alkyl carbon,aromatic carbon and carboxyl carbon increased,while the litter acid-unhydrolyzable residue of birch lost contributed to the reduction in methoxyl carbon,O-alkyl carbon and alkyl carbon contents.Forest gaps increased the contents of alkyl carbon,carboxyl carbon and aromatic carbon in the acid-unhydrolyzable residue,but had no significant effects on other functional carbons(7)Principal component analysis and cluster analysis of chemical components in litter results showed that after three years of decomposition,the chemical composition pattern of the 6 litters with different initial quality tends to become similar—water-soluble extractables,non-polar extractables and acid-soluble carbohydrates content decreased,while acid-unhydrolyzable residue content increased remarkably.Path analysis results showed that the formation of forest gap increased the forest floor temperature and lowered the frequency of freeze-thaw cycle.The increased temperature significantly reduced the content of water-soluble extractables and non-polar extractables in litters,while significantly increased the content of acid-unhydrolyzable residue,thereby reducing the litter remaining mass.The decreased freeze-thaw cycle frequency significantly increased the content of acid-soluble carbohydrates,however,there was no significant effect on the litter remaining mass.The correlation results showed that water-soluble extractables,non-polar extractables,dissoloved carbon/nitrogen/phosphorus,total phenol and condensed tannins were negatively correlated to mean temperature and positively correlated to freeze-thaw cycle,but acid-soluble carbohydrates,acid-unhydrolyzable residue,cellulose and lignin were the oppsite.In summary,after the formation of senesced litter in the subalpine forest,water-soluble and non-polar extractables were rapidly leached out of litters,accompanied by slow degradation of acid-soluble carbohydrates and accumulation of acid-unhydrolyzable residue.It indicated that litter decomposition left the chemical composition pattern of litters with initial quality differences to be consistent.The formation of forest gap promoted the mass loss by increasing the water-soluble and non-polar extractables,meanwhile,it inhibited the degradation of acid-soluble acrbohydrates and increased the accumulation and recalcitrance of acid-unhydrolyzable residue.The accumulated acid-unhydrolyzable residue was a complex polymer-"lignin alike" formed by litter chemical/biological products,while the loss of which was mainly due to the slow degradation of oxyalkyl carbon,methoxy carbon and alkyl carbon components.Therefore,forest gaps in alpine forests will accelerate the nutrient cycling and energy flow of soluble effective nutrients in plant-soil ecosystems through the influence of chemical changes during litter decomposition,in addition,promote the accumulation of acid-unhydrolyzable residue contributes to offering material basis for the formation of soil organic matter,which would be of great significance for maintaining soil fertility and sustainable management in alpine forests. |
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