| Litter decomposition is a key link in the material cycling of ecosystem.Along with the increases in stand age,the succession of understory vegetation and long-term growth of monospecific tree species are of great possibility to change the characteristics of litter,forest soil and microbial decomposers,and thus affecting the decomposition of litter in plantations.However,at present,the specific impact and underlying mechanisms of stand age increment on forest litter decomposition are still unclear.In the present study,10,20,33 and 43-yr Robinia pseudoacacia plantations in the Loess Hilly Region were taken as the research object.Based on the in-situ decomposition experiment,the change in chemical characteristics of litter,physical and chemical factors of forest soil,and bacterial and fungal decomposers and their extracellular enzyme activities with the increasing stand age were studied,and the pathway of the mentioned changes affecting the litter decomposition of the plantations and their contributions were analyzed.In order to figure out how the increases in stand age affects the material cycling of Rp plantations,and providing scientific basis for selecting more reasonable plantation management measures.The main results of this study are as follows:(1)With the increase of stand age,the decomposition rate of forest litter decreased first and then increased significantly,and the decomposition rate of 33-yr forest litter was significantly lower than the others(P < 0.05);The release rate of C and P increased,while the release rate of N and the degradation rate of lignin and condensed tannin decreased(P <0.05);The mixed decomposition of the litter of the dominant species in 10-yr plantation caused significant synergistic effect,that was,promoted the decomposition,however,the above synergistic effect disappeared with the increase of stand age.(2)The overall substrate quality of forest litter improved with the increase of stand age,which manifested as a trend of increasing nutrient elements and amino acids content,decreasing C/N and lignin/N ratio,and decomposition inhibitors(such as,condensed tannins and water-soluble phenols);in addition,the chemical richness and diversity of the forest litter decreased significantly as well(P < 0.05).(3)The temperature,moisture,available nutrients(ammonium and nitrate nitrogen,and available phosphorus)contents and p H did not exhibited regular changes with the increasing stand age,but in the early(summer)and middle(late spring of the next year)stages of decomposition,the changes in soil temperature and humidity tended to the disadvantage of litterdecomposition.(4)For the predominate genus of litter colonized mcirobes,with the increase in stand age,the abundance of Streptomyces increased continuously(early stage)or increased firstly and then decreased(middle and late stages);the abundance of Curtobacterium and Pantoea increased firstly and then decreased,the abundance of Allorhizobium-NeorhizobiumPararhizobium-Rhizobium show the opposite tendency,the abundance of Sphingomonas decreased continuously(early stage),the abundance of Amycolatopsis increased continuously(late stage).For fungi,the abundance of Alternaria,Lepista and Aspergillus increased firstly and then decreased(early and middle stages or middle and late stage),the abundance of Agaricus increased continuously(late stage),the abundance of Scytalidium show an opposite tendency(middle and late stage),the abundance of Trichothecium and Acremonium exhibited an N-shaped change trend(early and middle stage).With the increase in stand age,the species richness,diversity and evenness of litter colonizing bacteria exhibited fluctuantly decrease in the early stage of decomposition(P < 0.05),while in the middle and late stages of decomposition,the species richness and diversity decreased firstly and then increased,while the evenness decreased significantly(P < 0.05);The species richness,diversity and evenness colonizing fungi exhibited fluctuantly decrease in the early and late stage of decomposition,while the exhibited an opposite trend in the middle stage(P < 0.05).The community structure of colonized bacteria and fungi in litter of 10-yr plantation in the early stage of decomposition was significantly different from those in litter from other plantions(P < 0.05),however,this difference tended to disappear in the middle and late stages of decomposition.(5)For the predominate microbial genus in the litter corresponding surface soil of the plantations,with the increase in stand age,the abundance of RB41(Acidobacteria)increased firstly and then decreased during the whole sampling process;In fungi,the abundance of Mycosphaerella(early sampling stage)and Fusarium continuously decreased(middle sampling stage),the abundance of Gibberella,Hygrocybe,Leohumicola,Marasmius(late sampling stage)and Leucoagaricus(early sampling stage)increased firstly and then decreased(P < 0.05).With the increase in stand age,the species richness,diversity and evenness of soil bacteria increased in general(P < 0.05);In the early stage of decomposition,the diversity and evenness exihibited a fluctuat decreasing trend(P < 0.05),while in the middle and late stage of decomposition,the species richness of soil fungi exhibited a significant increasing trend(P < 0.05).The community structure of litter colonizing bacteria or fungi in different stand ages exhibited no significant difference in any decomposition stage(P < 0.05).(6)With the increase in stand age,the activity of litter laccase(early and late stage),manganese peroxidase(whole decomposition process),β-1,4-glucosidase(middle stage)and protease(late stage)decreased firstly and then increased again,the activities of cellobiose hydrolase(late stage)and alkaline phosphatase(whole process of decomposition)exhibited an opposite trend,while the activities of lignin peroxidase and N-acetyl-β-Aminoglucosidase(middle stage)and polyphenol oxidase(whole process of decomposition)exhibited a fluctuat increasing trend.At the same time,the activity of Lac(early sampling stage),Mn P(early and middle sampling stage),Li P(late sampling stage),βG(early stage of sampling)and NAG(middle stage of sampling)in litter corresponding soil increased firstly and then decreased,the activity of PPO decreased significantly(early and middle stages of sampling),while the activity of CBH(middle and late stages of sampling),Pro(early and late stages of sampling)and LAP(late stage of sampling)increased significantly.(7)With the increasing stand age,the improvement of litter substrate quality in Robinia pseudoacacia plantations would improve the activity of lignocellulases by affecting the litter colonized microbial community structure(mainly the fungal community structure in the early stage of decomposition and the bacterial community structure in the late stage of decomposition),and finally accelerate the decomposition of litter;At the same time,the decrease in chemical diversity of the forest litter might weaken the synergistic effect in the process of mixed decomposition,and thus hindering the decomposition of litter by reducing the activity of the mentioned enzymes;The changes in soil physical and chemical properties(mainly the increase of temperature in the early stage of decomposition,the deterioration of humidity conditions and the decrease in temperature in the late stage of decomposition)indirectly affected the community structure of litter colonized microorganisms by influencing the community of soil microorganisms(mainly fungi in the early and late stages of decomposition),and thus inhibitd the activity of lignocellulases and enzymes related to nutrient cycling,which was ultimately unfavorable to the decomposition of litter.During the in-situ decomposition of the litter from R.pseudoacacia plantation,the decomposition rate exhibited an 10-20 yr forest litter = 43 yr forest litter >33 yr forest litter trend,which was a result controlled by both the changes in litter and soil properties.Generally,the nutrient cycling of R.pseudoacacia plantation might be limited when afforestated after 33 years. |
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