| Detritus Input and Removal Treatment(DIRT)was conducted to examine the effects of shifting above-and belowground inputs on soil respiration,microbial biomass,microbial community composition and function in subtropical Pinus elliottii,Eucalyptus urophylla×Eucalyptus grandis,Acacia aulacocarpa and Casuarina equisetifolia coastal sandy plain forests,and to discuss the relationship among them.The experimental site was located at Changle Dahe State-owned Protection Forest Farm of Fujian Province(25°57'N,119°24'E).A completely randomized block design approach was conducted to set three 2 m×2 m subplots in each of four 20 mx20 m plots,including Root Trenching,Litter Removal and Control.Another four plots in P.elliottii plantation were added for Girdling treatment,and the four plots were also filled with three treatments mentioned above.In order to conclude the.effects of shifting input on soil respiration and its mechanism,soil respiration monthly after the treatments being built was measured,soil microbial community and enzyme activity was also tested in each senson.The main conclusions coming from the experiment were as follows:(1)Tree species have significant effects on soil respiration,mean soil respiration rate per month as well as annual soil respiration flux showed E.urophylla × E.grandis>A.aulacocarpa and C.equisetifolia>P.elliottii.Tree species affected soil microbial biomass,microbial community composition and soil enzyme activity significantly,and the effect contained obvious seasonal variety.(2)The difference of the effects of inputs treatments on soil respiration and the contribution to total soil respiration flux was significant.The removal of root and litter signifacantly reduced soil respiration rate,girdling in P.elliottii plantation didn't affect soil respiration obviously,the contribution of soil microbial respiration in P.elliottii plantation was the biggest(51.1%),the respiration flux derived from litter in E.urophylla x E.grandis plantation contributed the largest component.No matter in A.aulacocarpa or C.equisetifolia plantion,soil microbial respiration was at the top,but the difference derived from litter exclusion was small,root respiration was low.(3)Soil respiration rate had a significant exponential correlation with soil temperature,a significant linear correlation with soil moisture.The configuration status of soil temperature,water content leaded to the seasonal variation of soil respiration,two-factor model can explain 38%-67%variation across the plantations and the treatments.The temperature sensitivity of soil respiration in P.elliottii(1.537),C.equisetifolia(1.492)plantation was significantly lower than that of E.urophylla ×E.grandis(1.665)and A.aulacocarpa(1.878)plantation.The result was opposite to the conclusion of previous that the Q10 value of soil respiration in broad-leaved forest was smaller than that in coniferous forest,it was closely related to that the formation of stable organic carbon in sandy soil was diffidult.Root exclusion significantly increased Q10 value,removal of litter significantly reduced Q10 value,indicating that litter and root input processing caused changes of resisting ability to external environmental changes.(4)In P.elliottii plantation,litter removal significantly decreased the content of MBC,MBN,but root removal significantly increased MBC,soil MBC,MBN content in girdling plots didn't change significantly compared with the control plot,as to the exclusion of root and litter in girdling plot,both of MBC and MBN decreased.Soil MBC and MBN in E.urophylla × E.grandis,A.aulacocarpa,C.equisetifolia plantation changed little after removing root and litter,meaning that soil microbial biomass had high accmulation ability in these plantation,that may be related to the higher original organic matter content.In summer,PLFA content of all microbial groups in girdling plots was significantly lower than that in control plot,the number of microbe decreased signifiantly in root and litter removal plots in E.urophylla× E.grandis plantation.There was no signifiant variation in A.aulacocarpa and C.equisetifolia.The change law of soil microbe in different treatments was similar in other season.(5)Manipulation time was also an important aspect of affecting microbial biomass.With time passing,MBC content in P.elliottii plantation continuously decreased,in E.urophylla × E.grandis plantation decreased only in the last season,inC.equisetifolia and A.aulacocarpa plantation showed Winter>Summer>Spring,exactly corresponding to the intensity of soil microbe using the soil carbon pool.(6)The change of soil microbial community structure was significantly affected by above-and belowground inputs treatment.A year after treatments,the above-and belowground inputs significantly reduced the fungi abundance and the ratio of fungi to bacteria,increased the abundance of actinomyces in P.elliottii,A.aulacocarpa and C.equisetifolia plantations,while there was no obviously variation in E.urophylla ×E.grandis plantation.During the study,the biggest change of soil microbial community composition is the ratio of gram-positive bacteria to gram-negative bacteria,it largened in winter because of the higher stress resistance with gram-positive bacteria.Principal component analysis showed that time factor was an important condition to promote the evolution of microbial community structure.(7)The exclusion of litter,root and girdling treatment had significant effects on soil enzyme activities.After removing root,litter and girdling,hydrolytic enzyme activities in P.elliottii plantation were lower than control,the activity of oxidase under the removal of litter was significantly lower than control treatment.The effects of treatment on the activity of hydrolase in E.urophylla x E.grandis plantation fluctuated significantly between seasons and the activity of oxidase was not significant compared with control.Litter removal reduced the hydrolase activity in A.aulacocarpa and C.equisetifolia plantations,soil oxidase activity didn't change much.The variation of soil enzyme activity also had obvious seasonal difference.Except for P.elliottii,the soil enzyme activities of the other tree species were higher in spring than in other seasons.(8)Correlation analysis showed that there was signifacant negative correlation between cellulose hydrolase activity and gram-positive bacteria,arbuscular mycorrhizal fungi and fungal biomass.A positive correlation existed between acid phosphatase and total PLFAs,gram-positive bacteria and fungi biomass,N-acetyl-?-d-glucosaminidase was positively affected by fungi biomass,Polyphenol oxidase was positively related to arbuscular mycorrhizal fungi biomass.Significant positive correlation between Peroxidase activity and all kinds of microbial groups excluding gram-positive bacteria was discovered.Soil respiration rate was significantly associated with all soil microbial population,especially arbuscular mycorrhizal fungi,soil respiration rate and soil oxidase activity was significantly correlated,and the hydrolase enzyme whose activity was significantly related to respiration rate was different among different treatments.The significant relationship between soil microbial community and enzyme activity and soil respiration confirmed that the composition and function(enzyme activity)of soil microbial community was microscopic manifestations of soil respiration. |
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