| Fallen logs and litters are important components of forest ecosystem.Their decomposition process plays an important role in soil carbon turnover and nutrient cycling.It is also an indispensable part of forest regeneration.Taking the relict species T.longibracteata W.C.Cheng communities(pure T.longibracteata forest,T.longibracteata+hardwood mixed forest,T.longibracteata+Rhododendron simiarum mixed forest and T.longibracteata+Phyllostachys pubescens mixed forest)at Tianbaoyan National Nature Reserve in Fujian Province as objects,we investigated the storage of woody debris and litters in the four forest types.The main chemical properties,content of recalcitrant components,humidification characteristics of fallen logs and litters,microbial community structure of fallen logs and soil,the physical and chemical properties of soil were measured.Effects of fallen log or litter input on soil organic carbon mineralization and active soil organic matter were also studied using a laboratory incubation method.The main results as follows:(1)From 2012 to 2016,the average storage of woody debris in the four T.longibracteata forests was 11.21-17.37 t·hm-2.Woody debris species,decay class and position(fallen logs,snags,and stumps)ShannonWiener diversity index were 0.8717-1.2958,0.9796-1.3313,and 0.39361.0350,respectively.Woody debris species Shannon-Wiener diversity index in pure T.longibracteata forest was lowest in the four forest types.Decay class Shannon-Wiener diversity index was highest,while position diversity index was lowest in T.longibracteata+ hardwood mixed forest.The litter thickness of undecomposed and semi-decomposed layers of the four forest types were 1.8-4.0 cm and 2.1-4.3 cm,respectively.The storage of litter of the two decomposition layers were 3.30-9.33 t·hm-2 and 4.1711.73 t·hm-2,respectively.(2)The chemical properties of fallen logs were different in the four T.longibracteata forests.The organic carbon of fallen logs in pure T.longibracteata forest were the lowest.Content of total nitrogen in bark decreased with the decomposition.C/N increased with the increase of decay class in pure T.longibracteata forest,while other three forest types were the opposite.Total phosphorus content in pure T.longibracteata forest approximately increased with the decomposition,while in other forest types were stable.Total potassium content increased with the increase of decay class except for sapwood and heartwood in fallen logs of pure T.longibracteata forest.Content of dissolved organic carbon decreased with decomposition in the four forest types.Dissolved organic nitrogen of fallen logs in pure T.longibracteata forest and T.longibracteata+hardwood mixed forest increased,while in other two forest types decreased.Microbial biomass carbon in T.longibracteata+Phyllostachys pubescens mixed forest increased with decomposition.Microbial biomass nitrogen decreased in the four forest types.Content of organic carbon,total phosphorus,total potassium,dissolved organic carbon,dissolved organic nitrogen,microbial biomass carbon,and microbial biomass nitrogen of litter in semi-decomposed layer were higher than in the undecomposed layer.Content of litter total nitrogen was higher in semi-decomposed layer than in undecomposed layer of pure T.longibracteata forest and T.longibracteata+hardwood mixed forest,while in the other two forest types was the opposite.(3)Content of lignin,ratio of lignin/cellulose,lignin/N of fallen logs approximately increased with the decomposition,while content of cellulose,total phenol and condensed tannin decreased in the four forest types.Litter lignin content,cellulose content,lignin/cellulose,lignin/N,total phenol content and tannin content in semi-decomposed layer were 41.60-123.88 g·kg-1,296.87-319.27 g·kg-1,0.25-0.44,6.71-10.39,5.267.15 g·kg-1 and 26.32-100.46 g·kg-1,respectively.Litter lignin content,cellulose content,lignin/cellulose,lignin/N,total phenol content and tannin content in undecomposed layer were 92.81-111.29 g·kg-1,324.47-356.87 g·kg-1,0.26-0.34,7.56-11.63,3.15-8.04 g·kg-1 and 65.83-122.66 g·kg-1.(4)The contents of total PLFAs,bacterial PLFAs and fungal PLFAs in bark of fallen logs of decay class Ⅳ and Ⅴ were both significantly higher than that of other components.With decomposition,the content of PLFAs increased.The effect of fungi on the microbial community structure of bark and low decay class sapwood of fallen logs was significant.And the actinomycetes played a major role in the microbial community variation of heartwood and high decay class sapwood.Compared to bark,the association between chemical properties and microbial community structure in sapwood and heartwood was stronger.The association between recalcitrant components and fungal PLFAs content was strong in sapwood and heartwood.The content of macroelement such as carbon and nitrogen was related to bacterial PLFAs.(5)Contents of humus carbon in fallen logs were increased with the decomposition in the four forest types.Humic acid carbon contents also increased in sapwood.Contents of fulvic acid carbon in T.longibracteata+P.pubescens mixed forest were higher than other three forest types.⊿logK Value decreased with the increase of decay class,while E4/E6 increased.Humification degree of fallen logs showed an increase in pure T.longibracteata forest and T.longibracteata+Phyllostachys pubescens mixed forest,while in other two forest types decreased.Fulvic acid carbon content and humification degree in undecomposed layer of litter were higher than that in semi-decomposed layer.Contents of humic acid carbon in semi-decomposed layer were higher than that in undecomposed layer.(6)There was significant difference of soil physical and chemical properties between the four forest types except for pH value,bulk density,total potassium content and C/N.The contents of soil microbial biomass carbon and nitrogen were significantly higher in T.longibracteata+hardwood mixed forest soils than that in other three forest types.The contents of total PLFAs,Gram-positive bacteria,Gram-negative bacteria and fungi PLFAs in T.longibracteata+R.simiarum mixed forest soils were significantly higher than that in other three forest types.Content of actinomycetes PLFAs in T.longibracteata+hardwood mixed forest soils and content of protozoan in pure T.longibracteata forest soils were highest,respectively.The diversity of soil microbial community was high in T.longibracteata+R.simiarum mixed forest soils.The soil microbial community structure was most influenced by soil organic carbon and microbial biomass nitrogen.(7)The rate of soil organic carbon mineralization showed three stages after adding fallen logs or litters.At the first 10 days,the mineralization rate was very fast,then declined and became steady in the later period.During the incubation period,the decomposition rate of fallen logs and litters were 27.30%to 55.53%.The curves of the cumulative CO2-C released by fallen logs and litters were simulated according to the first order kinetic equation.It can be well described the CO2-C released amount.Adding fallen logs or litters could increase soil dissolved organic carbon,dissolved organic nitrogen,microbial biomass carbon and microbial biomass nitrogen.But there was no obvious effect on soil organic carbon and total nitrogen.The effect of adding litter and decay class V fallen log on the soil was more obvious than that of the other treatments. |
