Wood Litter Decomposition Characteristic And Its Bark Effect In Plantations Of Subtropical Degraded Red Soil Region

In forest ecosystems,litter decomposition is the primary source of soil organic matter and nutrients,and thus plays a crucial role in regulating a series of ecosystem service and function such as stand primary productivity,soil carbon sequestration potantial,and soil nutrient availability.Plantations are of great importance in maintaining agricultural production and ecological stability in subtropical degraded red soil region.In these forests,both the soil nutrient and organic matter contents are low due to the slow decomposition of litter,which leads to a series of problems such as low productivity and weak ecosystem stability.In order to improve soil fertility and ecological services,it is necessary to understand the decomposition of forest litter in this region.However,the existing studies mainly concentrated on leaf litter rather than wood litter such as branch and root.Moreover,it is unclear how the presence of bark affects wood litter decomposition.In addition,previous studies have primarily focused on microbial-mediated decay,and the leaching of wood litter is often ignored in forests.Here,we collected branches and coarse roots from common tree species from the subtropical plantations in the National Long-term Scientific Research Base for Forest Vegetation Restoration on Degraded Red Soil in the Jiangxi province,southern China.First,we investigated the effect of leaching on wood litter decomposition and associated dissolved organic matter(DOM)characteristics.Second,we used the litterbag method to assess wood litter decomposition dynamics in the field.Third,using a bark removal experiment,we determined the effect of bark on physical leaching and microbial decay during wood litter decomposition.This study aimed to reveal wood litter decomposition dynamics and their controlling factors,which would help understand carbon and nutrient cycles and enhance soil quality in plantations of subtropical degraded red soil region.The main results of this work are as follows:(1)Organ type and diameter class significantly affected microbial activity and mass loss during wood litter decomposition.Compared with tree branches,coarse roots had greater microbial activity and mass loss during decomposition.Regardless of tree branch and coarse root,both microbial activity and mass loss of wood litter increased with decreasing diameter class.During wood litter decomposition,mass loss showed a positive linear relationship with microbial activity.Meanwhile,both microbial activity and mass loss of wood litter were positively related to the initial N and P concentrations,but negatively related to the intial C:N and C:P ratios.(2)The presence of tree bark inhibited wood litter decomposition in the field.Irrespective of branch and coarse root,both microbial activity and mass loss were greater for tree bark than for wood litter(contain bark)and inside wood(wood litter without bark)during decomposition.Both microbial activity and mass loss of wood litter increased with decreasing diameter class during decomposition.For each wood litter type,tree bark presence inhibited microbial activity and mass loss during decomposition.Relative to tree branches,the inhibiting effect of bark on coarse root decay rate was greater.During wood litter decomposition,mass loss showed a positive linear relationship with microbial activity.Meanwhile,both microbial activity and mass loss of wood litter were positively related to the initial N,P,Ca,and Mn concentrations,but negatively related to the intial C:N and C:P ratios.(3)During leaching,organ type and diameter class significantly affected wood litter mass loss as well as DOM production and properties.Tree coarse roots had greater mass loss,dissolved organic carbon(DOC)production,dissolved total nitrogen(DTN)production,dissolved total phosphorus(DTP)production,and DOM aromaticity,but had lower DOM biodegradability than tree branches.Irrespective of tree coarse roots and branches,wood litter mass loss,DOC production,DTN production,DTP production,and DOM aromaticity increased with decreasing diameter class during leaching.However,both tree coarse root-and branch-leached DOM biodegradability decreased with decreasing diameter class.In addition,wood litter-leached DOM biodegradability negatively related with DOM aromaticity.(4)During leaching of tree branches,the presence of bark could not only reduce mass loss and DOM production,but also alter DOM biodegradability through its effect on DOM aromaticity.Tree bark had greater mass loss,DOC production,DTN production,and DTP production than the corresponding branch with bark and inside wood(branch without bark),and branch with bark had lower DOC,DTN,and DTP productions than corresponding inside wood.For broadleaf trees,bark had lower DOM biodegradability in the leachates than branch with bark and inside wood due to the higher DOM aromaticity.For coniferous trees,the greater DOM biodegradability was observed for bark than branch with bark and inside wood because of the lower DOM aromaticity.Moreover,the presence of bark lowered litter mass loss and DOC,DTN,and DTP productions during leaching.However,the effect of tree bark on branch-leached DOM aromaticity and biodegradability was dependent on tree functional group.For both evergreen and deciduous broadleaf trees,the presence of bark reduced branch-leached DOM biodegradability by increasing DOM aromaticity.In contrast,for evergreen coniferous trees,the presence of bark increased branch-leached DOM biodegradation by reducing DOM aromaticity.(5)The presence of bark significantly altered wood litter decomposition pathways.During leaching,tree bark had greater DOC production,DTN production,and DTP production than the corresponding branch with bark and inside wood(branch without bark).Similarly,bark had greater C loss through microbial respiration than the corresponding branch with bark and inside wood during decomposition.The presence of bark not only reduced the amounts of DOC leached from the corresponding tree branches,but also decreased C loss through microbial respiration.During incubation,mass loss of tree branches was inhibited by the presence of bark.In addition,the inhibiting effect of tree bark on leaching-induced DOC loss was greater than that of tree bark on microbial respiration-induced C loss during decomposition.In summary,wood litter decomposition was primarily regulated by organ type and diameter class.In both leaching and field decomposition experiments,coarse roots had greater decomposition rate than tree branches,and wood litter decomposition increased with decreasing diameter.Moreover,tree bark not only served as physical barrier to prevent the leaching of soluble organic C and nutrients from wood litter,but also inhibited microbial-mediated decomposition of branches.These findings will enrich the theoretical system of litter decomposition,and also contribute to the understanding of forest C and nutrient cycles in subtropical forests,which can provide theoretical guidance for nutrient management and sustainable management in subtropical plantations.