Effects Of Different Litter Inputs On Soil Microbial Residue Carbon In A Natural Forest And A Plantation Of Castanopsis Carlesii

The global soil carbon storage of the 0-3 m soil layer is about 2344 Gt,which is higher than that combined with vegetation and atmosphere.Therefore,the global carbon balance will be affected by small changes in forest soil carbon pool.The forest productivity and input of aboveground and underground litters will be changed by global climate change,thus affecting the soil carbon pool.For a long time,little known about the contribution of microorganism synthetic metabolism and microbial residues to soil organic carbon accumulation.A large number of natural forests are transformed into plantations which changes soil organic carbon storage,litter,fine root dynamics,and nutrient cycling in subtropical China.However,there are few reports about the effects of soil microbial residues under different litter inputs manipulation treatments in a subtropical natural forest and plantation.Therefore,based on the Castanopsis kawakamii Nature Reserve of the Sanming Forest Ecosystem and Global Change Research Station in Fujian Province,control(CT),root exclusion(NR),aboveground litter removed(NL),and no litter input(NI)and double litter addition(DL)were set up in a natural(NF)and plantation(AF)of Castanopsis carlesii.Soil samples of 0-10 cm soil layer were collected in April 2016 and 2019.Phospholipid fatty acids and amino sugars were used to characterize the composition of soil microbial community and microbial residual carbon,respectively.To explore the effects on soil organic carbon stability changes under litter and root inputs,the factors affecting the contribution of microbial-derived carbon to soil organic carbon were analyzed combined with soil physicochemical properties.The main results are as follows:(1)There is no significant difference in the content of total amino sugars(TAS),muramic acid(Mur A),and galactose(Gal N)under DL and CT treatment in April 2016 and2019.In a natural forest,soil TAS and glucosamine(Glu N)were significantly reduced under NR,NL and NI treatments in 2016 and 2019,indicating that soil microbial activity was affected by the amount of exogenous organic matters,and excluding exogenous carbon input was not conducive to the accumulation of soil organic carbon(SOC).In a plantation,there was no significant difference in soil TAS content between NL and CT treatments,but the TAS content decreased 20.6% and 17.1% under NR treatment in 2016 and 2019,respectively,indicating that the microbial-derived carbon was affected by the underground roots input in the plantation.In this study,the ratio of microbial residue carbon to soil organic carbon was 22.7% to 40.5% under different treatments.There was no significant difference in the ratio of microbial residue carbon,fungal carbon,bacterial carbon to soil organic carbon among different treatments in the natural forest in 2016 and 2019,indicating that exogenous carbon inputs did not change the contribution of microbial residual carbon to soil organic carbon.Fungal and bacterial carbon remained relatively stable in soil organic carbon pool.(2)Compared with CT,the content of SOC was significantly decreased in NI and DL treatments,but there was no significant difference in NR treatment in natural forest and plantation of Castanopsis carlesii in 2016,indicating that both no litter input and litter increased resulted in soil carbon loss.There was no significant difference in SOC content between DL and CT treatment in a natural forest,but the content of SOC was significantly increased in DL treatment compared with CT in plantation in 2019.Compared with CT treatment,SOC significantly decreased by 8.8% and 10.7% in a natural forest,while significantly decreased by 10.5% and 21.0% under NR treatment in a plantation in April2016 and 2019.The results showed that SOC was more influenced by root input in plantation than natural forest.NL and NI treatments significantly decreased the content of soil dissolved organic carbon(DOC)compared with CT both in a natural forest and plantation.Compared with CT in 2019,DOC and microbial biomass carbon(MBC)under NR and NL treatments were decreased by 21.8%,45.7% and 48.4%,28.8% respectively in a natural forest,and the content of DOC and MBC were decreased by 8.2%,35.2% and47.8%,22.4% respectively under NR and NL treatments in a natural forest.The results showed that compared with the underground roots,changing the number of litters above ground had a greater effect on soil dissolved organic matter,while microbial biomass carbon was more sensitive to underground roots in a natural forest and plantation of Castanopsis carlesii.(3)Compared with CT,total microbial biomass was significantly decreased under NR and NI treatments,but there was no significant difference under DL in April 2016 and 2019.In a natural forest,the total microbial biomass has no significant difference under NL and DL compared with CT,while decreased by 30.8% and 37.1% under NL in a plantation in2016 and 2019,respectively.It shows that no exogenous carbon input is not conducive to the growth and reproduction of microorganisms.In a plantation,soil microorganisms were more likely to respond quickly to the decrease than increase of aboveground litters input.In 2016 and 2019,the content of Gram-positive(GP-PLFAs)and Gram-negative bacteria(GN-PLFAs)were significantly decreased under NI treatment compared with CT.The fungal PLFAs content(Fungi-PLFAs)of NR treatment was significantly decreased by 51.9%than that under CT.There was no significant difference in Fungi-PLFAs content between CT and DL treatment,and in fungi/bacteria(F/B)among different treatments in a natural forest.The results indicated that the relative stability of fungi and bacteria did not affect by the change of exogenous carbon input in a natural forest.The biomass of Fungi-PLFAs was significantly decreased in a plantation under NR,NL,NI treatments,indicating that the activities of fungi in soil depended on aboveground and underground carbon input.FungiPLFAs under CT was higher in a natural forest than that in plantation,indicating that may be more refractory organic matter in a natural forest than a plantation.(4)Correlation analysis showed that SOC,MBC,total microbial biomass,GP-PLFAs,GN-PLFAs,Fungi-PLFAs were significantly positively correlated with TAS,Glu N,Mur A,and Gal N.DOC,SOC and Fungi were the environmental factors that significantly affected the amino sugar variables in a natural forest,while SOC and F/B were the main factors in a plantation.It shows that microbial residual carbon plays an important role in soil organic carbon storage,and the number of living microorganisms may determine the accumulation of microbial residual carbon in the soil.