Decomposition And Nutrient Release Of Four Typical Tree Leaf Litter On The Soil Surface And In The Air In Subtropical Plantations Of China

Plant litter decomposition is an indispensable component of carbon and nutrient cycling,and thus plays an essential role in maintaining ecosystem productivity,promoting the formation of soil organic matter,and soil nutrient supply in forest ecosystems.In the forest ecosystem,a considerable fraction of leaf litter is usually intercepted by tree branches or understory vegetation in the process of falling,resulting in litter decomposition both on the soil surface and in the air.However,previous studies have concentrated on plant litter decomposition on the soil surface,and little attention is paid to the dynamics of litter decomposition in the air.In this study,fresh leaf litter of four main afforestation tree species,Pinus massoniana,Pinus elliottii,Liquidambar formosana and Schima superba,were selected from typical subtropical plantations.Using the full factorial design,we adopted the litter bag method to study the changes of mass remaining and microbial activity of leaf litter during decomposition on the soil surface and in the air.Subsequently,we measured dissolved organic carbon(DOC),dissolved total nitrogen(DTN),and dissolved total phosphorus(DTP)production,and dissolved organic matter(DOM)spectral characteristics(SUVA₂₅₄,SUVA₂₈₀,SUVA₃₅₀,S_275-295,S_350-400,and S_R)in litter leachates,and assessed the mineralization/immobilization dynamics of total nitrogen(TN),total phosphorus(TP),manganese(Mn),calcium(Ca),and magnesium(Mg)during litter decomposition.The main results are as follows:(1)Litter mass remaining and microbial activity were significantly affected by decomposition position,sampling date,and tree species.When litter decomposition proceeded,litter mass remaining and microbial activity decreased.When litter decomposed on the soil surface,broad-leaved tree species had lower mass remaining and higher microbial activity than coniferous species,and litter mass remaining negatively correlated with microbial activity.When leaf litter decomposed in the air after 150 days,broad-leaved species had lower mass remaining and higher microbial activity than coniferous species,but no significant differences in mass remaining and microbial activity were observed among tree species after 360 days of decomposition.Moreover,there was no significant correlation between litter mass remaining and microbial activity in the air.Over 360 days of decomposition,tree leaf litter had lower mass remaining and higher microbial activity on the soil surface than in the air.In addition,home-field advantage occurred when tree leaf litter decomposed on the soil surface,especially after 150 days of decomposition,but disappeared when tree leaf litter decomposed in the air.(2)Over 360 days of decomposition,broad-leaved tree litter produced greater amounts of DOC,DTN,and DTP than coniferous tree litter both on the soil surface and in the air.However,SUVA₂₅₄,SUVA₂₈₀,SUVA₃₅₀,S_275-295,S_350-400 and S_R values of litter-derived DOM significantly varied with tree species,decomposition position,and sampling time.After 150 days of decomposition,tree litter had higher DOC,DTN,and DTP production and higher values of SUVA₂₅₄,SUVA₂₈₀,and SUVA₃₅₀ on the soil surface than in the air,indicating the greater aromaticity of DOM leached from the litter on the soil than in the air.After 360 days of decomposition,tree litter on the soil surface had higher DOC and DTP production and lower DTN production than that in the air,whereas DOM spectral characteristics had no obvious spatial change trend,but changed with varying tree species.With increasing incubation time,litter-derived DOC,DTN,and DTP productions showed inconsistent patterns among tree species and decomposition habitats on the soil surface,but showed an increasing trend in the air.In addition,the values of SUVA₂₅₄,SUVA₂₈₀,and SUVA₃₅₀ of litter-derived DOM increased with elevating incubation time both on the soil source and in the air.(3)The dynamics of TN,TP,Mn,Ca,and Mg in decomposing litter showed contrasting patters during litter decomposition.After 360 days of decomposition,the proportions of TN and TP released from decomposing litter were 26.4%-53.3%and46.6%-64.1%,respectively,while litter Mn,Ca and Mg releases were 8.3%-78.4%,50.8%-84.1%and 62.8%-86.8%,respectively.During decomposition,broad-leaved tree litter TN showed an initial immobilization,and subsequent mineralization on the soil surface and in the air,while coniferous tree litter showed a net TN release.However,the dynamics of TP in decomposing litter were dependent on tree species and decomposition habitats.Over 360 days of decomposition,litter Mn of P.elliottii initially increased,but then released,whereas the litter Mn of P.massoniana,L.formosana and S.superba showed a net release.In addition,litter Ca and Mg of P.massoniana,P.elliottii,L.formosana and S.superba showed a net release during decomposition on the soil surface and in the air.In summary,tree leaf litter on the soil surface had higher decomposition rates,greater microbial activity,and higher incidence of home-field advantages than that in the air;broad-leaved tree litter decomposed faster and had higher microbial activity than coniferous tree litter.Litter-derived DOM quantity and spectral characteristics were highly dependent on tree species,sampling date,and decomposition position.During 360 days of decomposition,the temporal patterns of nutrients in decomposing litter were inconsistent.Specifically,litter TN showed immobilization-mineralization patterns,while Mn,Ca and Mg showed a net release.The results of this study can help to deeply understand litter decomposition characteristics and soil nutrient cycling in forests,and also provide important theoretical guidelines for the improvement of forest soil fertility and sustainable forest management in subtropical regions of southern China.