The Influence Of Dissolved Organic Matter From Different Sources On Soil CO₂ Emission In Subtropical Forests

DOM(dissolved organic matter)is an important carbon and nutrient pool in soil,and can be an important factor regulating forest C mineralization as available carbon pool for microbial growth.Our objective in this study was to investigate how differences in the quantity and quality of DOM from fresh leaves,leaf litter and dead root of Cunninghamia lanceolata and Castanopsis carlesii affected soil CO2 fluxes,physical-chemical properties of soil and microbial community composition in a laboratory incubation experiment.This would provide valuable information on evaluating the function of DOM on forest ecosystem C and N cycle.Mineral soils(0-10 cm)from an 11-year-old C.lanceolata plantation in Sanming of Fujian province,China,were incubated for 80 days after adding the DOM from different sources.Carbon mineralization during incubation was determined using CO2 respiration method.Soil microbial biomass and community composition were analyzed using phospholipid fatty acid(PLFA)technique.The major results were summarized as follows:(1)The contents of dissolved organic carbon(DOC)and dissolved organic nitrogen(DON))increased with decreasing leaching ratio(1:10>1:5>1:2),specific ultraviolet absorbance(UV)and humification index(HIX)increased with increasing leaching ratio(1:2>1:5>1:10).Compared to the C extracted in cold water,hot water-extractable organic matter increased by 5-39 times(p<0.05).The UV and HIX values of the DOM from leaf litters were higher than those of fresh leaves(p<0.05),and the UV and HIX values of the DOM from dead root were higher than those of leaf litters,suggesting that dead root contains more aromatic substances and have a higher humification degree.UV and HIX of the DOM from coniferous species were lower than those of broadleaf species(p<0.05),E2:E3(UV250:UV365)of DOM were opposite,which suggested coniferous species have more low molecular weight and variable organic matter.(2)Rates of soil CO2 emission reached the maximum in 1 day following addition of different sources of DOM,then gradually tended to smooth,indicating that additions of DOM to the soil had significant but short-term influences on soil CO2 emission.The cumulative quantity of mineralized C following addition of DOM under leaching ratio of 1:2 was significantly greater than that under leaching ratio of 1:10 by day 10.The cumulative CO2 production increased by 57.2%-541.1%following addition of hot water-extractable DOM compared to additions of cold water-extractable DOM by day 10,while decreased to 5.5%-97.7%by day 80(p<0.05).After 31 days of incubation,the cumulative mineralized C following addition of DOM from fresh leaves of C.lanceolata was significantly greater than that from DOM of C.lanceolata leaf litter,while the cumulative mineralized C following addition of DOM from C.carlesii was opposite,while there was no significant difference in the cumulative CO2 production among DOM from different sources by day 80.(3)The soil physical-chemical properties were affected following addition of different sources of DOM(p<0.05).The content of MBC(microbial biomass carbon)and MBN(microbial biomass nitrogen)in soil were gradually reduced during the whole incubation period following addition of different sources of DOM,while the contents of DOC and DON,and mineral nitrogen content were increased,which can be explained by microbial activity.(4)Rates of soil CO2 emission reached the maximum in 2 hours following additions of DOM from dead roots of C.lanceolata and C.carlesii,while reached the maximum in 12 hours following additions of DOM from leaf litter of the two tree species.The PLFA contents of bacteria,gram-positive bacteria(G+),gram-negative bacteria(G-),actinomycetes and fungi following addition of DOM were significantly higher than those of control,and addition of DOM from leaf litter and dead roots of C.lanceolata were higher than those of C.carlesii.The PLFA contents of fungi/bacteria and G+/G-following additions of DOM from leaf litter of C.carlesii were different from those following additions of DOM from leaf litter of C.lanceolata,dead roots of C.carlesii and C.lanceolata,which suggested that there was significant dinfference in the types of PLFAs following additions of DOM from different sources.