Effect Of Alder (Alnus Sibirica) Expansion On Soil Organic Carbon In A Peatland Of The Da Xing'an Mountain

Northern peatlands,the peat marsh ecosystems distributed in the northern middle and high latitudes,account for less than 3% of the Earth's land surface,but store about 30% of the global soil organic carbon pool,and thus play an important role in the global carbon cycle.In the recent decades,symbiotic N2-fixing plants such as alder(Alnus spp.)have widely expanded into northern peatlands,due to climate warming and associated soil drying.Northern peatlands are extremely nutrient-poor ecosystems and respond sensitively to changes in nutrient availability.Therefore,the expansion of N2-fixing plants may exert marked influence on the stability of soil organic carbon and carbon storage potential.In order to assess the effect of N2-fixing plants on soil organic carbon in northern peatlands,we investigated plant community composition and biomass,soil organic carbon(C)mineralization rate,soil organic C stability,and soil C pool dynamics in the A.sibirica tree islands and adjacent open peatlands in the north Da Xing'an Mountain,Northeast China.The main conclusions are:(a)The expansion of A.sibirica increased plant biomass,but decreased biodiversity.Compared with the open peatlands,A.sibirica tree islands had greater aboveground biomass and lower biodiversity,although there was no significant difference in root biomass.At the plant functional group level,A.sibirica tree island had greater deciduous shrub biomass and lower biomass of grass,moss,and lichen than the open peatlands,albeit there was no significant difference in aboveground biomass of evergreen shrub.The dominances of deciduous shrub,evergreen shrub,grass,moss,and lichen in A.sibirica tree islands were lower than those in the open peatlands,due to the huge amount of A.sibirica biomass.In addition,the expansion of A.sibirica caused losses of moss and lichen,and thereby decreased species richness,Simpson index,Shannon index,and Pielou index in northern peatlands.(b)A.sibirica expansion increased soil N availability,and thus promoted soil organic C mineralization rate and altered chemical composition of soil organic C.A.sibirica tree islands had lower soil water content,soluble polyphenol concentration in the 20-40 cm soil layer and soil microbial biomass C:N,and higher soil inorganic N(NH4+-N,NO3–-N)concentration,net N mineralization rate,microbial biomass C and N,and soil organic C mineralization rate than the open peatlands.In addition,A.sibirica tree islands had lower proportion of aromatic C,carbonyl C,aromaticity and hydrophobic C / hydrophilic C in the 10-20 cm soil layer,but had greater proportion of aliphatic C / aromatic C and alkyl-C in the 10-20 cm soil layer than open peatlands,indicating that A.sibirica expansion simplified the structure of soil organic C and reduced soil organic C stability.(c)A.sibirica expansion caused soil carbon loss in the northern peatland.A.sibirica tree islands had lower soil organic C pool and phosphorus(P)pool,especially in the deep soil layer(20-40cm).This may be cuased by the priming of soil organic C mineralization in the deep soil layer induced by A.sibirica roots.However,A.sibirica tree islands had higher soil N pool and lower soil C:N than the open peatlands.Furthermore,A.sibirica expansion generally increased ?13C and ?15N content in plant litter and soil.As northern peatlands has a huge soil C pool,soil C loss caused by alder expansion may exert a profound impact on global C cycling and atmospheric chemical composition.