Effects Of Throughfall Exclusion On Soil Aggregate And Carbon Sequestration In Two Subtropical Plantations

Global climate change has altered the temporal and spatial patterns of regional precipitation.Many parts of the world are experiencing a trend of drought,which may affect the carbon cycle of terrestrial ecosystems.Forest is the main component of terrestrial ecosystems,and the response of forest soil carbon cycle to precipitation reduction,in particular,drought event,has become one of the global ecological research frontier fields.However,there is no clear understanding of the precipitation reduction effects on soil aggregate,which is the most important physical protection factor of soil organic carbon(SOC).As so far,adaptive plantation management has been a promising way for mitigating climate change.Identifying the effects of precipitation reduction on soil aggregate and its role in SOC sequestration in plantations,is helpful to reveal the response and mechanism of soil carbon to precipitation reduction.It will further enhance the carbon sink function of plantation ecosystems and benefit the formulation of climate change adaptation oriented plantation management strategy.Therefore,aiming at enhancing the soil carbon sequestration in plantations,50% throughfall reduction treatment(TRT)was conducted to simulate the precipitation reduction in two representative plantations(Pinus massoniana and Castanopsis hystrix)in subtropical China.The treatments started from 2012 in parallel.The specific objective of this study is to examine the effects of precipitation reduction on the composition of soil aggregate and its role in SOC distribution.Methods including located observation,soil physicochemical analysis,indoor soil incubation,microorganism high-throughput sequencing were used.The TRT effects on the contents of total organic carbon(TOC)and its labile components,including dissolved organic carbon(DOC),particulate organic carbon(POC),and easily oxidizable organic carbon(EOC)in soil and aggregates,with the underlying mechanisms,were explored.The changes in soil physicochemical properties,aggregate composition and organic carbon distribution,SOC mineralization,microbial community structure and function in the dry and wet seasons were integratedly analysed.The main results and conclusions are as follows:(1)Compared with the P.massoniana plantation,the quantity of broadleaf litterfall,fine root biomass at 0-5 cm soil layer,and the contents of soil TOC,DOC,POC and EOC were higher in the C.hystrix plantation.However,the aggregate-associated organic carbon and its contribution rate which can significantly affect SOC were not significantly different between the two plantations.Carbon input from litter and the distribution of aggregate organic carbon affect the SOC content in the two plantations.(2)TRT significantly reduced the average,maximum,and minimum daily moisture of topsoil.Compared with the control,TRT reduced the average daily soil moisture in the P.massoniana plantation,by 14.5%,12.1%,and 16.9% in the whole year,dry season,and wet season,respectively.The corresponding average daily soil moisture was reduced by 20.4%,20.1% and 20.7% in the C.hystrix plantation.However,TRT did not significantly affect soil temperature.(3)Compared with the control,after 4 years of treatment,TRT did not significantly affect the quantity of total litterfall and its main components in the two plantations.However,as a whole year,TRT significantly increased the fine root biomass of P.massoniana at 0-5 cm soil layer by 19.1%,while reduced the fine root biomass of C.hystrix at 0-5 cm soil layer by 24.1%.In the P.massoniana plantation,TRT increased the soil EOC content in dry and wet seasons;and fine root biomass was positively correlated with soil EOC content.However,in the C.hystrix plantation,TRT reduced the contents of soil DOC in dry season and EOC in wet season;the soil DOC content positively correlated with fine root biomass,and the soil EOC content positively correlated with broadleaf litterfall quantity and fine root biomass.(4)Compared with the control,there is a trend for large sized soil aggregates to break into small sized soil aggregates under TRT.In the P.massoniana and C.hystrix plantations,TRT reduced the soil mean weight diameter(MWD)by 15.1% and 16.0%,respectively.In addition,TRT increased the EOC content and its contribution rate in 0.106-0.25 mm aggregate of P.massoniana soils;while TRT decreased the EOC content but increased its contribution rate in the 0.106-0.25 mm aggregate of C.hystrix soils.For each plantation,the EOC contribution rate in this sized aggregate was positively correlated with soil EOC content.Therefore,TRT-induced changes in 0.106-0.25 mm aggregate-associated EOC content and its contribution rate are one of the reasons for the changes in soil EOC contents in the two plantations under TRT.(5)The cumulative mineralization of soil and aggregate organic carbon increases with incubation duration,and the cumulative mineralization of organic carbon in micro-aggregates(<0.25 mm)was less than those of macro-aggregates(>0.25 mm).TRT did not significantly affect the cumulative mineralization of organic carbon.However,TRT significantly affected the SOC mineralization characteristics and kinetic parameters of P.massoniana soils in dry season,and those in both dry and wet seasons for C.hystrix soils.For the P.massoniana soils,TRT inhibited labile carbon mineralization while promoted refractory carbon decomposition in the dry season.For C.hystrix soils,TRT favoured the accumulation of refractory carbon in dry season and the decomposition of micro-aggregate associated organic carbon in wet season.(6)Compared with the control,TRT reduced the abundance of cbbL gene in the P.massoniana and C.hystrix soils in the dry season.The cbbL gene abundance impacts the soil DOC and EOC contents of C.hystrix.In the P.massoniana plantation,TRT also reduced the abundance of 16 S rRNA in the dry and hot seaosns,while increased the Shannon index of soil fungal communities in the hot season.However,the TRT effects were not significant on the total fungal abundance.In the study area,Acidobacteria was dominated in soil bacterial community.Compared with control,TRT increased the relative abundance of Actinobacteria in P.massoniana soils,while reduced the relative abundance of Planctomycetes in C.hystrix soils.The relative abundance of Planctomycetes was positively correlated with soil POC and EOC contents.In addition,Basidiomycota,Ascomycota,and Zygomycota were the most dominated phylums in soil fungal community.In the P.massoniana plantation,compared with control,TRT reduced the relative abundance of Zygomycota,which was affected by soil temperature and moisture.The abundance of Zygomycota can influence soil MWD.The declines in soil bacterial abundance and free Al oxide content,together with the changes in soil moisture led to the decrease of MWD.(7)Soil TOC contents in the subtropical plantations were not significantly affected by TRT.However,the TRT effects were significant on labile SOC through integrated impacts on micro-aggregate organic carbon,SOC input,microbial abundance,and SOC mineralization.In the P.massoniana plantation,TRT increased soil EOC content through increasing 0.106-0.25 mm aggregate-associated EOC contribution rate and fine root biomass,but reducing the mineralization rate of soil labile organic carbon.In the C.hystrix plantation,a)in dry season,TRT reduced soil DOC content through reducing soil MWD,the abundance of cbbL gene,and fine root biomass;b)in wet season,TRT reduced soil EOC content through increasing the 0.106-0.25 mm aggregate-associated EOC contribution rate,but reducing the relative abundance of Planctomycetes and fine root biomass.This study indicates that throughfall reduction alters the composition of soil aggregates and reduces aggregate structural stability in the two subtropical plantations,resulting from decreased soil moisture and free Al oxide content,and altered microbial composition.The precipitation reduction increased the contents of labile SOC in the coniferous plantation,but reduced labile SOC contents in the broadleaf plantation.These changes were influenced by the soil micro-aggregate associated organic carbon,carbon input from litter,specific soil microbial taxonomic group abundance and SOC mineralization rate.This study suggests that,under the future changing precipitation pattern,close-nature management aiming to achieve multiple ecosystem services with less disturbance at forest soils should be encouraged in subtropical coniferous plantations,so as to increase the SOC stability and soil carbon sink.