Response Of Soil Carbon Processes To Throughfall Exclusion In Two Subtropical Plantations In Southern China

In recent two hundred years, the climate change caused by anthropogenic activities withthe main characteristics of global warming has been changing the global precipitation patternand water cycling. Extreme precipitation events ocurred frequently, and there exsits a trend ofglobal drought events, which has a significant impact on the carbon cycle process of terrestrialecosystems. Being the main component of terrestrial ecosystems, forest ecosystems are playingan important role in regulating terrestrial carbon cycle by which to mitigte climate change.However, the effects of rainfall pattern changes on forest ecosystem carbon sink/source isincreasingly concerned, and thus has become an important research topic in the field of globalchange ecology. There are wide range distribution of planted forests in southern China, andafforestation/reforestation have become the effective measures to cope with global climatechange by sequestering more CO2from the atmosphere. Globle and regional rainfall patternchange will be likely to have a great impact on carbon sequestration of plantations and wouldjeopardize its carbon sink capacity, while there is still considerably uncertain about its impacts.Therefore, there is a clear need to investigate the ecosystem carbon cycle and its underlyingmechanism of planted forests China, which helps evaluate the future role of planted forests inglobal carbon cycle and its carbon sink capacity under the global climate change. In thisstudy, two types of planted forests, including Castanopsis hystrix (CH) and Pinus massoniana(PM) plantaitons with the same age and similar site conditions in Experimental center oftropical forestry (Chinese Academy of Forestry) in Pingxiang city, Guangxi Autonomousregion were selected to explore impacts of the in situ experimental drought treatment throughthroughfall exclusion on ecosystem carbon cycle and soil microbial community by combiningsoil conventional physical and chemical analysis, soil drilling method, chloroform fumigationmethod, with soil CO2flux infrared gas analysis and phospholipid fatty acid analysis (PLFA).The objectives of the study are to explore:(1) the effects of thgouhfall exclusion on soil carbon stock and soil physical and chemical properties,(2) the responses of soil respiration tothroughfall exclusion and its controlling factors,(3) the impacts of throughfall exclusion onsoil microbial community and the underlying mechanism.The main reults are as follows:(1) The manipulation experiment of throughfall exclusionhas been successfully implemented and performed, with the expected outcome of effectivelyintercepted throughfall under the canopy and consequently significant changes in soil moisture.The annual total amont of intercepted throughfall by the manipulation treatment in CHplanation was655.36mm, taking up36.84%and49.82%of the annual total precipitation andthroughfall, respectively. While in PM plantion, the total annual amount of throughfallinterception was733.51mm, taking up41.24%and50.06%of the annual total precipitationand throughfall, respectively. The rate of throughfall interception to the total throughfallannually range between44.92%～54.68%, showing a better consistency throughout theexperiment. Furthermore, throughfall exclusion significantly changed the soil moisture, theannual average soil moisture in CH and PM plantations were significantly reduced by37.73%and37.73%, respectively, relative to the controls.(2) throughfall exclusion led a significant decline in soil organic carbon at surface soillayer (0～10cm) by altering soil bulk density and organic carbon density in CH plantation. Inaddition, throughfall exclusion significantly increased soil pH and NH4+-N content at surfacesoil layer (0～10cm), but sinificantly decreased soil microbial biomass carbon (MBC) contentin CH plantation at rainy season and sinificantly decreased soil microbial biomass nitrogen(MBN) content in PM plantation at dry season. The responses of fine root biomass in twoplantaitons to throughfall exclusion were different, with the significant decrease in fine rootcarbon biomass at surface layer (0～10cm) in CH plantation, but the significant increase infine root biomass in PM plantation. The results indicated that throughfall exclusion caused adecline in SOM and fine root biomass at surface layer (0～10cm) in CH plantation, resultingin a reduction in soil organic carbon stock, while in PM plantation, soil organic carbon stockmight be increased with the increase in fine roots carbon inputs. (3) Throughfall exclusion did not change seasonal dynamics of soil respiration in eitherCH or PM plantations, but with the different responses of soil respiration rate. In addition,throughfall exclusion significantly inhibited soil respiration rate, leading a decline by9.70%inannual accumulative total respiration flux in CH plantation. However, Throughfall exclusionsignificantly increased soil respiration rate in PM plantation, leading a increase by6.55%inaverage annual soil respiration flux. Furthermore, throughfall exclusion did not affect therelationship between soil temperature and humidity, and soil respiration. Soil respiration rateswere significantly corelated with soil temperature and humidity in two plantations. Moreover,soil temperature at5cm depth can explain more than70%of the total variation of soilrespiration, while soil moisture can explain less than30%.In CH plantations, the throughfall exclusion caused a significant decline in the annualaverage temperature sensitivity (Q10)(1.46±0.19)(P<0.05) compared to that in control plots(1.98±0.16). However, throughfall exclusion significantly increased Q10in PM plantation.Furthermore, This study also found that soil respiration flux showed significant linearcorrelations with the multiple factors such as fine root biomass, MBC, soil organic carbon andC/N. The results showed that different effects of throughfall exclusion on fine root productivity,soil organic matter content in different plantations can account for the different responses ofsoil respiration in CH and PM plantations.(4) Throughfall exclusion significantly influenced soil microbial PLFAs, but its influencedegree variated with season and plantation type. In CH plantation, throughfall exclusiontreatment increased soil microbial PLFAs biomass in rainy season, but decreased in dry season.In PM plantation, throughfall exclusion treatment decreased soil microbial PLFAs biomass inrainy season, but there was no significant effects at dry season. During rainy season,throughfall exclusion only significantly influenced the relative abundance of few mcirobialcommunies, but the relative abundances of most mcirobial communies were impacted in thedry season. Generally, throughfall exclusion significantly reduced the relative abundance ofarbuscular mycorrhizal fungi as well as the ratio of the relative abundance of fungal PLFAs to bacterial PLFAs, but significantly incresed the relative abundance of bacterial PLFAs andgram-positive bacterial PLFAs and the ratio of the relative abundance of gram-positivebacterial PLFAs to gram-negative bacterial PLFAs. In the present study, soil microbialcommunity structure was significantly correlated with NH4+-N, soil pH, soil temperature, soilwater content. Meanwhile, the stress index of soil microbial communites showed linearrelationship with the above mcirobial variables. The results showed that the alterations of soilhydrothermal condition, pH, and nitrogen (total nitrogen, ammonium nitrogen and nitratenitrogen) as well as other factors caused by experimental throughfall exclusion producedselection pressure on soil microorganisms, which was one of the important reasons accountingfor the altered soil microbial community.