The Coupling And Development Of TRIPLEX-MICROBE Model And Its Simulation About Global Soil Organic Carbon And Microbial Carbon

Soil organic carbon?SOC?is the largest carbon pool in the terrestrial biosphere and plays an important role in the global carbon cycle.Microbial physiology plays a critical role in the biogeochemical cycles of the Earth system.However,most traditional soil carbon models are lacking in terms of the representation of key microbial processes that control the soil carbon response to global climate change.In this study,the improved process-based modelTRIPLEX-GHGwasdevelopedbycouplingthenewMEND?Microbial-ENzyme-mediated Decomposition?model to estimate total global soil organic carbon?SOC?and global soil microbial carbon?MBC?,and to predict the trend of SOC and MBC in the future climate change scenarios in 21st century.The MEND model was divided into three parts for the model integration.First,additional parameters and variables pertaining to the MEND module were defined,and the code was written into the TRIPLEX-GHG before the soil biogeochemistry loop.Next,MEND module equations were incorporated within an hourly loop.Finally,the codes for model outputs were embedded in the previously defined NetCDF format.There are four main inputs?soil temperature,soil moisture,soil pH,and litterfall?in the MEND model.As a result,the soil temperature,soil moisture and soil pH were estimated from the Land Surface module,and the litterfall was derived from the Vegetation Dynamics module of TRIPLEX-GHG,and these were all converted into an hourly scale.This study compiled observed SOC data based on many previous studies and used these observations for model parameter fitting and calibration.For the simulations,the CRU-TS 3.1Climate Database was adopted to construct monthly climate input data for the observed sites and global soil.For the RCP2.6,RCP4.5 and RCP8.5 future scenarios input data,we selected the ESM IPSL-CM5A-LR input data.Selected variables included cloud cover,diurnal temperature range,precipitation,temperature,vapor pressure and wet-day frequency.For other input data,such as CO₂ concentration,the soil classification map and the global grid area,we used the same datasets as those of TRIPLEX-GHG.The main conclusions of this paper are as follows:1)The new model?TRIPLEX-MICROBE?shows considerable improvement over the previous version?TRIPLEX-GHG?in simulating SOC.This study estimated the global soil carbon stock to be approximately 1195 Pg C,with 348 Pg C located in the high northern latitudes,which is in good agreement with the well-regarded Harmonized World Soil Database?HWSD?and the Northern Circumpolar Soil Carbon Database?NCSCD?.This study also estimated the global soil microbial carbon to be 21 Pg C,similar to the 23 Pg C estimated by Xu et al.?2013?.This study found that the microbial carbon quantity in the latitudinal direction showed reversions at approximately 30°N,near the equator and at 25°S.A sensitivity analysis suggested that the tundra ecosystem exhibited the highest sensitivity to a 1°C increase or decrease in temperature in terms of dissolved organic carbon?DOC?,microbial biomass carbon?MBC?and mineral-associated organic carbon?MOC?.The responses of Enzyme,MBC and T_MGMR to a 10%increase/decrease in precipitation were most pronounced in the desert,open shrubland and grassland,respectively.2)Great uncertainty still exists in the response of global land soil organic and microbial carbon to future climate change.The global SOC and MBC simulated by TRIPLEX-MICROBE which is able to consider and predict the microbe activity under three future representative concentration pathways?RCPs?scenarios in the 21st century.The SOC simulated by TRIPLEX-MICROBE between 2013 and 2100 decreased about 6.1%,9.4%and15.9%in the three RCPs,respectively.The MBC increased between 2013 and 2100 in the three RCPs,accounting for 13.8%,20.3%and 39.6%,respectively.The SOC stored in the Arctic will experience a process of decline under the priming effect,but as the climate warming,from the equator to mid-latitude gradually,the northern hemisphere becomes a larger carbon sink,which could offset the carbon loss from high latitudes and high altitude.The latitude when MBC reached the maximum in relatively coldest season of the year in the northern hemisphere moves northward about 10 degrees under the three RCPs scenarios compared to the history period.Tropical and temperate areas of vegetation gradually northward expand with warming of the climate change,and vegetation fixed more carbon when gradually northward to offset the increasing atmospheric CO₂.There are still some limitations in this study.The carbon-nitrogen coupled MICROBE module?MEND?is currently under development,and it does not consider vertically resolved soil layers and seasonal variation of litterfall.