Soil　acts　as　both　a　major　sink　and　source　of　carbon　in　terrestrial　ecosystems.　Soil　microorganisms　play　an　important　role　in　carbon　sequestration　as　well　as　carbon　mobilization.　Either　substrate　to　microbe　or　microbe　to　substrate　transport　are　important　in　barren　soil　environments.　Available　external　carbon　sources　can　be　utilized　before　some　soil　organic　matter　components　are　activated.　In　this　process,　external　carbon　source　availability　markedly　determines　soil　microorganism　activity　(e.g.,　the　distance　between　the　carbon　source　and　soil　microorganisms).　Here,　I　investigated　the　role　of　space　as　a　factor　influencing　carbon　utilization.　I　designed　a　soil　incubation　device　that　both　separated　the　carbon　source　from　soil　microbes　and　ensured　free　carbon　movement.　Dextran　was　applied　to　avoid　mass　flow　and　maintain　a　balanced　water　potential.　An　isotope-labeling　technique　was　used　to　quantitatively　study　carbon　diffusion.　The　microcosm　experiment　verified　that　the　carbon　source　was　gradually　utilized　by　the　soil　microorganisms　according　to　the　distance　gradient.　Soil　microorganisms　close　to　the　carbon　source　initially　used　glucose,　whereas　more　remote　microbes　waited　for　glucose　to　be　transported.　The　microbial　biomass　carbon　(MBC)　content　was　much　higher　at　smaller　(0-0.5　cm)　distances　than　they　did　at　larger　(0.5-1　or　1-2　cm)　ones,　which　indicated　that　the　higher　concentration　of　available　carbon　source　in　the　space　closer　to　the　glucose　was　conducive　to　the　rapid　acquisition　and　assimilation　of　microorganisms.　The　findings　of　this　study　establish　a　methodology　and　identify　potential　applications　for　further　analysis　of　the　spatial　effects　of　carbon　source　availability　on　soil　microbial　activity.