Priming　effect　(PE)　is　increasingly　recognized　as　an　important　mechanism　in　the　microbial　degradation　of　dissolved　organic　matter　(DOM)　from　freshwater　to　the　ocean.　However,　potential　PE　during　the　mixing　of　DOM　from　different　sources　and　the　effects　on　different　DOM　constituents　are　still　largely　unknown.　This　study　examined　the　PE　after　adding　DOM　from　typical　natural　and　anthropogenic　sources　(rainwater,　fresh　plant,　leaf　litter,　and　wastewater)　into　pre-aged　river　DOM,　using　dissolved　organic　carbon　(DOC)　measurement,　absorption　spectroscopy,　and　fluorescence　excitation-emission　matrices-parallel　factor　analysis　(EEMs-PARAFAC).　The　plant-derived　DOM　had　a　low　humic　content　and　was　dominated　by　benzoic　acid-like　and　tyrosine-like　fluorescent　components　(C4　and　C5),　which　showed　a　high　DOC　bioavailability　of　80%.　DOC　in　rainwater　and　wastewater　also　had　high　bioavailabilities　(45%–50%),　while　DOM　in　the　leaf　litter　leachate　showed　high　aromaticity,　average　molecular　weight,　and　humic　content　but　low　DOC　bioavailability　(12%).　There　was　generally　limited　PE　(　©　2020　Elsevier　Ltd