To understand the effects of biogas slurry on soil labile organic carbon of poplar plantation in a coastal area, we designed four addition treatments of biogas slurry, i.e., CK (no addition), L (125 m³·hm^-2), M (250 m³·hm^-2) and H (375 m³·hm^-2) at Dongtai Forest Farm in Jiangsu Province. The results showed that applying biogas slurry increased the concentration of dissolved organic carbon (DOC), while decreased soil microbial biomass carbon (MBC). Moreover, biogas slurry significantly decreased soil pH and increased NO₃^--N concentration. There was a significant negative correlation between pH and NO₃^--N concentration, whereas NH₄⁺-N was mainly the form of N in biogas slurry, indicating that the change of pH might be caused by input of ammonium nitrogen. Biogas slurry also increased soil MBN and decreased soil MBC/MBN, suggesting changes in soil microbial community.