Agricultural　and　industrial　activities　have　increased　atmospheric　nitrogen　(N)　deposition　to　ecosystems　worldwide.　N　deposition　can　stimulate　plant　growth　and　soil　carbon　(C)　input,　enhancing　soil　C　storage.　Changes　in　microbial　decomposition　could　also　influence　soil　C　storage,　yet　this　influence　has　been　difficult　to　discern,　partly　because　of　the　variable　effects　of　added　N　on　the　microbial　enzymes　involved.　We　show,　using　meta-analysis,　that　added　N　reduced　the　activity　of　lignin-modifying　enzymes　(LMEs),　and　that　this　N-induced　enzyme　suppression　was　associated　with　increases　in　soil　C.　In　contrast,　N-induced　changes　in　cellulase　activity　were　unrelated　to　changes　in　soil　C.　Moreover,　the　effects　of　added　soil　N　on　LME　activity　accounted　for　more　of　the　variation　in　responses　of　soil　C　than　a　wide　range　of　other　environmental　and　experimental　factors.　Our　results　suggest　that,　through　responses　of　a　single　enzyme　system　to　added　N,　soil　microorganisms　drive　long-term　changes　in　soil　C　accumulation.　Incorporating　this　microbial　influence　on　ecosystem　biogeochemistry　into　Earth　system　models　could　improve　predictions　of　ecosystem　C　dynamics.