Soil organic matter (OM) is an important feature of natural wetlands (NWs) often lacking in created wetlands (CWs). Some have suggested that OM amendments be used to accelerate development of edaphic conditions in CWs. Our objective was to investigate microbial and geochemical responses to compost amendments at a CW. Five levels of amendments were incorporated into drier and wetter zones of the CW to test two hypotheses: 1) microbial biomass carbon (MBC) and denitrification potential will increase with increasing levels of amendments; and 2) phosphorus (P) sorption will decrease with increasing levels of amendments. Regression indicated that pH, MBC, and P sorption had linear relationships, while bulk density (BD) had an exponential relationship with amendment level. Denitrification enzyme assay (DEA) had highest values at intermediate amendment levels. Analysis of variance indicated amendment effects for BD, MBC, DEA, and P sorption, and wetness effects for pH and MBC. Amendment levels between 60–180 Mg ha⁻¹ were ideal for microbial development and denitrification, while not sacrificing P sorption, and would be more logistically and economically feasible than levels of 200–300 Mg ha⁻¹. However, responses to amendments were complex and optimizing amendments for certain functions may detrimentally affect other functions.