Many wet tropical forests, which contain a quarter of global terrestrial biomass carbon stocks, will experience changes in precipitation regime over the next century. Soil microbial responses to altered rainfall are likely to be an important feedback on ecosystem carbon cycling, but the ecological mechanisms underpinning these responses are poorly understood. We examined how reduced rainfall affect- ed soil microbial abundance, activity, and community compo- sition using a 6-month precipitation exclusion experiment at La Selva Biological Station, Costa Rica. Thereafter, we ad- dressed the persistent effects of field moisture treatments by exposing soils to a controlled soil moisture gradient in the lab for 4 weeks. In the field, compositional and functional re- sponses to reduced rainfall were dependent on initial condi- tions, consistent with a large degree of spatial heterogeneity in tropical forests. However, the precipitation manipulation sig- nificantly altered microbial functional responses to soil mois- ture. Communities with prior drought exposure exhibited higher respiration rates per unit microbial biomass under all conditions and respired significantly more CO2 than control soils at low soil moisture. These functional patterns suggest that changes in microbial physiology may drive positive feed- backs to rising atmospheric CO2 concentrations if wet tropical forests experience longer or more intense dry seasons in the future.