The Detritus Inputs and Removal Treatments (DIRT) were established at the H.J. Andrews Experimental Forest to examine the effects of organic matter manipulations on soil organic matter (SOM) chemistry and nutrient cycling. In 2007, after ten years of manipulations, isotope dilution methods were used to estimate gross N mineralization and nitrification rates among the six treatments that control the rate and quality of SOM inputs (control, double wood, double litter, no roots, no litter, and no inputs). We hypothesized that these long-term SOM manipulations have altered the rate and dynamics of microbial nitrogen turnover. The isotope dilution incubations showed that respiration was lower in organic matter exclusion plots relative to the control, double litter and woody treatments. In the double wood treatment, NH4+ production and consumption decreased relative to the control (1.4 mg N-NH₄⁺ kg^-1 soil day^-1), but did not significantly differ among any other treatments. Double wood treatment was the only treatment with gross nitrification significantly greater than zero. Greater rates of NH₄⁺ consumption relative to NO₃^- production suggest immobilization was the primary pathway for NH₄⁺ consumption. Microbial assimilation was further supported by an increase in ¹⁵N from time zero to 24 hours within post extraction soils. Rates of NH₄⁺ consumption were greater than NO₃^- consumption suggesting preferential microbial usage of NH₄⁺. Overall, the low nitrification relative to high immobilization rates suggests that the microbial communities remain N limited after 10 years of altering SOM inputs.