Soil nitrogen transformations are intricately linked to carbon transformations. We utilized two existing organic matter manipulation sites in western Oregon, USA and Hungary to investigate these linkages. Our questions were: (1) what effect does the quantity and quality of organic matter have on net and gross N processing in mineral soil? and (2) do these effects vary across sites with very different climate, soils, vegetation, and N deposition status? The organic matter manipulations had small if any effects on gross and net N cycling rates. Gross N cycling rates under low N deposition increased with increasing soil C and N, but C:N ratio had no correlation with gross N cycling rates. Soil ammonium concentrations under high N deposition, however, were higher in the organic matter manipulation plots without roots and lower in plots with double litter, indicating a tree root effect and a litter immobilization effect, respectively, but did not differ significantly under low N deposition. Net ammonium production was lowest in the litter removal and root removal plots and highest in the litter addition plots at both sites. Gross and net N cycling rates and mineral nitrogen pool sizes were generally higher under higher N deposition and lower C:N ratio soil, which was consistent with past studies over gradients of N deposition. By looking at organic matter manipulations in two very different sites we gained some insight into the role that C:N ratio as well as total C and total N have in controlling N and C cycling in forest soils.