As an alternative to chemical fertilizers, growers routinely apply organic amendments to provide crop nutrition, but lack information on which combinations best increase nitrogen (N) availability rates and stimulate beneficial soil micro-organisms. The objectives of this study were to characterize how organic amendments and amendment combinations influenced 1) bacterial communities, 2) nematode communities and 3) N availability and plant growth. Dry amendments (pelleted poultry manure and compost) and liquid amendments (fish hydrolysate and food hydrolysate), were tested alone and combined in laboratory incubations and a greenhouse experiment. Pelleted poultry manure caused rapid increases of bacterial taxa associated with N cycling such as Nitrosospira, Pseudomonas, Pseudoxanthomonas and Flavobacterium as well as increases in opportunistic bacterial feeding nematodes such as Panagrolaimus. The abundance of many bacterial taxa, including some N cyclers, showed strong relationships with nematode genera and nematode indices of nutrient processing. Compost increased the prevalence of bacteria such as Chryseolinea, Sporocytophaga and Phenylobacterium, which are known to degrade complex C-compounds. Both liquid amendments caused slight changes in microbial communities. While pelleted poultry manure resulted in the highest net N availability and greatest plant growth compared to controls, compost reduced plant growth unless combined with liquid amendments. Results suggest that C:N ratio and C complexity influence nematode and bacterial community compositions as well as N availability, with implications for optimizing biologically based N cycling.