Interactions between nitrogen availability, bacterial communities, and nematode indicators of soil food web function in response to organic amendments

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.