Changes in nitrification and bacterial community structure upon cross-inoculation of Scots pine forest soils with different initial nitrification rates

Nitrification occurs slowly in many acid Scots pine forest soils. We examined if bacterial community structure and interactions between members of the bacterial community in these forest soils prohibit growth of ammonia-oxidising microorganisms and their nitrifying activity. Native and gamma-irradiated Scots pine forest soils known to have low net nitrification rates were augmented with fresh soils or soil slurries from nitrifying Scots pine forest soil, and vice versa. Augmentation of native non-nitrifying soils with nitrifying soils induced net nitrification, although no significant changes in bacterial community structure, as measured by 16S rRNA gene-based denaturing gradient gel electrophoresis (DGGE), were observed. In sterilised soils, the inoculum, i.e. native nitrifying soil or non-nitrifying soil, determined the occurrence of net nitrification and bacterial community structure, and not the origin of the sterilised soils. Our results demonstrate that low net nitrification rates in acid Scots pine forest soils cannot be (solely) explained by unfavourable abiotic soil conditions, but that still uncaptured biotic factors contribute to suppression of nitrification.     

Local variation in belowground multitrophic interactions

A growing number of studies point at the involvement of root herbivores in influencing plant performance, community composition and succession. However, little is known about the factors that control root herbivore abundance and the role of local variation in the effectiveness of these factors. Here, we performed a full factorial experiment with plants, root-feeding nematodes and rhizosphere microbial communities from two dune sites, to test the hypothesis that the outcome of belowground multitrophic interactions depends on local differences between the interacting organisms. The organisms included the marram grass Ammophila arenaria, the cyst nematode Heterodera arenaria, microbial plant pathogens and natural enemies of the nematodes from two coastal foredune systems, one in The Netherlands and one in Wales. The two plant populations differed at the molecular and phenotypic level, and the microbial communities from the two dune sites differed in the composition of the dominant soil fungi but not of the dominant bacteria. Plants were negatively affected by the rhizosphere microorganisms from one of the sites. Nevertheless, nematode performance was not affected by the origin of both the host plants and the microbial communities. The reproductive output of the cyst nematode depended on the presence of microorganisms, as well as on inter-population variability in the response of the nematode to these natural enemies. In the absence of microorganisms, the two nematode populations differed in the number and size of the produced cysts, although maternal effects cannot be excluded. Inter-population differences in the host plant were a secondary factor in the nematode-microorganisms interactions, and did not influence bottom-up control of the cyst nematodes. Our results did not reveal strong signals of coevolution in belowground multitrophic interactions of plants, cyst nematodes and soil microbial communities. We conclude that the interactions between the studied organisms do not necessarily depend on their local vs. non-local origin. Nevertheless, we were able to show that local variation in soil organism community composition can be an important factor in determining the outcome of interactions in belowground multitrophic systems.