Condensed tannins mediate the effect of long-term nitrogen addition on soil nematodes in a boreal spruce forest

Fertilization and combustion of fossil fuels have increased nitrogen (N) deposition over the last decades. Most boreal forests are N-limited; thus, increased N availability may have considerable implications for boreal forest food web structure and functioning. Soil biota are important drivers of ecosystem processes through their effect on carbon (C) and nutrient cycling. Among these, nematodes are ubiquitous in soil and respond quickly to environmental changes. They are thus useful in determining changes in ecosystem status. Boreal forest plants produce large quantities of condensed tannins (hereafter tannins), and these may decrease following N addition. As defence compounds, tannins may alter soil community structure by reducing microbial activity and deter microfauna. Using nematodes as indicators, we here investigated the linkages between N-induced changes in soil food web and soil tannin concentration. We utilized an experimental site where N has been added since 2003 at a rate of ca. 37 kg N ha-1 year ? 1. This have resulted in a shift in the understory, and consequently a 23 % lower soil tannin concentration. Fertilization led to a less structured nematode community dominated by bacterivores. Channel Index was 35 % lower in fertilized plots, indicating an increasing decomposition dominance by bacteria. Furthermore, the dominance of bacteria appears to have been inhibited by soil tannin concentrations in the fertilized plots. Overall, our study demonstrate that soil community structure strongly changes upon increased N availability, and lower soil tannin concentrations further facilitate the dominance of bacteria.