Long-term nitrogen addition in a boreal forest affects wood-inhabiting fungal communities and influences wood decomposition

Abstract

Addition of nitrogen (N) to forest soil may alter wood decay rates and fungal community structure and richness. In a northern Sweden Pinus sylvestris L. forest, two levels of ammonium nitrate were applied annually from 1971 to 2008. In 2007 we initiated an investigation into wood decay (assessed through mass loss) and fungal responses using stakes of trembling aspen (Populus tremuloides Michx.) and loblolly pine (Pinus taeda L.) wood installed at different soil depths in plots where zero (N0), low (34 kg; N1), or high (68 kg; N2) levels of N were applied. Stakes were located either horizontally on the surface of the organic horizon, at the interface between the mineral and organic horizons, or vertically in the mineral soil. For litter and mineral soil, fertilizer treatment was not significant for any soil chemical or physical property. Overall, pine and aspen wood stake mass loss was less than 35 % three years after deployment. Notably, the N1 treatment had the most pronounced effect on wood decay, and significantly accelerated aspen decomposition at the organic horizon surface in the second and third years. Analysis of fungal DNA extracted from the wood stakes revealed fluctuations in fungal richness and community composition depending upon stake location and duration since deployment. Fungal richness was notably higher in surface aspen stakes under N0 and N1 treatments and in surface pine stakes under N0 in the second year, though richness generally decreased with time as stake decay increased. Fungal community composition also varied by stake location and time since deployment. These results indicate that prolonged N addition can affect fungal richness, which may in turn affect wood decomposition rates. Further research is needed to clarify the nature and persistence of long-term soil N-addition effects on organic matter decomposition and soil microbial communities.