Application of nitrogen fertilizer to a boreal pine forest has a negative impact on the respiration of ectomycorrhizal hyphae

Aims There is evidence that increased N inputs to boreal forests, via atmospheric deposition or intentional fertilization, may impact negatively on ectomycorrhizal (ECM) fungi leading to a reduced flux of plant-derived carbon (C) back to the atmosphere via ECM. Our aim was to investigate the impact of N fertilization of a Pinus sylvestris (L.) forest stand on the return of recently photoassimilated C via the ECM component of soil respiration. Methods We used an in situ, large-scale, C-13-CO2 isotopic pulse labelling approach and monitored the C-13 label return using soil gas efflux chambers placed over three different types of soil collar to distinguish between heterotrophic (R-H), autotrophic (R-A; partitioned further into contributions from ECM hyphae and total R-A) and total (R-S) soil respiration. Results The impact of N fertilization was to significantly reduce R-A, particularly respiration via extramatrical ECM hyphae. ECM hyphal flux in control plots showed substantial spatial variability, resulting in mean flux estimates exceeding estimates of total R-A, while ECM contributions to R-A in N treated plots were estimated at around 30%. Conclusion Significant impacts on soil C cycling may be caused by reduced plant C allocation to ECM fungi in response to increased N inputs to boreal forests; ecosystem models so far lack this detail.