Relations among soil microbial community composition, nitrogen turnover, and tree growth in N-loaded and previously N-loaded boreal spruce forest

Nitrogen (N) deposition on forests decreases the decomposition of organic matter, and may therefore decrease N mineralization. However, a higher N supply also meets the N demands of soil microbes, which may increase N mineralization, especially if the trees reduce their carbon allocation to mycorrhizal fungi and other biotic N sinks in the rhizosphere. We report effects of high continuous N load and of termination of N load on stem wood volume, foliar and soil chemistry, gross N turnover, and soil microbial communities in a boreal Norway spruce forest in Sweden. Ammonium nitrate had been applied annually for 42 years at average rates of 34 kg ha−1 year−1 of N to the N1 plots, and for two decades at 73 and 108 kg ha−1 year−1 of N to N2 and N3 plots, respectively. Treatments N2 and N3 were terminated 19 and 17 years before the study, which enabled us to study the recovery from the previous high N load.Stem growth responded strongly to N additions and correlated to gross N mineralization and microbial community composition. Total stem volume measured in 2010 was 299, 483, 470, and 376 m3 ha−1, in N0 (control), N1, N2, and N3 plots, respectively. In N-limited control plots with high soil C/N ratio and low soil and needle N concentrations, we observed lower gross N mineralization rate and lower recovery of non-immobilized tracer in the soil ammonium pool, which indicates a high microbial N immobilization capacity. Commonly, microbial biomarkers decline after N addition, which was also found here, whereas total abundance was highest in high N plots with thick mor-layer. Contrary to our expectation the fungi/bacteria ratio was not lower, because fungi and bacteria declined concurrently and to the same extent. As we expected, soils in N amended plots had higher relative amounts of gram-positive bacteria than in control plots. Termination of N additions caused a reduction in gross N mineralization and needle N. Lower 15N abundance in needles compared with the mor-layer indicated a recovery of the function of ectomycorrhizal symbiosis.We propose that the higher gross N mineralization rate in N-loaded forest is not caused by greater decomposition of organic matter, but a higher internal turnover of microbial N.

► We studied a 42-year-old N fertilization experiment in a boreal spruce forest.
► We examined soil microbial community composition, N turnover, and stem growth.
► The stem growth rate was higher than in control at a N load of 34 kg ha−1 year−1.
► Gram-positive bacteria and microbial N turnover increased after decadal N load.
► After termination of N load ectomycorrhiza tended to recover and N turnover to decrease.