40 years of restoration: Temporal effects on nitrogen levels and carbon stability of post-extracted bogs

• Restoration success of post-extracted bogs was investigated for up to 43 years. • Soil bulk density was restored after three decades. • NH 4 + and NO 3 – stocks decrease with restoration time. • Heterotrophic soil respiration increases in the restored topsoil. • Restoration does not change the temperature sensitivity of peat decomposition rates. Peatlands store one-third of terrestrial carbon (C). However, despite being major C sinks, some peatlands are disturbed by peat extraction. In the last decades, restoration attempts were undertaken to restore extracted peatlands, nevertheless, it remained unclear whether, to which degree and how fast restoration can achieve original peat characteristics. We hypothesized that i) soil bulk density decreases with stabilisation of the water table after rewetting and that ii) mineral nitrogen (N min ) stocks are highest soon after restoration and decrease with restoration time. Further, we assumed that iii) with the development of a new vegetation cover, heterotrophic respiration increases over time, yet with decreasing temperature sensitivity (Q10). To test these hypotheses, we sampled chronosequences of two post-extracted and restored bogs in Germany with restoration ages ranging from 2 to 43 years. We determined bulk density, N min stocks, basal respiration and temperature sensitivity of heterotrophic soil respiration via incubation experiments. We found that bulk densities and hence also ammonium (NH 4 + ) and nitrate (NO 3 – ) stocks decreased with restoration age and that low NO 3 – stocks were found at high water tables. While basal respiration of the topsoil increased with restoration age, Q10 values remained high, ranging from 2.1 to 3.2. Respiration was correlated with total N ( r = 0.56) and bulk density ( r = -0.58), while Q10 values were neither influenced by degradability of organic matter nor by N min content. We conclude that a stable high water table and dense vegetation decrease bulk density and N min stocks within three decades after restoration, while there is no restoration effect on Q10 within the first four decades.