Carbon and nitrogen accumulation rates in ombrotrophic peatlands of central and northern Alberta, Canada, during the last millennium

Northern peatlands sequester carbon (C) and nitrogen (N) over millennia, at variable rates that depend on climate, environmental variables and anthropogenic activity. The ombrotrophic peatlands of central and northern Alberta (Canada) have developed under variable climate conditions during the last hundreds to thousands of years, while in the course of the twentieth century, some regions were also likely subjected to anthropogenic disturbance. We aimed to quantify peat C and N accumulation rates for the last millennium from seven peatlands to estimate the relative influence of climate and anthropogenic disturbance on C accumulation dynamics. Peatlands have accumulated C at an average rate of 25.3 g C m−2 year−1 over the last millennium. Overall, climate was likely a major factor as, on average, highest apparent rates of C accumulation were found around 1100 CE, during the warmer Medieval Climate Anomaly, with lowest rates during the Little Ice Age, around 1750 CE. Local factors, such as disturbance, played a role in C sequestration at the site scale. The average N accumulation rate was 0.55 g N m−2 year−1, with high inter- and intra-site variability. In general, N accumulation mirrored patterns in C sequestration for peat deposited pre-1850 CE. However, higher N accumulation rates observed after 1850 CE, averaging 0.94 g N m−2 year−1, were not correlated with C accumulation. Moreover, some of the historically strongly accumulating sites may have become less efficient in sequestering C, and vice versa. All seven sites showed a marked decrease in δ15N when comparing pre- and post-1850 timeframes, consistent with increasing post-1850 N additions from an atmospheric source, likely biological N fixation. Overall, N was not a driving factor for C accumulation.