Disentangling climate variability and human activity in past nitrogen cycling is key to understanding ecosystems. Previous studies in Ireland observed a widespread, permanent shift in terrestrial nitrogen cycling during Later Prehistory, potentially linked to intensifying land-use. Here, we investigate whether this shift was climatically- or anthropogenically-driven by analysing three raised bogs – highly sensitive to climate change – using pollen, testate amoebae-inferred water-table depth, and geochemical proxies (δ15N, δ13Corg, Corg:N). We identify shifts in bog surface wetness, based on radiocarbon- and tephra-constrained age-depth models: wet conditions ~4700–4450 cal BP, a dry interval ~4400 cal BP, wetter conditions ~4300–3150 cal BP, two drought phases ~3150–3050 and ~2850–2700 cal BP (Late Bronze Age and Terminal Dowris droughts), and renewed wet conditions ~2600 cal BP, consistent with the 2.7 ka event. Bulk peat δ15N values broadly tracked bog surface wetness but were more strongly influenced by secondary decomposition, highlighting post-depositional controls. Overall, our results show that Bronze Age droughts did not produce lasting changes in peat δ15N, indicating sustained nitrogen cycle shifts observed by others after ~3000–2600 cal BP likely reflect cumulative human impacts rather than climate.
Ferrandin et al. (Fri,) studied this question.