Decoupling climate and human impacts on the nitrogen cycle during the Irish Bronze Age

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.