Temperature controls bryophyte-associated nitrogen fixation in super-humid temperate forests in New Zealand

Nitrogen (N) fixation performed by certain prokaryotes (diazotrophs) is a key source of new N in pristine ecosystems. Diazotrophs associate with various plants, including bryophytes-the dominant plant group in northern ecosystems such as boreal forests. There, bryophyte-associated N₂ fixation contribute half of total ecosystem N input. However, bryophyte-associated N₂ fixation has been scarcely studied outside northern, cold ecosystems, leaving current ecosystem nutrient budgets incomplete. To assess how widespread bryophyte-associated N₂ fixation is in temperate, wet climates, we sampled bryophytes along nested climate gradients-along three mountains along a north-south gradient in New Zealand-and measured N₂-fixation rates in situ and ex situ. We found a high diversity of bryophytes along the gradients, all exhibiting N₂-fixation activity. Bryophyte diversity followed the classical pattern of decreasing diversity with elevation and similarly, bryophyte-associated N₂ fixation declined with elevation. The warmest, not the wettest, mountain had the highest N₂-fixation rates across the investigated bryophytes. We show that N₂ fixation is widely distributed across diverse bryophyte hosts in temperate habitats, with temperature being the key climate control in temperate rainforests.