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

Abstract

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