Biological Nitrogen Fixation and Nitrogen Accumulation in Peatlands

Peatlands cover about 3% of the Earth's surface and are regarded as a vital carbon (C) pool and sink. The formation of peatland is supported by continuously supplied nitrogen (N) but the sources of this N remain unclear. Here, we first review N stocks and the rate they accumulate in peatlands, then we present the sources of N, especially through biological nitrogen fixation (BNF). We found that global peatlands store 5.9-25.9 Gt N. In the past millennia, northern peatlands have a lower N accumulated rate than tropical undisturbed peatlands. BNF rate is approximately 1.9 +/- 2.7 g m(-2) yr(-1) in northern peatlands, higher than the rate of N deposition, 0.5 +/- 0.4 g m(-2) yr(-1). For tropical peatlands, BNF observation has hardly been reported yet and needs further investigation. This review provides a broad picture of peatland N cycling and suggests that there are large uncertainties, due to limited observations of BNF and N fluxes by inflow and outflow runoff. Therefore, we call for more efforts contributing to field observations and modelling of the N budget in peatlands.Systematic Review Registration: (website), identifier (registration number).