Nitrous oxide emissions from inland waters: Are IPCC estimates too high?

Abstract

Nitrous oxide (N2O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. N2O emissions are typically estimated using emission factors (EFs), defined as the proportion of the terrestrial nitrogen (N) load to a water body that is emitted as N2O to the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) has proposed EFs of 0.25% and 0.75%, though studies have suggested that both these values are either too high or too low. In this work, we develop a mechanistic modeling approach to explicitly predict N2O production and emissions via nitrification and denitrification in rivers, reservoirs and estuaries. In particular, we introduce a water residence time dependence, which kinetically limits the extent of denitrification and nitrification in water bodies. We revise existing spatially explicit estimates of N loads to inland waters to predict both lumped watershed and half‐degree grid cell emissions and EFs worldwide, as well as the proportions of these emissions that originate from denitrification and nitrification. We estimate global inland water N2O emissions of 10.6–19.8 Gmol N year−1 (148–277 Gg N year−1), with reservoirs producing most N2O per unit area. Our results indicate that IPCC EFs are likely overestimated by up to an order of magnitude, and that achieving the magnitude of the IPCC's EFs is kinetically improbable in most river systems. Denitrification represents the major pathway of N2O production in river systems, whereas nitrification dominates production in reservoirs and estuaries.

Natural Lakes Are a Minor Global Source of N2O to the Atmosphere

“…As reviewed in Maavara et al (), N 2 O production and/or emissions have in earlier studies been estimated from nitrification/denitrification rates or from N loads based on empirically derived emission factors ( EFs ). The wide range of published EFs is one of the main reasons for the wide range of existing N 2 O emission estimates.…”

“…HydroLAKES distinguishes three different types of SWBs: (1) natural lakes without dam constructions, (2) reservoirs, and (3) natural lakes hydrologically regulated by dam constructions. The latter two types of water bodies cover only 6,796 cases and were taken from the Global Reservoir and Dam database (Lehner et al, ), which was used in the study of Maavara et al (). For our study, we use the classification of SWBs as defined in HydroLAKES but, as long as not indicated otherwise, combine classes (1) and (3) as “natural lakes” (i.e., natural origin).…”

“…Recently, Maavara et al () developed a process‐oriented model to consistently estimate N 2 O emissions from a broad range of inland waters, including reservoirs. Based on a spatially explicit representation of the river‐reservoir network and point and nonpoint sources of nitrogen (N) and phosphorous (P), this model made it possible to explore and quantify the global‐scale spatial patterns in riverine and reservoir N 2 O emissions in a consistent manner.…”

“…Based on a spatially explicit representation of the river‐reservoir network and point and nonpoint sources of nitrogen (N) and phosphorous (P), this model made it possible to explore and quantify the global‐scale spatial patterns in riverine and reservoir N 2 O emissions in a consistent manner. In this study, we apply the approach of Maavara et al () to a global data set of 1.4 million SWBs, which are classified into natural lakes and reservoirs (Messager et al, ). Model results for N 2 O emissions from SWBs are evaluated against regional, observation‐driven estimates.…”

Land Use, Not Stream Order, Controls N2O Concentration and Flux in the Upper Mara River Basin, Kenya

“…Increasing atmospheric N 2 O concentration is of global concern, as N 2 O has 265 times greater impact on climate warming than CO 2 on a molecular basis (Intergovernmental Panel on Climate Change [IPCC], ) and has been increasing in the atmosphere at a rate of ~0.73 ppb/year over the last three decades (IPCC, ). Estimated global riverine N 2 O flux contributions are, however, highly uncertain with estimates ranging from 32.2 to 2,100 Gg N/year (Beaulieu et al, ; Hu et al, ; Kroeze & Seitzinger, ; Maavara et al, ). While older IPCC and modeled estimates suggested that rivers and estuaries contribute up to 20–35% of global anthropogenic N 2 O emissions (Kroeze & Seitzinger, ; Mosier et al, ), some studies later suggested that streams and rivers contribute considerably more to global N 2 O flux than was previously accounted for in the IPCC estimates (Beaulieu et al, ; Turner et al, ; Venkiteswaran et al, ).…”