Global Biogeochemical Cycles - July 2021

Global uptake of atmospheric methane by soil from 1900 to 2100.

Murguia-Flores F., Ganesan A.L., Arndt S. and Hornibrook R.C.

Soil methanotrophy is the only biological process that removes CH4 from the atmosphere. There is good agreement about the size of the global sink but great uncertainty about its interannual variability and regional responses to changes in key environmental drivers. We used the process-based methanotrophy model MeMo v1.0 and output from global climate models to simulate regional and global changes in soil uptake of atmospheric CH4 from 1900 to 2100. The annual global uptake doubled from 17.1±2.4 to 37.2±3.3 Tg yr-1 from 1900-2015 and could increase further to 82.7±4.4 Tg yr-1 by 2100 (RCP8.5), primarily due to enhanced diffusion of CH4 into soil as a result of increases in atmospheric CH4 mole fraction. We show that during the period 1980-2015 temperature became an important influence on the increasing rates of soil methanotrophy, particularly in the Northern Hemisphere. In RCP-forced simulations the relative influence of temperature on changes in the uptake continues to increase, enhancing the soil sink through higher rates of methanotrophic metabolic activity, increases in the global area of active soil methanotrophy and length of active season. During the late 21st century under RCP6.0, temperature is predicted to become the dominant driver of changes in global mean soil uptake rates for the first time. Regionally, in Europe and Asia, nitrogen inputs dominate changes in soil methanotrophy, while soil moisture is the most important influence in tropical South America. These findings highlight that the soil sink could change in response to drivers other than atmospheric CH4 mole fraction.

Global Change Biology - March 2021

Leaching of dissolved organic carbon from mineral soils plays a significant role in the terrestrial carbon balance.

Nakhavali M., Lauerwald R., Regnier P., Guenet B., Chadburn S. and Friedlingstein P.

The leaching of dissolved organic carbon (DOC) from soils to the river network is an overlooked component of the terrestrial soil C budget. Measurements of DOC concentrations in soil, runoff and drainage are scarce and their spatial distribution highly skewed towards industrialized countries. The contribution of terrestrial DOC leaching to the global‐scale C balance of terrestrial ecosystems thus remains poorly constrained. Here, using a process based, integrative, modelling approach to upscale from existing observations, we estimate a global terrestrial DOC leaching flux of 0.28 ± 0.07 Gt C year−1 which is conservative, as it only includes the contribution of mineral soils. Our results suggest that globally about 15% of the terrestrial Net Ecosystem Productivity (NEP, calculated as the difference between Net Primary Production and soil respiration) is exported to aquatic systems as leached DOC. In the tropical rainforest, the leached fraction of terrestrial NEP even reaches 22%. Furthermore, we simulated spatial‐temporal trends in DOC leaching from soil to the river networks from 1860 to 2010. We estimated a global increase in terrestrial DOC inputs to river network of 35 Tg C year−1 (14%) from 1860 to 2010. Despite their low global contribution to the DOC leaching flux, boreal regions have the highest relative increase (28%) while tropics have the lowest relative increase (9%) over the historical period (1860s compared to 2000s). The results from our observationally constrained model approach demonstrate that DOC leaching is a significant flux in the terrestrial C budget at regional and global scales.

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Nature Communications - January 2021

Mineral phosphorus drives glacier algal blooms on the Greenland Ice Sheet.

McCutcheon J., Lutz S., Williamson C., Cook J.M., Tedstone A.J., Vanderstraeten A., Wilson S.A., Stockdale A., Bonneville S., Anesio A.M., Yallop M.L., McQuaid J.B., Tranter M. & Benning L.G.

Melting of the Greenland Ice Sheet is a leading cause of land-ice mass loss and cryosphere-attributed sea level rise. Blooms of pigmented glacier ice algae lower ice albedo and accelerate surface melting in the ice sheet’s southwest sector. Although glacier ice algae cause up to 13% of the surface melting in this region, the controls on bloom development remain poorly understood. Here we show a direct link between mineral phosphorus in surface ice and glacier ice algae biomass through the quantification of solid and fluid phase phosphorus reservoirs in surface habitats across the southwest ablation zone of the ice sheet. We demonstrate that nutrients from mineral dust likely drive glacier ice algal growth, and thereby identify mineral dust as a secondary control on ice sheet melting.

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Nature Communications - January 2021

Around one third of current Arctic Ocean primary production sustained by rivers and coastal erosion.

Terhaar J., Lauerwald R., Regnier P., Gruber N. & Bopp L.

In this study, a international team of scientists from the Institut Pierre Simon Laplace in Paris, the Université Libre de Bruxelles, and the Eidgenössische Technische Hochschule in Zürich, provides a first comprehensive estimate of nutrient inputs from rivers and coastal erosion to the Arctic Ocean, and a quantitative assessment of the importance of these two sources of nutrients for Arctic Ocean primary production. The riverine fluxes are based on observed monthly fluxes from the six largest Arctic rivers, whereas the fluxes from coastal erosion were calculated using satellite images of the Arctic coastline and measurements of the nutrient content in these eroding soils. The riverine and coastal erosion fluxes were then used to force a state-of-the-art high-resolution ocean-biogeochemical model. The results suggest that terrigenous nutrients sustain 28-51 % of the total Arctic Ocean productivity, with rivers accounting for 9-11 % of total Arctic Ocean and coastal erosion for 19-41 %.

This study suggests a much more prominent imprint of terrestrial inputs on Arctic Ocean productivity compared to previous studies and highlights the important role of coastal erosion for the Arctic Ocean ecosystem. Terrigenous nitrogen input from both rivers and coastal erosion will likely increase over the 21st century, which may further augment Arctic Ocean productivity through a reduced nutrient stress on its marine ecosystem.

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Global Biogeochemical Cycles - December 2020

Sedimentary Nutrient Supply in Productive Hot Spots off the West Antarctic Peninsula Revealed by Silicon Isotopes.

Cassarino L., Hendry K.R., Henley S.F., MacDonald E., Arndt S., Freitas F.S., Pike J. and Firing Y.L.

In this study we evaluate the benthic fluxes of silicic acid along the West Antarctic Peninsula (WAP). Silicic acid (DSi) is one of the macronutrients essential in fuelling biological hot spots of diatom‐dominated primary production along the WAP. Here we measure the concentration and stable silicon isotopic composition of DSi in porewater profiles, biogenic silica content (BSi), and diatom abundance from sediment cores collected along the WAP. We couple these measurements with reaction‐transport modeling, to assess the DSi flux and the processes that release this key nutrient from the sediment into the overlying waters. Our results show that the benthic DSi flux is dominated by the diffusive flux, which is estimated to be equivalent to 26.7 ± 2.7 Gmol yr−1 for the WAP continental shelf. The DSi isotope profiles reveal the important impact of sedimentary processes on porewater DSi and suggest that biogenic silica dissolution is the main source of DSi in porewaters and consequently of the benthic fluxes. Our integrated data‐model assessment highlights the impact of surface productivity on sedimentary processes and the dynamic environment of core‐top sediments where dissolution and reverse weathering reactions control DSi exchanges.

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