Page 258 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 11. Integration and outlook
The important link between the carbon and the nitrogen cycle
The carbon cycle cannot be studied on its own. Within global nutrient cycles, there is a connection between carbon, nitrogen, but also hydrogen, oxygen, phosphorus, and sulfur, which are all conserved and recycled in the atmosphere, and in terrestrial and aquatic ecosystems (Zhang, Sekar and Visser 2020). Several global models integrate the link between the carbon and nitrogen cycle in the terrestrial biosphere (Thornton et al. 2007; Bonan and Levis 2010; Zaehle and Friend 2010).
There are many recent research efforts on linking effects of changes within the nitrogen cycle to changes in the carbon cycle. A study by Mao et al. (2020) investigated nitrogen cycling in active permafrost of the Tibetan Plateau. In contrast to the prevailing view, deeper alpine permafrost layers had lower available N content and lower net N mineralization rates than the active layer. A microcosm approach on the effects of nitrogen load on swamp meadow soils on 13CH4 turnover found strong effects of ammonia and urea on methanotroph diversity (He et al. 2019a). This highlights the need to quantify nitrogen inputs of permafrost systems to accurately predict future GHG emissions, especially with the link to alterations in N2O emissions (Ramm et al. 2020).
There is evidence that strong increases in nitrogen load can reduce the CH4 uptake capacity of soils. Meta-regression analyses on N additions in boreal forests indicated negative effects above threshold nitrogen loads of 48 kg N ha-1 yr-1. This is currently well above the average nitrogen deposition in the Netherlands in 2018: <25 kg N ha-1 (RIVM 2019; Xia et al. 2020). Since current N deposits are generally well below this threshold in pristine ecosystems, this could probably only have effects in agriculture-affected areas, where deposition can exceed 50 kg N ha-1 yr-1 (RIVM 2019).
A direct link between the nitrogen and methane cycle is made by nitrite- and nitrate- dependent methane oxidizing prokaryotes (Ettwig et al. 2010; Haroon et al. 2013). Currently, there is little evidence that these AOM organisms provide important CH4 sinks in pristine peatlands and permafrost due to low nitrogen loads of pristine ecosystems (Steven et al. 2006; Finger et al. 2016; Salmon et al. 2016; Chen et al. 2018). However, AOM organisms are often detected in these types of ecosystems. In our studies on Alaskan thermokarst lakes in Chapter 7, we observed sequences related to “Candidatus Methanoperedens nitroreducens”, but no stimulation of nitrite-/nitrate-dependent AOM occurred within the timeframe of microcosm
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