16. januar 2020

Mosses are the gatekeepers of nitrogen input to ecosystems

Terrestrial Ecology

New research shows that mosses are crucial in regulating the nitrogen flow between atmosphere and ecosystem, but the results suggest a strong dependency on climatic factors such as temperature and precipitation, which climate change are likely to alter.

Mosses comprise most of the understory in subalpine and boreal coniferous forests, where net primary productivity is limited by the availability of nitrogen (N). Despite their high abundance in many ecosystems, it is still unknown how efficient mosses are in N recycling and how much is lost to the ecosystem via e.g. leaching.

- “To aid in understanding the N dynamics of mosses we established and tested a conceptual framework describing the possible pathways that mosses recycle or lose N during senescence. Moreover, we followed the fate of N in two dominant mosses using stable isotopes in old-growth fir forests in the eastern Tibetan Plateau”, says first author Xin Liu from the Chengdu Institute of Biology, Chinese Academy of Sciences, at the moment visiting researcher at Department of Biology, University of Copenhagen. 

The study was carried out as a collaboration between scientists from the Department of Biology at the University of Copenhagen and the Chengdu Institute of Biology, Chinese Academy of Sciences. The results have recently been published in the widely recognized scientific journal Annals of Botany.

Field experiment in old-growth fir forest in eastern Tibetan Plateau. The understory of the forest is dominated by mosses. Stable isotopes of N were used to trace the N dynamics along moss shoots. Photo: Xin Liu.

Plants withdraw nutrients such as nitrogen (N) from dying tissue for use in the plant parts that are still alive and active. This process of nutrient resorption is not only an important function for the plant itself as it supplies crucial nitrogen for growth, but also for the ecosystems’ nutrient budget. However, if mosses recycle nitrogen as efficiently as other plants is not known. Mosses can cover up to 100% of the ground in forest ecosystems and play a key role in ecosystem nitrogen cycling. This is due to the facts that mosses are extremely efficient at taking up nutrients from atmospheric deposition and harbor N2-fixing bacteria that can contribute substantially to the plants’ and ecosystems’ nutrient pool. Besides these traits, new research shows that mosses recycled the majority of their tissue N internally, suggesting that mosses are key in regulating N flow between atmosphere and ecosystem.

Mosses are unique and differ from higher plants, such as forbs, shrubs, and trees. For example, most mosses do not have vascular tissue which main role is to transport water and nutrients, and mosses typically have simple one-cell-thick leaves which are prone to lose nutrients during changes in the microclimate such as sudden rain events.

- “We have obtained results showing that both dominant mosses recycled over 50% of N within the moss tissue, and they lost up to 33% of N from live tissue via leaching, while ca. 15% of N remained in the dead tissue”, states Xin Lui.

Common moss species in the understory layer of subalpine forests in the eastern Tibetan Plateau. From left to right: Hylocomium splendens, Actinothuidium hookeri, Ptilium crista-castrensis, and Rhytidiadelphus triquetrus. Photo: Xin Liu.

The results highlight the crucial role mosses play in N sequestration of ecosystems since N retention (recycled and sequestered in senescent tissue) outweighed N losses via leaching. Hence, the moss carpet is the regulating boundary layer between atmosphere and ecosystem. However, N loss from moss carpets will increase given the rising probability and magnitude of extreme weather events that promote nutrient leaching from moss tissue. This will ultimately lead to changes in ecosystems’ N pools and thereby, N turnover with consequences for ecosystem productivity and carbon sequestration.