Aging Mosses Can Be Nitrogen Sinks: Tracking Tissue Nitrogen During Senescence

Old or dying leaves move nutrients to living tissues (i.e. resorption) but some nutrients remain in the dying tissues and are recycled in the environment via decomposition. One of the nutrients involved in resorption and can be ‘tracked’ within an ecosystem is nitrogen (N). 

Liu and colleagues (2019) at the Chengdu Institute of Biology of the Chinese Academy of Sciences studied the N resorption efficiency (NRE) of two moss species with different growth rates. The authors found that mosses are important N sinks in a forest but the sink strength depends on N leaching.

The team studied two dominant moss species with 3-year lifespan, Actinothuidium hookeri and Hylocomium splendens, in the Dague Nature Reserve in China at 3,700m altitude. Samples were collected for three years and ¹⁵N concentrations were measured in growing, senescing and senesced segments in individual shoots. The total tissue N content (N pool) was calculated from biomass weight and shoot number counts and over the years, N loss was estimated from resorption by younger segments and leaching.

The mean NREs of A. hookeri and H. splendens were between 52-61% and the two mosses lost N from live tissue at the annual rate of 8.7 and 13.2 % respectively. Overall, there was more N loss via leaching from living tissue than N retention in senesced tissue. 

The authors write “Our findings suggest that understorey mosses are currently sinks of N due to their high NRE,” but they also suggest that, “This N sink, however, can turn into an N source due to the predicted changes in temperature and precipitation patterns that increase N losses from mosses during freezing–thawing and drying–rewetting events.”

This study provides a unique insight into how mosses play a crucial role in the N cycle within an ecosystem and how much remains unknowns about the impact of climate change and other moss species.