Bryophytes enhance nitrogen content in decaying wood via biological interactions

Abstract

An increase in the nitrogen (N) content in coarse woody debris (CWD) facilitates its decomposition, affecting the cycling of N and other nutrients in forest ecosystems. Bryophytes may increase the N content by transferring the N in bryophyte tissues to the underlying CWD. This study examined whether and how bryophytes increase N content in the underlying CWD using N-stable isotope ratios (δ15N) as tracers for N sources. The N content and δ15N values in CWD with bryophytes were significantly higher than those in CWD without bryophytes. However, the δ15N values in CWD with bryophytes differed significantly from those in bryophytes on CWD, demonstrating that the N in bryophytes did not cause the increase in N content in CWD. The analyses using δ15N further indicated that the high N content in CWD with bryophytes may be attributed to increased N supply from wood-decomposing fungi and N-fixing bacteria. This increase in N content may result from the enhanced moisture content in CWD beneath bryophytes, which facilitates the activity of wood-decomposing fungi and N-fixing bacteria. Notably, the influence of bryophytes on the N content in CWD differed between bryophyte life forms: bryophytes that form dense mats increased the N content in CWD, whereas those with loose mats decreased the N content. This difference can be explained by the greater humidity experienced by CWD with bryophytes forming dense mats than that experienced by CWD with bryophytes forming loose mats. Given that the N content in CWD affects the decay processes, the results highlight the importance of biological interactions associated with bryophytes in forest ecosystems.