Nitrogen Import During Early Decomposition Stages

Additions of nitrogen increased both fungal respiration and biomass (chitin assay) as well as weight loss of Scots pine needles inoculated with M. epipterygia in laboratory systems (Boberg and Lindahl, unpublished data), indicating that the needle decomposer was constrained by low nitrogen availability. Furthermore, cellulase activity increased in response to nitrogen additions to litter in the field (Carreiro et al., 2000). When litter-filled mesh bags were incubated in the field, the nitrogen content of leaf and wood litter increased during the first years of decomposition, not only in relative concentration, but also in absolute amounts (Figure 1; Berg et al., 1982; Fahey et al., 1985; Dighton and Boddy, 1989). Using 15N tracer isotope, Hart and Firestone (1990) and Frey et al. (2000) demonstrated vertical translocation of nitrogen into surface litter in a forest and an agricultural soil. Thus, fungi appear to translocate nitrogen into fresh, nitrogen-poor litter, and thereby promote their growth and activity. This supposition was supported by a positive correlation between the amount of imported nitrogen and fungal biomass increase (Berg and Soderstrom, 1979) as well as litter weight loss

(Berg and Staaf, 1981). Furthermore, nitrogen import has been shown to be more pronounced in litter with low initial nitrogen content (Berg and Staaf, 1981).

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