Direct Measurement Of Litter Breakdown

In deciduous forests with annual pulses of leaf drop, it is possible to measure litter breakdown directly from litter samples taken through time. If combined with estimates of the mass of litterfall, these samples provide a good measure of the dynamics of litter breakdown (Fig. 5.6) (Witkamp and van der Drift, 1961). As the year progresses, the litter layer mass, L, becomes transformed into F layer (see Chapter 1). Sampling over several years reveals year-to-year variation in masses of

Unclassified ORNL-LR-DWG 65862R

Unclassified ORNL-LR-DWG 65862R

Soil Carbon Breakdown

Forest floor (gC/m2)

FIGURE 5.5. Estimates of decomposition rate factor k for carbon in evergreen forests, from the ratio of annual litter production (L) to (approximately) steady-state accumulation of forest floor (XSS) (from Olson, 1963, with permission).

Forest floor (gC/m2)

FIGURE 5.5. Estimates of decomposition rate factor k for carbon in evergreen forests, from the ratio of annual litter production (L) to (approximately) steady-state accumulation of forest floor (XSS) (from Olson, 1963, with permission).

FIGURE 5.6. (a) Rate of disappearance of litter in mull and mor. (b) Amount of litter on a cleaned surface in mull and mor. (c) Amount of fresh oak (O), birch (B), poplar (P), and alder (A) litter in mull (from Witkamp and van der Drift, 1961).

litter input and rates of breakdown (Table 5.1). It should be noted that two systems of organic layer horizonation are used in the soils literature: L, F, and H refer to litter, fermentation (or fragmented), and humus layers, respectively (see Green et al., 1993, for a complete description of this biologically based system); these are equivalent to the Oi, Oe, and Oa layers often used in forest soils literature.

Rates of litter breakdown are measured more easily by using confined leaf litter. Mesh bags (litterbags) containing a known mass of leaf litter are placed on the forest floor at the time of leaf drop. Litterbags are then collected on a time schedule and the remaining mass is measured (Fig. 5.7). Litterbags have been a valuable tool for comparative studies of rates of litter breakdown (Fig. 5.8). Such studies include mass loss rates by different tree species, and have shown the importance of elemental contents, lignin, carbon-nitrogen ratios, and other resource quality factors (Table 5.2) (Blair, 1988a; Melillo et al., 1982). Decomposition rates also vary between habitats and forest types, and litterbags have proved to be useful in delineating and analyzing differences (Table 5.3) (Cromack, 1973; Heneghan et al., 1998, 1999).

TABLE 5.1. Summary of Litter Decomposition Experiments Conducted on Clear-cut (WS 7) and Control (WS 2) Watersheds, at the Coweeta Hydrologic Laboratory"

74-75 75-77 77-78

WS 7, pre-cut WS 2, pre-cut WS 7, post-cut WS 2, post-cut

% % % %

Species

Rate

Remaining

Rate

Remaining

Rate

Remaining

Rate

Remaining

Liriodendron tulipifera

0 0

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