Litter decomposition provides the primary source of mineral nitrogen (N) for biological activity in most terrestrial ecosystems. A 10-year decomposition experiment in 21 sites from seven biomes found that net N release from leaf litter is dominantly driven by the initial tissue N concentration and mass remaining regardless of climate, edaphic conditions, or biota. Arid grasslands exposed to high ultraviolet radiation were an exception, where net N release was insensitive to initial N. Roots released N linearly with decomposition and exhibited little net N immobilization. We suggest that fundamental constraints on decomposer physiologies lead to predictable global-scale patterns in net N release during decomposition.

Decomposition is a critical process in global carbon cycling. During decomposition, leaf and fine root litter may undergo a later, relatively slow phase; past long-term experiments indicate this phase occurs, but whether it is a general phenomenon has not been examined. Data from Long-term Intersite Decomposition Experiment Team, representing 27 sites and nine litter types (for a total of 234 cases) was used to test the frequency of this later, slow phase of decomposition. Litter mass remaining after up to 10 years of decomposition was fit to models that included (dual exponential and asymptotic) or excluded (single exponential) a slow phase. The resultant regression equations were evaluated for goodness of fit as well as biological realism. Regression analysis indicated that while the dual exponential and asymptotic models statistically and biologically fit more of the litter type-site combinations than the single exponential model, the latter was biologically reasonable for 27-65% of the cases depending on the test used. This implies that a slow phase is common, but not universal. Moreover, estimates of the decomposition rate of the slowly decomposing component averaged 0.139-0.221 year À1 (depending on method), higher than generally observed for mineral soil organic matter, but one-third of the faster phase of litter decomposition. Thus, this material may be slower than the earlier phases of litter decomposition, but not as slow as mineral soil organic matter. Comparison of the long-term integrated decomposition rate (which included all phases of decomposition) to that for the first year of decomposition indicated the former was on average 75% that of the latter, consistent with the presence of a slow phase of decomposition. These results indicate that the global store of litter estimated using short-term decomposition rates would be underestimated by at least one-third.

Woody detritus biomass estimates are most often achieved by determining the volume of dead wood and then converting to mass by use of density values. There have been few studies on how density (mass/volume) of this material changes during the decay process. The goal of this study was to synthesize both published and unpublished data on woody detritus density so as to improve estimates of coarse woody detritus (CWD) and fi ne woody detritus (FWD) biomass across the diverse forests of the United States. In the case of CWD, a total of 88 species were found to have data on densities for fi ve decay classes that had been published and/or collected in North America from the boreal to the tropical zones. In general, density declined from decay class 1 to 5 with at least fi ve density reduction patterns observed. For FWD, our search indicated approximately 25 species had been sampled. FWD density was a function of piece diameter and the general state of decay. We determined that by sampling representative species within a genus, the uncertainty of CWD estimates could be reduced by up to 50 percent over not having sampled a genus. Our analysis indicated that the uncertainty of FWD mass estimation ranged from 12 to 19 percent when FWD relative density was estimated. We conclude that a more systematic sampling of CWD and FWD density is needed for major species if uncertainties of mass estimates nationwide are to be reduced.