Element dynamics along a decay continuum in a red spruce ecosystem in Maine, USA

Element dynamics (C, N, P, Ca, Mg, Mn, Fe, and Al) were examined along a decay continuum from freshly fallen litter to soil organic matter in a red spruce ecosystem in Maine. The continuum was defined using previously reported data on litterfall chemistry and the early stages (i.e., first 24 mo) of decay, combined with new studies on an additional 33 mo of decay and an evaluation of forest floor organic matter at the same site. Carbon concentrations decreased over time as fresh litter was transformed into soil organic matter; N, Fe, and Al concentrations increased; and P, Ca, Mg, and Mn concentrations showed variable patterns of increased and decrease. In general, homogeneity of the litter increased as fresh litter of different initial chemistries was transformed into chemically more uniform soil organic matter. Ecosystem budgets indicated that after 57 mo of decay, litter had lost 63% of its original mass and had released C (65%),N (17%), P (79%), Ca (50%), Mg (80%), and Mn (74%), and had accumulated Al (431%) and Fe (353%). Evidence is presented indicating that the retention of Al (and Fe) is primarily controlled by abiotic adsorption onto litter exchanges sites. Overall results are consistent with a two-phase model of decay: in the first phase (0 to 3-4 yr), mass loss fits a negative exponential model of decay, an inverse linear relationship exists between N concentration and mass loss, and rapid changes are observed in litter chemistry; the second phase (3-4+ yr) is characterized by markedly slower changes in mass loss and litter chemistry.