Relationships between litter quality and nitrogen availability in Rocky Mountain forests

Abstract

We assessed the relationships between net nitrogen mineralization, litter quality, and litter decomposition rates in 30 Colorado forests to test three hypotheses of the LINKAGES forest ecosystem model: (i) nitrogen release from litter decay in the forest floor correlates with total (forest floor plus mineral soil) net nitrogen mineralization; (ii) net nitrogen mineralization can be predicted from litter chemistry; and (iii) lignin:nitrogen ratio is the litter quality parameter most closely correlated with decomposition rates. Net nitrogen mineralization was measured for 1 year in 10 stands each of lodgepole pine (Pinuscontorta var. latifolia Engelm.), aspen (Populustremuloides Michx.), and Engelmann spruce–subalpine fir (Piceaengelmannii Parry and Abieslasiocarpa (Hook.) Nutt.), using a sequence of on-site incubations in closed-top tubes. We analyzed foliage litter and fine-root samples for initial concentrations of nitrogen, lignin, carbon, and cellulose, and determined litter mass remaining after 2-, 4-, and 6 months of laboratory decomposition. Net nitrogen mineralization in the forest floor correlated highly with total net nitrogen mineralization both within and across species. Net nitrogen mineralization also correlated significantly with litter quality across species, as did litter mass remaining after each decay period. Initial lignin:nitrogen ratio showed as strong a relationship with net nitrogen mineralization and litter decay rates as any of the litter quality variables assessed. The three tested hypotheses were thus broadly supported in these forest ecosystems.