Nitrogen and phosphorus dynamics during decomposition of multiple litter types in temperate coniferous forests

Graduation date: 2009Litter nutrient dynamics contribute significantly to biogeochemical cycling in\ud forest ecosystems. These dynamics may be influenced by site attributes, litter nutrient concentrations, and soil nutrient availability either independently or synergistically. Litter nutrient dynamics were examined in two decomposition studies in temperate coniferous forests of Oregon. I used ¹⁵N-labelled litter of three species in a comparative study of how site environment and initial substrate quality influence\ud decomposition and nitrogen (N) dynamics of fresh foliage, fine roots, and twigs at Cascade Head Experimental Forest and H. J. Andrews Experimental Forest. There were no site differences with respect to N dynamics, and N mineralization patterns were species-specific. Although N immobilization did occur early in the decomposition process, the general trend for all litter was net N mineralization throughout the study without a net N immobilization phase. For several litter x species combinations the difference between gross N mineralization and net N mineralization was significant, with gross N mineralization ~7 to 20% greater than net\ud mineralization. These results suggest that initial litter chemistry is a more important\ud driver than site environmental differences of the N dynamics associated with decomposition. I also assessed whether litter phosphorus (P) concentrations and soil P availability influenced decomposition rates and litter nutrient dynamics in N-rich\ud Douglas-fir forests in the Oregon Coast Range using a factorial P fertilization\ud experiment. Over the course of 2 years, fresh foliage, fine root, and twig litter from Douglas-fir seedlings were decomposed at three sites. Litter mineralized P at a rapid rate early in the decomposition process compared to N, which was mineralized more slowly or immobilized. Decomposition rates and mineralization of N and P were\ud strongly correlated with initial litter chemistry. Initial litter element ratios between control and P-fertilized litter differed, but over the 2 years element ratios (C:N, C:P, N:P) converged to similar values across treatments. These studies confirm that net mineralization of N and P may occur early in the decomposition process and that litter may decompose without exhibiting a net N immobilization phase. Further, initial litter nutrient concentrations and element ratios may be important predictors of nutrient transformations during decomposition