Control of decomposition processes and nutrient flow in a California forest and grassland.

摘要

This dissertation work focused on two major factors that potentially control decomposition processes and nitrogen (N) release from plant detritus in forests and annual grasslands with Mediterranean-type climates: substrate quality (degradability) and spatial compartmentalization of available carbon (C) and N within the soil profile. Despite a lower initial quality of Pinus ponderosa needle litterfall in an old- compared to a nearby young-growth mixed-conifer forest, no difference in decomposition rates was found between litter types when incubated in either stand. The extremely low rates of decomposition in these forest (0.08 and 0.18 yr{dollar}sp{lcub}-1{rcub}{dollar}, respectively) appear to be due to the combined effects of low quality litter and the seasonal displacement of warm temperatures from moist conditions in Mediterranean-type climates. {dollar}sp{lcub}14{rcub}{dollar}C-, {dollar}sp{lcub}15{rcub}{dollar}N-labeled leaf and root materials of Bromus mollis and Pinus ponderosa, having a range in initial C:N ratios within each litter type, were applied to field microplots to study the effect of the initial C:N ratio of litter on the rate of release, flow, and fate of C and N within the soil-plant system. In both ecosystems, the initial C:N ratio generally altered the first-year rate of {dollar}sp{lcub}14{rcub}{dollar}C and {dollar}sp{lcub}15{rcub}{dollar}N release from the plant materials; materials having lower initial C:N ratios released {dollar}sp{lcub}14{rcub}{dollar}C and {dollar}sp{lcub}15{rcub}{dollar}N faster. Analysis of {dollar}sp{lcub}14{rcub}{dollar}C and {dollar}sp{lcub}15{rcub}{dollar}N recoveries in the microbial biomass and other soil and plant pools indicated that the initial C:N ratio affected the temporal flow and fate of {dollar}sp{lcub}14{rcub}{dollar}C and {dollar}sp{lcub}15{rcub}{dollar}N within the soil-plant system, regardless of whether rates of {dollar}sp{lcub}14{rcub}{dollar}C and {dollar}sp{lcub}15{rcub}{dollar}N release were altered. The significance of spatial compartmentalization of C and N within the soil profile as a control on soil N cycling was assessed by injecting small amounts of {dollar}sp{lcub}15{rcub}{dollar}NH{dollar}sb4sp+{dollar} in solution into the mineral soil (relatively low C:N ratio), and tracing the accumulation of {dollar}sp{lcub}15{rcub}{dollar}N in the overlying organic horizons (relatively high C:N ratio) over time. Calculated annual N-fluxes (9 and 3 kg-N ha{dollar}sp{lcub}-1{rcub}{dollar} yr{dollar}sp{lcub}-1{rcub}{dollar} for the old-growth forest and grassland, respectively) were similar in magnitude to other ecosystem N flows. Further, N mineralization and immobilization occurred concurrently within these organic layers, suggesting that: net changes in N mass within organic horizons substantially underestimate their actual N dynamics; and these organic horizons act as both sources and sinks for N within the mineral soil.