Carbon and nitrogen distribution and processes in forest and agricultural ecosystems: A study involving solid- and liquid-state NMR and pyrolysis GC/MS.

摘要

Forest and agricultural ecosystems comprise a large portion of the United States. Soil organic matter (SOM) is viewed as the organic source of C and N for plants and microorganisms as well as an important C and N sink in these soils. To obtain a better understanding of C and N cycling in forest and agricultural ecosystems, one needs to understand humification, transformations, the chemistry of the various inputs and the chemical structural changes that these materials are subjected to on their path towards soil formation.; For the forest study, a vertical transect of samples was collected from the forest canopy to the subsoil in the Harvard Forest before and after ten years of chronic N fertilization to determine compositional changes caused by ten years of fertilization and assess the effects of N loading.; Currently, little is known about carbon structural types, quantity or quality preserved as a result of agricultural management practices. Soil samples and various OM inputs (before and after degradation) were collected from agricultural soils under conventional and organic management practices. Samples from both ecosystems were characterized using elemental analysis, solid-state 13C and 15N NMR, and pyrolysis GC/MS.; 13C NMR spectra from all samples contain signals associated with paraffinic, carbohydrate, aromatic and carboxyl structures. NMR spectra of soil inputs are dominated by carbohydrate carbons, while these intensities are severely diminished in the soil. An important observation is that the dominant persistent structures are paraffinic-type carbons in forest and agricultural ecosystems and, to a lesser extent, lignin structures in the agricultural system. However, comparison of the paraffinic signatures of the major OM inputs to the soils suggests that these are not the primary source of persistent paraffinic structures. In the forest system, N fertilization caused an increase in lipid-extractable paraffinic structures in oak leaves, an increase in N content in leaves, and slower litter decomposition in O and A soil horizons under pine and hardwood trees. In the agricultural system, minimal differences were observed between organic and conventionally managed soil, while crop residue lignin-to-nitrogen ratio differences were observed between management practices.