Characteristics of soil organic matter in two forest soils Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/pn89d9417

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  • Soil organic matter (SOM) is the terrestrial biosphere's largest pool of organic carbon (C) and is an integral part of C cycling globally. Soil organic matter composition typically can be traced directly back to the type of detrital inputs; however, the stabilization of SOM results as a combination of chemical recalcitrance, protection from microbial decomposition within soil structure, and organo-mineral interactions. A long-term manipulative field experiment, the Detrital Input and Removal Treatment (DIRT) Project, was established to examine effects of altering detrital inputs (above- vs. below-ground source, C and nitrogen (N) quantity, and chemical quality) on the stabilization and retention of SOM. Surface mineral soil was collected from two DIRT sites, Bousson (a deciduous site in western Pennsylvania) and H.J. Andrews (a coniferous site in the Oregon Cascade Mountains), to examine the influence of altering detrital inputs on decomposability and mean residence time of soil organic matter and different organic matter fractions. Soil organic matter was physically separated into light fraction (LF) and heavy fraction (HF) organic matter, by density fractionation in 1.6 g mL⁻¹ sodium polytungstate (SPT). Density fractionation in SPT resulted in the mobilization and loss of ~25% of total soil organic C and N during the physical separation and rinsing of fractions during recovery, which was also the most easily decomposed organic matter present in the bulk soil. At H.J. Andrews, this mobilized organic matter had a short mean residence time (MRT), indicating that it originated from fresh detrital inputs. In contrast, at Bousson, the organic matter mobilized had a long MRT, indicating that it originated from organic matter that had already been stabilized in the soil. Mean residence times of LF from Bousson varied widely, ~3 y from doubled litter and control plots and 78-185 y for litter removal plots, while MRT of HF was ~250 y and has not yet been affected by litter manipulations. Results from long term incubation of LF and HF material supported these estimates; respiration was greatest from LF of doubled litter and control plots and least from HF of litter removal plots. In contrast, MRT estimated for LF and HF organic matter from H.J. Andrews were similar to each other (~100 y) and were not affected by litter manipulation. These estimates were also supported by the incubation results; there was not a difference in cumulative respiration between detrital treatments or density fractions. The results from the coniferous site may be due to a legacy of historically large inputs of coarse woody debris on the LF and it may be decades before the signal of detrital manipulations can be measured. Alternatively, these highly andic soils may be accumulating C rapidly, yielding young HF ages and C that does not differ substantially in lability from coniferous litter-derived LF. The DIRT Project was intended to follow changes in soil organic matter over decades to centuries. As expected, manipulation of detrital inputs has influenced the lability and mean residence time of the light fraction before the heavy fraction organic matter; however, it will be on much more lengthy time scales that clear differences in organic matter stabilization in response to the alteration of detrital inputs will emerge. Soil CO₂ efflux is a compilation of CO₂ from many sources, including root respiration and the decomposition of different organic matter fractions, roots, and exudates. If the sources of CO₂ have different isotopic signatures, the isotope analysis of CO₂ efflux may reveal the dominant sources within the soil profile. In a short incubation experiment of density fractions from both sites, respired CO₂ reflected the isotopic signature of the organic matter fraction after 30 days, but was more enriched in ¹³C. Initially CO₂ was isotopically depleted in ¹³C relative to the organic matter fraction and the period of depletion related to the amount of easily degraded organic matter present at H.J. Andrews only.
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