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. / Graduation date: 2006
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28854 |
| Date | 16 March 2006 |
| Creators | Crow, Susan E. |
| Contributors | Lajtha, Kate |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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