| dc.description.abstract | Soil respiration is an important source of atmospheric CO2, with the potential for
large positive feedbacks with global warming. The size of these feedbacks will depend
on the relative sensitivity to temperature of very large global pools of highly
stable soil organic matter (SOM), with residence times of centuries or longer. Conflicting
evidence exists as to the relationships between temperature sensitivity of
respiration and stability of SOM, as well as the temperature sensitivity of individual
stabilisation mechanisms.
This PhD considers the relationship between different stabilisation mechanisms
and the temperature sensitivity of SOM decomposition. I used physical fractionation
to isolate SOM pools with a variety of turnover rates, from decadal to centennially
cycling SOM, in a peaty gley topsoil from Harwood Forest. Mean residence times of
SOM as determined by 14C dating was most strongly affected by depth, providing
stability on a millienial scale, while OM-mineral associations and physical protection
of aggregates provided stability to around 500 years.
Chemical characteristics of organic material in these fractions and whole soils (13C
CP-MAS NMR spectroscopy, mass spectrometry, FTIR spectroscopy, thermogravimetric
analysis, ICP-OES) indicated the relative contribution of different stabilisation
mechanisms to the longevity of each of these fractions. Two long-term incubations of
isolated physical fractions and soil horizons at different temperatures provided information
about the actual resistance to decomposition in each SOM pool, as well as the
temperature sensitivity of respiration from different pools. Naturally 13C-labelled labile
substrate additions to the mineral and organic horizons compared the resistance
to priming by labile and recalcitrant substrates. Manipulation of soil pore water
was investigated as a method for isolating the respiration of SOM from physically
occluded positions within the soil architecture.
Contadictory lines of evidence emerged on the relative stability of different SOM
pools from 14C dating, incubation experiments and chemical characterisation of indicators
of stability. This led to the interpretation that physical aggregate protection
primarily controls SOM stability within topsoils, while mineral and Fe oxide stability
provides more lasting stability in the mineral horizon. Less humified and younger
SOM was found to have a higher sensitivity to temperature than respiration from
well-humified pools, in contrast to predictions from thermodynamics. | en |
