Abstract:
A major uncertainty in soil carbon studies is how inputs of fresh plant-derived
carbon affect the turnover of existing soil organic matter (SOM) by so-called
priming effects. Priming may occur directly as a result of nutrient mining by
existing microbial communities, or indirectly via microbial population
adjustments. Soil type and conditions may also influence the intensity and
direction of priming effects. However the mechanisms are poorly understood.
The objectives of this study were (1) to investigate how additions of labile C4
substrate affected SOM turnover in two contrasting unplanted C3 soils (clayey
fertile from Temple Balsall, Warwickshire (TB) and sandy acid from
Shuttleworth, Bedfordshire (SH) using13 C isotope shifts; (2) to investigate the
influence of rhizodeposition from plant roots on SOM turnover in the same two
soils planted with a C4 grass; (3) to assess an automated field system for
measuring soil temperature, moisture and photosynthesis sensitivities of SOM
turnover in the same two soils over diurnal to seasonal time scales. I used a
combination of laboratory incubation, glasshouse and field experiments.
In the soil incubation experiment, I made daily applications of either a maize
root extract or sucrose to soil microcosms at rates simulating grassland
rhizodeposition, and followed soil respiration (Rs) and its δ13 C over 19 days. I
inferred the extent of priming from the δ13 C of Rs and the δ13 C of substrate and
soil end-members. There were positive priming effects in both soils in response
to the two substrates. In the SH soil there were no differences in priming effects
between the substrates. However in the TB soil, sucrose produced greater
priming effects than maize root extract, and priming effects with sucrose
increased over time whereas with maize root extract declined after the first
week. I explain these effects in terms of the greater fertility of the TB soil and
resulting greater microbial nitrogen mineralization induced by priming. Because
the maize root extract contained some nitrogen, over time microbial nitrogen
requirements were satisfied without priming whereas with sucrose the nitrogen
demand increased over time.
In the glasshouse experiment, I planted C4 Kikuyu grass (Pennisetum
clandestinum) in pots with the same two soils. The extent of rhizodeposition by
the plants was altered by intermittently clipping the grass in half the pots (there
were also unplanted controls) and priming effects were inferred from the δ13 C of
Rs and the δ13 C of plant and soil end-members. Unclipped plants in both soils
generated positive priming effects, while clipping reduced priming in TB soil and
produced negligible PEs in SH soil. Microbial nutrient mining of SOM again
explained the observed PEs in this experiment. Photosynthesis was a major
driver of priming effects in the planted systems.
In the third experiment, I found that the tested automated chamber system
provided reliable measurements of Rs and net ecosystem exchange (NEE), and
it was possible to draw relations for the dependency of Rs and NEE on key
environmental drivers.
Collectively, the results contribute to a better understanding of the mechanisms
of priming effects and highlight possibilities for further research. The methods
developed here will allow high temporal and spatial resolution measurements of
Rs and NEE under field conditions, using stable isotope methods to separate
fluxes into plant- and soil-derived components.
Keywords: Soil respiration, soil moisture, soil temperature, Isotope ratio,
maize root, flux chamber, climate change, organic matter, rhizodeposition.