Document Type

Thesis

Date of Degree Completion

Winter 2024

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Carey Gazis

Second Committee Member

Lisa Ely

Third Committee Member

Karl Lillquist

Abstract

Soil is an important terrestrial carbon sink. Through regenerative land management, which includes minimizing soil disturbance, the carbon content of soil can be raised towards preindustrial levels. This feat requires effectively storing carbon in a natural system that is inclined to cyclicity. The most stable form of soil organic matter is mineral-associated organic matter (MAOM). This carbon labelling experiment was designed to identify short-term CO2 fluxes within the soil-plant-atmosphere system, particularly those related to MAOM turnover in the rhizosphere. Basil (Ocimum basilicum) was planted on a plot at the campus farm at Central Washington University. Another plot of bare soil was used as an environmental control. The basil was pulse labelled by allowing photosynthesis within a 13CO2-rich atmosphere for a total of nine hours. Measurements were made with clear and opaque flux chambers to determine flux of CO2 from the soil and into basil via photosynthesis. Soil and respiration samples were taken before and after labelling; biomass was also sampled following labelling. Samples were analyzed with an Isotope Ratio Mass Spectrometer for carbon ratios. After-labelling δ13C values were compared to before-labelling δ13C values to determine where the 13C spike had moved within the plant and soil over the course of the experiment. Soil samples were collected so that turnover estimated through soil respiration could be compared with any observed change in MAOM carbon content. Over the month-long timescale of this study, there was no measurable change in MAOM and the 13C spike was not detected in the soil. However, the heavy carbon appeared in respiration gas collected soon after the last labelling interval as well as in the basil’s roots.

To love is to mind the ebb and flow

and to use warily the garden hoe.

With more neutrality, one may peer

belowground a function of the atmosphere.

We wished to find breath from the mineral

but our methods were not so liberal.

Even still, we return with knowledge

of basil’s carbon haulage.

Example: our tapestry of sandy clay loam

was drier of heavy nectar from the phloem

than expected,

with roots and shoots reasonably affected.

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