ABOUT
Early crop agriculture is thought to have taken place on naturally waterlogged soils such as floodplains bordering rivers, where periodic flooding provided a natural form of irrigation for early crop plants. We are hoping to use the sulphur isotopic composition of plants and seeds as a natural tracer of waterlogging during plant growth, by researching the effects of waterlogging-induced microbial sulphate reduction on the sulphur isotopic composition of plants grown on periodically waterlogged soil. We are going to grow rice, barley and Brome grass under controlled experiments to test this.
Representation of a sulphate molecule showing the number of protons and neutrons in the nuclei of 32S and 34S isotopes
Microbial sulphate reduction is ubiquitous in anaerobic environments including saturated soil, and is carried out by a range of different organisms. These organisms use sulphate in place of oxygen for respiration. Sulphur in the environment is made up of isotopes; forms of sulphur which differ in the number of neutrons in their nuclei. The most abundant sulphur isotopes are the stable isotopes 32S and 34S, which don't undergo radioactive decay. 34S has two extra neutrons in its nucleus. When sulphate is respired by microbes, the ratio between the isotopes 32S and 34S is altered.
We will measure the ratios of 32S to 34S of sulphate in our growing soil/ compost mix to see if there is a change in the isotope ratios indicating that microbial sulphate reduction has occurred. If we also see a similar change in isotope ratios in the sulphur in the plants that we grow in waterlogged conditions, but not in the plants or soil without waterlogging, we can deduce that the sulphur isotope ratios are showing the effect of waterlogging, and can be used as a tracer. We can then apply this natural tracer technique to archaeobotanical samples of rice, barley and Brome grass, to see if the sulphur isotope ratios in these samples indicate that these ancient plants were grown in waterlogged conditions.
