Assessing spatial variability of C, Fe and Al concentrations in gleyed soils as a means of understanding the stabilisation of soil organic carbon.

Globally soils store more carbon than the atmosphere and vegetation combined; this carbon could otherwise be present as greenhouse gases in the atmosphere. Maintaining soil conditions such that they continue to store organic carbon is very important; however, our understanding of how carbon is stored in soils is incomplete. It is thought that certain minerals in the soil, including iron and aluminium oxides, help stabilise organic carbon preventing microbial attacking and its conversion to carbon dioxide or other green house gases. However, it is not clear how exactly iron oxides and organic carbon interact to prevent organic carbon being metabolised by soil organisms. Gleyed soils occupy ca. 900 million ha globally, and are soils that are subject to permanent or temporary waterlogging; this results in distinctive patterns whereby iron oxides are concentrated in some areas and are absent from others. This spatial patterning of iron oxide minerals makes these soils useful for researchers trying to understand the mechanisms of organic carbon stabilisation. Bulk chemical analysis can provide valuable information about total concentrations of organic carbon, iron, and aluminium, and about the nature of the organic matter that is present, but these traditional bulk chemical approaches do not provide information about the small scale spatial differences in carbon and iron oxide concentration. Microscopic analysis of such soils would allow us to directly examine the concentration of iron oxides and organic carbon, thus helping to elucidate the mechanisms of organic carbon stabilisation by iron oxides in soil over small distances. However, the microscopic scale of these measurements can make it difficult to extrapolate the findings to whole soil profiles and larger soil volumes. The proposed pilot project aims to bring together for the first time specific bulk and microscopic analytical techniques in order to study the mechanisms of organic carbon stabilisation in gleyed (waterlogged) soils. Undisturbed and bulk samples will be taken from soils at Harwood Forest, Northumberland with contrasting drainage conditions. Bulk samples will be analysed to determine background soil conditions, total concentrations of carbon, iron and aluminium oxides, and the nature of the organic matter present. A sequential series of increasingly aggresive extractions will be carried out on the soils, and the solution recovered after each extraction will be analysed again to determine the nature of the organic carbon bound to different types of iron oxide minerals (weakly crystalline and strongly crystalline). Undisturbed blocks of soil will be stabilised in resin, sliced and lapped to create thin sections which can be examined using petrological and electron microscopes. Areas of iron oxide concentration and depletion can be mapped and the ratio of Fe, Al, O, and C determined across cm, mm and micrometer scales. These sections will then be subjected to the same treatments as the bulk soils in order to remove iron, aluminium and organic carbon associated with first weakly and then strongly crystalline iron oxides. This novel approach will add to our understanding of how carbon is stabilised in gleyed soils and how sensitive it might be to changing soil conditions as a result of climate change. This will provide valuable information for modellers of soil carbon turnover. The project will also refine the analytical methods available to researchers which will be useful for the study of similar processes in different soils; this could be particularly useful for the study of carbon storage and soil processes in agricultural systems, which make a significant contribution to global greenhouse gas emissions.