Examining the protection of organic carbon in mineral soils of managed landscapes

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Recent changes in atmospheric carbon dioxide concentrations are linked to human activities that accelerate the movement of carbon from long-term global reservoirs into the atmosphere as carbon dioxide, contributing to planetary warming. Soils represent one of the largest actively cycling terrestrial carbon reservoirs, and hold three times as much carbon as the atmosphere or living vegetation. Changes in land use such as conversion of forests to agricultural land and repeated harvesting of forests can cause carbon stored in soils as organic matter to be released to the atmosphere. This can have additional consequences for overall soil health and fertility. The bulk of carbon stored in soils is held within the mineral soil, up to half of it below 20cm depth. It was long assumed that carbon held within the mineral soil had a complex chemical structure that made it difficult for microbial communities to decompose. Recent studies, however, show that much of the older carbon held in soils is actually readily decomposable, but is protected from decomposition by interactions with soil minerals. These interactions may result in the loose adsorption of organic matter to mineral surfaces, or the combining of organic matter with minerals in new stable compounds of varying complexity. In soils where conditions are unaltered over time, this form of carbon can be quite stable, and remain in soils for hundreds to thousands of years. However, when soil conditions are altered by factors such as land surface disturbance and changes in climate, the mechanisms that protect soil organic matter can be reversed, making it available to be decomposed and returned it to the atmosphere. Little is known about how carbon associated with minerals is released when subjected to an altered physical and chemical environment arising from changes in land use and/or climate. Likewise, there is little information about the potential of soils to recover these carbon stores when these disturbances cease. This proposal outlines a research plan to investigate how carbon is partitioned within a range of mineral-associated carbon pools of varying strength and complexity in soils typical of Eastern Canada. Forests of varying ages will be examined, and compared to local agricultural soils to determine the size and variability of these soil carbon pools. Soils will be collected from existing stored archives and sampled in the field. Experiments will subject a subset of these soils to long term increases in soil temperature, and measure how the carbon held in these pools and its chemical composition changes. The goals are to understand how susceptible this protected carbon is to changes in soil conditions, and to be able to make predictions about how this carbon will respond to altered soil conditions. It also seeks to identify important measurements that can be used as standard indices for monitoring changes in soil organic matter within mineral soils.