Soils and intertidal sediments are often subjected to wet-dry cycles. Most surface soils are wetted in summer and dried in winter. Salt marsh and intertidal sediments can have daily wet-dry cycles due to the rising and falling tide. Under dry conditions, dehydration can alter the three dimensional (3D) structure of soils and sediments; thus dried sediments may behave differently than never-dried sediments. For example, water repellency (or hydrophobicity) is often seen in many types of soils as a result of drying. Furthermore, dried soil organic matter is more easily degraded than non-dried soil. Similarly, the organic matrix of fruits and vegetables can collapse irreversibly after dehydration.
With support from this Small Grant for Exploratory Research (SGER), researchers at the State University of New York at Stony Brook will investigate the role play by the structure of organic matter in its decompositional reactivity. They hypothesize that the three-dimensional structure and the polarity of organic matter are important factors controlling decomposition in salt marsh sediment. Hydrophobic groups of organic matter coating mineral surfaces or existing as detritus are exposed to seawater, and have a highly hydrated 3D structure. The organic molecules in the 3D structures are hydrophilic and polar, allowing other polar organic compounds to be absorbed by or partitioned into its 3D structure via electrostatic forces. During the drying process, polar functional groups of organic molecules are forced to interact with each other or with mineral surfaces, thus causing the polar moieties to turn inside out and expose more hydrophobic groups. The drying process makes organic matter change its conformation, shrink its volume, and become more hydrophobic. Rewetted organic matter is more hydrophobic than never-dried material and thus has a stronger sorption capacity for nonpolar organic compounds than the original organic matter. In environments with wet and dry cycles, the distribution of hydrophobic or hydrophilic compounds between solution and particulate phases could thus be influenced by the 3D structure and polarity of organic matter. This should greatly affect the rate of remineralization of the organic matter.
The investigators will further explore some preliminary findings that the three-dimensional architecture of organic matter is critically important to sorption behavior of coastal sediments. Specifically, they will investigate (1) the importance of terrestrial versus marine organic matter in this 3D structure, and (2) the importance of the 3D nature to remineralization rates of organic matter.
As a clear broader impact, this project has societal implications by contributing to a better mechanistic understanding of organic matter remineralization and thus the global carbon cycle. If a more sophisticated understanding of the ocean's response to increased levels of carbon dioxide can be developed, then more reasonable choices between political alternatives are possible. Coastal sediments contain much of the sedimentary organic carbon in the world oceans.
