This element is found at these locations (XPath):

For a dataset, the abstract element can appear at the resource level or the project level. The <abstract> element will be used for full-text searches, and it should be rich with descriptive text. In particular, descriptions should include information that does not fit into structured metadata, and focus on the “what,” “when,” and “where” information, general taxonomic information, as well as whether the dataset is ongoing or completed. Some general methods description is appropriate, and broad classes of measured parameters should also be included. For a large number of parameters, use categories instead of listing all parameters (e.g. use the term “nutrients” instead of nitrate, phosphate, calcium, etc.), in combination with the parameters that seem most relevant for searches.