Agricultural soils in the Chesapeake Bay region are often rich in nutrients such as phosphate. However, some forms of phosphate are bound to iron in soils and are not available for plant uptake (McQueen et al. 1986). As a result of saltwater intrusion (SWI), or the underground movement of ocean water to inland areas, coastal farmland around the region has begun to transition to wetland habitat. This has caused shifts in the dominant pools of iron found in the soils along these transitional zones (Tully et al. 2019; Williams et al. 2014). The changing structure of iron from crystalline to amorphous and dissolved forms can release phosphate from soils and allow it to wash farther downstream with outgoing tides. The resulting excess of phosphorus in the water can lead to nutrient pollution problems. Therefore, the underlying chemical mechanisms that mediate interactions between saltwater and iron in coastal soils is important to study.
Dani and I spent the summer and into the fall semester simulating SWI on a farm soil in the lab. First, we collected agricultural soil from an actively farmed field on the Eastern Shore of Maryland. Then we weighed it into separate beakers and treated it with eight different combinations of ions found in saltwater. We set the treatments to 15ppt to simulate brackish water intruding onto an agricultural field. We purged half of the microcosms with nitrogen gas and sealed them off to simulate anaerobic conditions as agricultural soils transition to wetlands. The other half were left open to the air. Then, we extracted total dissolved iron from water and total and amorphous iron from soil sampled from the microcosms at 0, 15 and 30 days. We finished the iron extractions last week and have run two of three sets of extractions on the Flame Atomic Absorption Spectroscopy (AAS). One more to go!
Our initial plan was to finish the analysis by the end of September. However, due to some serious technical issues, we could not run our samples until October. Although encountering technical problems was not a pleasant experience, the troubleshooting process improved my problem-solving skills and enhanced my conceptual understanding of the AAS. Furthermore, discussing AAS methods with Dani was rewarding. I not only enhanced my analytical skills but learned the importance of choosing appropriate methods for analyzing samples in order to collect reliable data.
We are currently in the process of analyzing the data, and I am looking forward to our results. I hope that our work will help others understand the effects of SWI on iron transformations in coastal soils. This research could lead to improved environmental policies on agricultural land affected by SWI.
McQueen D.J., Lean D.R.S. & Charlton, M.N. (1986), The effects of hypolimnetic aeration on iron-phosphorus interaction. Water Res. 20:1129-1135.
Tully, K.L., Weissman, D., Wyner, W.J., Miller, J. & Jordan, T. (2019). Soils in transition: saltwater intrusion alters soil chemistry in agricultural fields. Biogeochemistry. 142:339-356, DOI: 10.1007/s10533-019-00538-9
Williams, A.A., Lauer, N.T. & Hackney, C.T. (2014). Soil Phosphorus Dynamics and Saltwater Intrusion in a Florida Estuary. Wetlands, 34:535-544. DOI: 10.1007/s13157-014-0520-7
- By Tia Ouyang