Advantages of Using a Carbon-Rich Substrate in a Constructed Wetland for Agricultural Water Treatment: Carbon Availability and Biota Development

Proper bed substrate selection is essential for the growth of microorganisms and plants, which are key elements in constructed wetlands (CWs) performance. This is even more important in CWs that treat irrigated agricultural drainage water characterised by a high nitrogen (N), but low carbon (C) and phosphorus (P) concentration. This nutrient imbalance compromises biota activity and limits water treatment processes. Three substrates were tested in a CW field-scale pilot plant for 2 years: gravel (100%) and two mixed substrates, gravel + 30% natural wetland soil and gravel + 10% biochar. While gravel is one of the most commonly used substrates in subsurface flow CWs, soil has hardly been employed, and field-scale biochar application studies are scarce. We analysed the effect of adding a C-rich substrate to gravel on both nutrient imbalance correction and the biotic performance of CWs. Adding natural soil or biochar increased P availability in beds. However, the dissolved organic C concentration in interstitial water was only enhanced by soil addition. Unlike gravel, microbial density and activity, and Phragmites australis plant growth, were higher in beds with soil, followed by those with biochar. A similar pattern was observed for plant tissue quality. Although biochar proved positive for biota, it had a short-term effect as a source of C and P. Therefore, employing a suitable gravel-soil mixture provides notable advantages for the irrigated agricultural water treatment challenge. Even after considering some limitations, our results represent an important step to design CWs that treat this wastewater type and for eutrophication control in sustainable agroecosystems.