Assessment of nutrient storage and translocation in winter harvested Typha latifolia from free-water surface treatment wetland mitigating diffuse agricultural pollution

Wetland macrophytes play a critical role in the performance of treatment wetlands (TWs), primarily through nutrient uptake. However, this retention is temporary, as nutrients are released back into the water upon the decomposition of plant litter. The removal of stored nutrients from TWs can be efficiently achieved by harvesting plants during the peak of the growing season, albeit with significant ecological disturbance. Therefore, winter harvesting is recommended, although the specific amounts of nitrogen (N) and phosphorus (P) removed during this period remain uncertain. This study aimed to evaluate the effectiveness of winter harvesting in removing substantial nutrient amounts compared to belowground storage. Experimental harvesting was conducted over five winters (2018, 2019, 2021, 2022, and 2023) at the Vända free-water surface TW system in Estonia, focusing on above-ice biomass (stems, leaves, flowers) and below-ice biomass (roots and rhizomes). The dry weight and nutrient content of these biomasses were analysed. Findings indicated a gradual increase in nutrient pools within Typha latifolia plants, without significant differences between the two subsequent wetlands or a clear correlation with vegetation cover. Winter harvesting of above-ice biomass removed approximately 50 % of plant biomass, and about 30 % of N and P accumulated in the macrophytes, as most nutrients were already stored in the rhizomes by the end of the growing season.