Rooting and plant density strongly determine greenhouse gas budget of water hyacinth (Eichhornia crassipes) mats

Abstract. Water hyacinth occurs in numerous tropical and subtropical countries, either as a native or as an invasive exotic species, where it can establish large and dense mats. The plant is also frequently used for water purification and bioremediation purposes. Although it is a free-floating species, the plant roots into the sediment of shallow waters, tapping into the sediment nutrient pool. Its long and extensive root system strongly increases nutrient absorption, resulting in high growth rates and concurring high carbon sequestration rates. On the other hand, the plants may also fuel methane (CH₄) production as dense mats may deplete oxygen in the surface water and sediment below, which in combination with the high production of organic matter creates favorable conditions for methanogenesis. We hypothesize that water hyacinth vegetation acts as a strong greenhouse gas (GHG) sink due to its high growth rates, especially when (sediment) nutrient availability is high. Still, this sink may be counterbalanced by CH₄ release, which will be most pronounced when the plants are rooting in the sediment due to potential CH₄ shuttling from the sediment through the roots and leaves into the atmosphere (chimney effect). To mechanistically unravel the influence of water hyacinth on nutrient dynamics and greenhouse gas fluxes, we performed an aquarium experiment in which plant density and root access to the sediment were manipulated. Although plant cover led to lower concentrations of dissolved total phosphorus (DTP) and phosphate, there were no effects of density or rooting. We found no vegetation effect on the ebullition of CH₄, but its diffusion was 4.5 times higher at high plant coverage. Rooting increased CH₄ diffusion by 1.3 (high density) and 4 times (low density), demonstrating the chimney effect that we hypothesized. Independent of rooting, however, water hyacinth at high density sequestrated less carbon compared to low density, possibly due to space limited growth and self-shading. Overall, water hyacinth enhanced CH₄ emissions, especially when rooted. Due to water hyacinth's high CO₂ sequestration rates, the overall GHG budget in terms of CO₂ equivalents still resulted in water hyacinth mats being near-neutral or even a GHG sink, depending on water hyacinth density. Our results show that the effect of water hyacinth mats on GHG fluxes strongly depends on both plant density and contact with the sediment. This indicates that, when making regional GHG balances, not only plant presence but also its density and water depth – regulating sediment-root contact – should be taken into account.