Impact of aquatic macrophyte decomposition on sedimentary nutrient and metal mobilization in the initial stages of ecosystem development

The shifts and extent of aquatic macrophyte-mediated P and N regeneration, crucial in the initial stages of ecosystem development, were investigated using immature sediments which were poor in organic matter, P and N, but rich in Fe and Mn, obtained from a five-year-old artificial pond ‘Chicken Creek’, in east Germany. During the decomposition of indigenous plant materials (Phragmites australis (Cav.) Trin. ex Steudel, Potamogeton natans L., Potamogeton pectinatus L.) in a laboratory experiment, the concentrations of P, ammonium (NH4+), Fe and Mn in the sedimentary pore water increased with increasing fresh plant biomass amendments, and over time. Based on weighted average of the biomass added, there were no differences between species in the enrichment of pore water P (51- to 60-fold) and Mn (19- to 26-fold), compared to pure sediments. There was, however, a clear sequence in the NH4+ and Fe enrichment of pore water between control sediment and plant additions with P. pectinatus < P. natans < P. australis, increasing from 25- to 86-fold and 85- to 239-fold. In all cases, the extent of Fe mobilization exceeded that of P, as indicated by an increasing Fe:P ratio. Independent of the origin of solid Fe, this suggests a tendency towards the regeneration of mobile redox-sensitive Fe. Macrophytes can thus accelerate the transition from rather geochemically inert to more biogeochemically reactive sediments (P priming effect) by enhancing the mobilization and release of P to other biota. At a current relative Fe surplus, the equilibrium between the release and binding of P, after plant decomposition, shifts towards redox-sensitive P-binding, indicating a progressive P mobilization and availability.

► The role of three macrophyte species in initial ecosystem development was studied.
► I examined plant decomposition influencing sediment nutrient and metal mobilization.
► Beside N and P, P binding partners Fe and Mn were also significantly regenerated.
► Plant decomposition favoured solid Fe mobilization towards redox-sensitive Fe.
► Higher proportion of redox-sensitive Fe will increase P mobility and availability.