The interaction between decomposition, net N and P mineralization and their mobilization to the surface water in fens

Worldwide, fens and peat lakes that used to be peat-forming systems have become a significant source of C, N and P due to increased peat decomposition. To test the hypothesis that net nutrient mineralization rates may be uncoupled from decomposition rates, we investigated decomposition and net mineralization rates of nutrients in relation to sediment and pore water characteristics. We incubated 28 non-calcareous peat sediments and floating fen soils under aerobic and anaerobic conditions. We also tried to find a simple indicator to estimate the potential nutrient mobilization rates from peat sediments to the water layer by studying their relation with sediment and pore water characteristics in 44 Dutch non-calcareous peat lakes and ditches.Decomposition rates were primarily determined by the organic matter content, and were higher under aerobic conditions. However, highly decomposed peat sediments with low C:P and C:N ratios still showed high net nutrient mineralization rates. At Fe:PO4 ratios below 1 mol mol−1, PO4 mobilization from the sediment to the water layer was considerable and linearly related to the pore water PO4 concentration. At higher ratios, there was a strong linear correlation between the Fe:PO4 ratio and PO4 mobilization. Hence, measuring Fe and PO4 in anaerobic sediment pore water provides a powerful tool for a quick assessment of internal PO4 fluxes. Mobilization of mineral N was largely determined by diffusion. Total sediment Fe:S ratios gave an important indication of the amount of Fe that is available to immobilize PO4. Pore water Fe concentrations decreased at ratios <1 mol mol−1, whereas pore water PO4 concentrations and PO4 mobilization to the water layer increased.As PO4 mobilization rates from the sediment to the water layer contribute to almost half of the total P load in Dutch peat lakes and fens, it is of pivotal importance to examine the magnitude of internal fluxes. Dredging of the nutrient-rich upper sediment layer will only be a useful restoration measure if both the influx of P-rich water and its internal mobilization from the newly exposed, potentially more reactive peat layer are sufficiently low.