Accumulation rates of soil organic matter in wet dune slacks on the Dutch Wadden Sea islands

Background and aims A long-term monitoring program (ranging from 16 to 77 years) on the Dutch Wadden Sea Islands provided well-documented examples of vegetation
succession in wet dune slacks.We used this opportunity to study soil organic matter (SOM) accumulation in relation to vegetation succession. The aim of this paper is to
identify the factors which regulate accumulation rates of SOM in wet dune slacks.
Methods We used several soil chronosequences using data from the monitoring program together with data from a long-term research activity and more recent measurements. We used several soil chronosequences using data from the
monitoring program together with data from a long-term research (up to 150 years) and more recent measurements.Fieldmeasurements included pH, soil organicmatter, above ground standing crop and water levels.Water level regimes (inundation duration and mean minimum water level), were simulated using a hydrological model. Capable of simulating inundation duration and water-level fluctuations, this
model used field measurements collected over more than 5 years, as well as precipitation and evapotranspiration data collected over a period of 25 years.
Results Sampling two synchronic chronosequences showed that SOM accumulations increased linearly during the first 50-60 years and then levelled off. Sampling various diachronic chronosequences over time showed a wide variation in accumulation rates. Slacks with low productive species, such as Littorella uniflora, showed low accumulation rates (0.02-0.08 kg/m2/year), and persisted even over a period of more than 90 years. In contrast, slacks dominated by high productive species, such as Phragmites australis, showed ten times higher accumulation rates (0.17-0.26 kg/m2/year) over a similar time period and comparable annual inundation periods (176-240 days). A multiple linear regression showed that variation in SOM accumulation rates was best explained by above-ground biomass of the
vegetation. Conclusions We conclude that the rate of SOM accumulation
in wet dune slacks is primarily controlled byplant above-ground biomass. Both above-ground biomass and SOM accumulation can remain very low over a long period of time when dune slacks are flooded during most of the year and plants with adaptive traits are able to maintain vegetation succession at a pioneer stage.