Changes in nutrient concentration and carbon accumulation in a mediterranean restored marsh (Ebro Delta, Spain)

Eutrophication is now a serious environmental problem worldwide because it disrupts the metabolism of aquatic ecosystems. In the Ebro Delta, intensive rice farming during the 20th century has increased coastal eutrophication and caused ecological and economic impacts. Marsh restoration is as an effective economic and ecological tool to remove nutrients from agricultural runoff, thereby limiting coastal eutrophication impacts and also providing other ecosystem services. The objective of this experimental study was to assess overall N and P concentration reduction, C accumulation and Si buffering in an oligohaline restored marsh receiving nutrient and sediment inputs from river irrigation and rice field drainage waters under different water levels. We established the experimental restored marsh in abandoned deltaic rice fields from August 2009 to June 2012. The study of changes in nutrient concentration was performed in 2010 from June to November. The study of nutrient and carbon accumulation was performed from August 2009 to May 2011. We used two freshwater input type treatments (riverine irrigation and rice field drainage water) and three water level treatments (10, 20 and 30 cm). Our results showed that higher water nutrient concentrations from rice fields caused significantly higher N- NH4+ and P- PO43− concentration reduction (80.76 ± 1.8% and 17.99 ± 3.92% respectively). There was also an export in TP and P- PO43− (−45.08 ± 13.12 and −23.85 ± 8.15%, respectively) in experimental marsh units receiving river irrigation waters. Significantly lower soil redox conditions and higher total maximum aboveground biomass in the IW treatment were associated with lower N- NO3− concentration reduction and higher Si-SiO2 concentration reduction (94.14 ± 0.72% and 58.54 ± 1.08% respectively) than the DW treatment. Higher sediment concentrations from rice fields were associated with higher C accumulation rates (126.10 ± 6.25 g m−2 y−1) compared with experimental marsh units receiving river irrigation waters (99.44 ± 8.23 g m−2 y−1). Higher water levels also increased significantly P- PO43− and Si-SiO2 concentration reduction and C accumulation rates within both water type treatments. Our experimental study showed how multiple mechanisms control N and P concentration reduction, Si buffering and C accumulation. Plant growth may decrease the ability to reduce the input concentration of N- NO3− possibly due to denitrification inhibition via plant oxygenation of marsh soils. Plant uptake may favor Si buffering in the restored marsh, although high water levels may also control Si buffering through higher residence time for diatom uptake. This study indicates that Mediterranean oligohaline restored marshes removed N and P using both river irrigation and rice field drainage waters and also provide C accumulation and Si buffering services. The use of agriculture runoff as a primary source of nutrient and sediment is beneficial for marsh restoration projects focused on C accumulation. In general, higher water levels (20–30 cm) were better for nutrient concentration reduction and C accumulation, but higher water levels were also associated with lower plant biomass.