R_Lord_et_al.mp4

Recording of presentations at the Suricates project closing conference

Title : Soil carbon and nutrient addition from La Rance sediment application to agricultural land – environmental benefits and synergies for dredging and Net Zero.

Authors : Richard LORD*1, Keith TORRANCE1 , Marion CHAPALAIN2 , Auxane HUBERT2 , Valérie FOUSSARD2

1 University of Strathclyde, 16 Richmond Street, Glasgow, G1 1XQ, Scotland (UK) 2 Etablissement Public Territorial de Bassin Rance, Frémur, Baie de Beaussais, Rue Gambetta, 22100 Dinan (France) * Corresponding author e-mail: richard.lord@strath.ac.u

Summary :

Beneficial use of sediment on agricultural land is a common option for uncontaminated, freshwater or desalinated estuarine sediments. The favourable mixture of particle sizes and decaying organic matter content provides the basis of a loamy soil texture and a slow-release source of major nutrients, including nitrogen, phosphorous, and potassium, together with neutralisation potential from biogenic carbonate. What is less clear is the fate of the organic carbon, its residence time and future storage potential as soil carbon. If the whole system carbon footprint of dredging is to be considered under net zero targets, then this must also be compared to the likely counterfactual, including the benefits of avoided greenhouse gas emissions from the sediment while still in situ, offset against any future contribution to blue carbon stores had the sediment been left in place.

In September 2022 as part of the Interreg NWE Suricates Project (Sediment Uses as Resources in Circular And Territorial EconomieS) the receiving soils at two agricultural sediment application sites, at la Croix du Frêne and le Vaugarni (both near Dinan) were sampled, together with the stockpiled dewatered sediment delivered to the two sites from the La Hisse treatment facility ready for spreading. At both sites the soils were resampled in May 2023 following incorporation of the sediment into soil by the respective farmer and continued rotational crop planting.

Our soil organic carbon analyses indicate that the placed sediment, which had been stored at La Hisse since January 2020, has variable organic matter contents (3.1-1.9%), now as low as half that in fresh sediment collected from the Lyvet sediment trap (4.4%), but at similar levels to the receiving soils at la Croix du Frêne and le Vaugarni respectively (2.4-2.2%). Intermediate values were returned after incorporation and overwintering (2.6-2.1%). The placed sediments also contain significant inorganic carbon content (2.4-2.5%), presumably from the marine shells observed while sampling. For the 500m3 .ha-1 wet application rate (assuming 60% dry matter) this corresponds to 21-19t of total biogenic C addition per hectare (or 79-68t CO2 equivalent). If this total amount of C could be considered as stored this would represents a value of over €2k.ha-1 (OECD C price 2018-21).

The placed sediments also contain significant total major nutrients in rank order K > N > P. Measured available nutrients were only <4% for N and P, compared to > 10% for K. The equivalent value of mineral fertilizers (UK prices April 2023) for the total nutrient addition present in the sediment would be €10k-€8k.ha-1 . Further work is required to ascertain the long-term C dynamics and nutrient release of organic matter contained in sediment place on land. However, assuming complete decomposition will occur over the 10-year interval between permitted spreading operations, the loss of temporary soil C storage will be balanced by the, much larger, value of nutrients released as the organic matter decomposes, roughly equal to the actual cost of sediment application (€7k.ha-1 ).

Acknowledgements: The SURICATES project is funded by the European Regional Development Fund (ERDF) as part of the Interreg NWE programme 2014-2020