How can we use circular fertilizers in grain production without increasing heavy metal risks?

The project Boost Nordic Biogas https://www.slu.se/institutioner/vaxtproduktionsekologi/research/vaxtodling-inriktning-grovfoder/forskningsprojekt-grovfoder/boost-nordic-biogas/ will conduct two fertilisation experiments 2023-2025 with different digestate products. Some of these have heavy metal contents that limit the annual amount of fertiliser allowed for spreading. The concentrations and amounts of six metals (Pb, Cr, Cu, Cd, Ni och Zn) are limited by Swedish law to avoid accumulation in agricultural soils. Plant uptake , however also is affected by for example pH and interactions with soil organic matter. The heavy metals in different fertilisers also have different solubility. Sewage sludge biochar, for example can bind heavy metals very hard. The project encompass both literature studies, data analysis and some lab work. During 2023-2025, two field experiments with grain crops will be conducted at Röbäcksdalen in Umeå. The masters student will analyse data from harvested grain crops and investigate soil properties. The grain crops are fertilized with liquid or pelletized digestate or sludge biochar or dewatered digestate combined with recycled ammonium sulphate or mineral fertilizers. The first task will be to formulate a hypothesis and to select treatments for sampling. An external lab will make the metal analyses. The main part of the work can be done at a distance during autumn and winter 2024- 2025, but the experiment will be harvested in September 2024, and soil samples for lab studies need to be taken either before November 2024 or in May 2025.