Kan utvinning av maerl försvaras ur ett hållbarhetsperspektiv? En jämförande litteraturstudie av maerl och bergkalk som jordförbättringsmedel

Sammanfattning

Liming is an important agricultural practice for counteracting soil acidification and improving crop production in acidic soils. Conventional agricultural lime is usually derived from geological limestone, but maerl, loose-lying calcified red algae, has been used both historically and in modern times as a raw material in calcified seaweed-based soil and horticultural products. This creates a sustainability issue because maerl is not only a carbonate material but also forms slow-growing marine habitats with high ecological importance. The aim of this literature study was to assess whether maerl extraction can be justified from a sustainability perspective by comparing maerl and limestone-based agricultural lime and by evaluating possible alternatives. The study is based on a qualitative review of literature concerning maerl and rhodolith beds, maerl extraction, agricultural liming, limestone quarrying, lime production and alternative carbonate materials. The results show that maerl can increase soil pH and supply calcium, magnesium and carbonate. However, its agronomic function largely overlaps with other liming materials. The potential benefits of maerl appear context-dependent and are not clearly unique enough to justify extraction from marine habitats. The ecological literature shows that maerl beds are sensitive to physical disturbance. Dredging and bottom-towed activities can remove or bury living maerl, alter sediment structure, affect associated species and disturb marine carbonate and carbon flows. Recovery is often slow because the habitat develops over long time scales. Limestone-based lime also has environmental impacts through quarrying, transport, energy use and process emissions, especially when limestone is calcined. Nevertheless, limestone-based agricultural lime is more established and scalable than maerl. Shell-based waste materials from aquaculture and the food industry may provide regional carbonate sources, but they are unlikely to replace conventional liming materials. Overall, maerl extraction appears difficult to justify as a liming strategy when other carbonate materials already provide the same basic soil function with less direct impact on long-lived marine habitats.