Summary
This 30-year field experiment on two Swedish farms examined the long-term fertility effects of repeated sewage sludge application at rates of 0, 4, and 12 Mg dry matter ha⁻¹ every 4 years. Sewage sludge improved soil structure and increased organic carbon stocks by up to 17 Mg ha⁻¹, with modest yield gains especially for spring barley, but nitrogen and phosphorus use efficiency were low (3–8% and ~2% respectively). Whilst heavy metals accumulated moderately in soil, crop uptake remained negligible, suggesting sewage sludge offers value primarily for soil organic matter and structure improvement rather than nutrient cycling efficiency within the study timeframe.
UK applicability
The findings are directly relevant to UK agricultural contexts where sewage sludge recycling is regulated under the Quality Protocol and Environmental Permitting Regulations. The study's evidence on low nutrient use efficiency but structural benefits may inform UK policy on sludge application rates and expectations, though UK soil types and climate conditions may moderate some outcomes.
Key measures
Soil organic carbon stocks (to 0.40 m depth); soil bulk density; soluble phosphorus concentration; nitrogen use efficiency; copper, zinc, and mercury soil concentrations; crop yield; carbon retention rate (18–20%); phosphorus and nitrogen use efficiency (2% and 3–8% respectively)
Outcomes reported
The study measured soil organic carbon stocks, soil structure (bulk density), soluble phosphorus, nitrogen use efficiency, heavy metal accumulation, and crop yield responses following 30 years of repeated sewage sludge applications at two Swedish field sites.
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