Balancing nutrients and metal contaminants: sustainable land application rates for livestock manures in Ireland

Livestock manures are valuable nutrient sources for crop production, but variable composition and inappropriate application rates can cause nutrient imbalance, phosphorus accumulation and trace-metal loading, increasing the risks to water quality and the wider environment. This study evaluated manure-specific application rates by considering nutrient and trace-metal composition, crop requirements and soil phosphorus status. It was applied to cattle, sheep and chicken manures used for perennial ryegrass ( Lolium perenne L.) and spring wheat ( Triticum aestivum L.) under Irish nutrient-management recommendations. Maximum application rates ranged from 0.96 to 7.62 t dry matter ha −1 yr −1 for perennial ryegrass and from 0.99 to 9.76 t dry matter ha −1 yr −1 for spring wheat across Morgan's soil phosphorus indices 1 to 3, representing increasing soil phosphorus status. No manure application was permitted at index 4. Cattle manure permitted the highest application rates because of lower total phosphorus concentration, whereas sheep manure supported the lowest. The governing nutrient constraint varied with manure composition, crop nutrient requirements and soil nutrient status, with phosphorus generally limiting cattle and sheep manure application, while nitrogen limited chicken manure application under specific conditions in TN-based calculations. Regulated trace-metal loadings remained below the annual permissible limits. Zinc and copper reached approximately 31% and 14% of their respective limits, while cadmium remained below 10% and chromium, nickel and lead below 2%. These findings demonstrate the importance of considering manure nutrient composition, crop requirements, soil nutrient status and trace metal loading limits together when determining manure application rates to protect crop productivity and water quality.