Framework for estimating toxic releases from the application of manure on agricultural soil: National release inventories for heavy metals in 2000-2014.

Livestock manure is commonly applied on agricultural land for its fertilising properties. However, the presence of toxic substances in animal manure such as pathogens, antibiotics and heavy metals, can result in damages to ecosystems and human health. To date, although relevant for policy-making, e.g. regulation framing, their releases to agricultural land have been incompletely and inconsistently quantified at global and national scales. Here, we thus developed a generic framework for estimating such releases based on the quantities of manure applied and concentrations of toxic substances. Applying this framework, we built a global release inventory for arsenic, cadmium, chromium, copper, mercury, nickel, lead and zinc differentiated into 215 countries and 15years (period 2000-2014). Comparisons with more narrowly-focused inventories showed overall consistency in our inventory results, although a number of uncertainties and limitations were identified. In particular, the need for harmonising sampling and analytical methods for estimating heavy metal contents in manure and generating more country-differentiated data, especially for developing countries, should be prioritised by future research studies. Using life cycle impact assessment methods, it was additionally found that mercury, zinc and copper are the substances contributing the most to the toxic impacts on human health and freshwater ecosystems resulting from manure application to land. While countries such as China, India, Russia, Brazil and the United States of America contributed to half the heavy metal releases from manure application worldwide, the impact intensity per area of agricultural land was observed to be highest for island countries, the European Union and South-East Asia because of higher per-area applications of manure. These findings demonstrate the need to perform country-specific impact assessment to support policy-making regulating the concentrations of toxic substances such as heavy metals in utilised manure.