Livestock manure fertilization as a 'source' or a 'sink' of soluble heavy metals in soil: Insights from geochemical modeling

Livestock manure application is viewed as a cost-effective, sustainable waste-recycling strategy. However, its function as a source or sink for soluble heavy metals across different soil types and the governing mechanisms remain unclear. This study addressed these issues via a 180-day incubation experiment of pig manure amendment in a fluvo-aquic soil (pH 7.66) and a paddy soil (pH 4.15). Pig manure acted as a source of soluble Cd in the fluvo-aquic soil (3-4.5-fold increase) but as a sink in the paddy soil (2.6-4.5-fold reduction), while serving as a source of soluble Cu in both soils (2.0-17.4-fold increase). The multi-surface speciation model (MSM) simulations indicated that manure-derived organic matter (MOM) exhibited a lower adsorption affinity for Cd and Cu than soil organic matter (SOM). Thus, Cd and Cu mainly bound to SOM in both soils, with 0.8-58.3 % of these metals shifted to MOM-bound forms upon manure applicated. Notably, the proportion of MOM-bound Cd and Cu was higher in fluvo-aquic soil (up to 58.3 %) than in paddy soil (up to 18.0 %). Following pig manure application, the decrease of DOM-bound Cd/Cu over time led to a decrease of the soluble Cd in the fluvo-aquic soil and soluble Cu in both soils. However, in the paddy soil, this decrease was counteracted by the increase in inorganic soluble Cd. The MSM predicted that manure's contrasting Cd source/sink roles were controlled by soil-specific mechanisms: elevated dissolved organic matter (DOM) in the fluvo-aquic soil increased soluble Cd (acting as a "source"), whereas increased pH in the paddy soil decreased soluble Cd (acting as a "sink"). Conversely, manure application elevated soluble Cu concentrations in both soils ("source" role) due to increased levels of soil total reactive Cu and DOM. This study provides crucial insights for developing optimized pig manure application strategies in agricultural soils.