Phosphorus-Based Variable-Rate Pig Slurry Application Reduces Greenhouse Gas Emissions and Improves Phosphorus Plant Availability

Excessive or insufficient application of phosphorus (P) fertilisers can lead to soil P build-up or reduced crop productivity, respectively. Variable-rate P fertilisation offers a sustainable solution to this challenge. This study compared the efficiency of pig slurry (PgS) as a P fertiliser to a mineral P fertiliser (superphosphate, SPP) by evaluating their impacts on soil P availability, greenhouse gas (GHG) emissions, and nutrient leaching in different laboratory experiments. PgS was applied at three increasing rates (R1, R2 and R3) to soils with varying P levels: very low (VL), low (L) or medium (M). A control (CTRL) without P application was included. Results showed PgS was as efficient as, or superior to, SPP in increasing soil extractable P content (1%–104%). Excessive PgS application indicated to soil P build-up. CO₂ emissions were highest with PgS (204.85 mg C kg⁻¹ soil) compared with SPP (171.26 mg C kg⁻¹ soil), though no significant differences in N₂O and CH₄ emissions were observed. GHG emissions were influenced by original soil P values, with the highest emissions in VL soil (1.36 g CO_2-eq kg⁻¹ soil). Optimal fertilisation (R2 for L soils) minimised emissions (647.56 mg CO_2-eq kg⁻¹ soil). Total P (TP) leaching was exacerbated in our selected sandy soils and increased with higher PgS application (11.95 mg TP kg⁻¹ soil in R3) and higher soil P levels (8.18 mg TP kg⁻¹ soil in soil M). Similar trends were observed for N leaching. This study highlighted PgS as a vial alternative to mineral P fertilisers and underscored the importance of site-specific variable-rate P application, to optimise fertiliser efficiency while minimising environmental impacts.