Pig effluent-P application can increase the risk of P transport: two case studies

Abstract

Land application of piggery effluent (containing urine, faeces, water, and wasted feed) is under close scrutiny as a potential source of water resource contamination with phosphorus (P). This paper investigates 2 case studies of the impact of long-term piggery effluent-P application to soil.

A Natrustalf (Sodosol) at P1 has received a net load of 3700 kg effluent P/ha over 19 years. The Haplustalf (Dermosol) selected (P2) has received a net load of 310 000 kg P/ha over 30 years. Total, bicarbonate-extractable, and soluble P forms were determined throughout the soil profiles for paired (irrigated and unirrigated) sites at P1 and P2, as well as P sorption and desorption characteristics.

Surface bicarbonate (P _B , 0–0.05 m depth) and dilute CaCl ₂ -extractable molybdate-reactive P (P _C ) have been significantly elevated by effluent irrigation (P1: P _B unirrigated 231, irrigated 2906; P _C unirrigated 0.030.00, irrigated 23.90.2; P2: P _B unirrigated 7248, irrigated 39501960; P _C unirrigated 0.70.0, irrigated 443287 mg P/kg; means.d.). Phosphorus enrichment to 1.5 m, detected as P _B , was observed at P2. Elevated concentrations of CaCl ₂ -extractable organic P forms (P _OC ; estimated by non-molybdate reactive P in centrifuged supernatants) were observed from the soil surface of P1 to a depth of 0.4 m. Despite the extent of effluent application at both of these sites, only P1 displayed evidence of significant accumulation of P _OC .

The increase in surface soil total P (0–0.05 m depth) due to effluent irrigation was much greater than laboratory P sorption (>25 times for P1; >57 times for P2) for a comparable range of final solution concentrations (desorption extracts range 1–5 mg P/L for P1 and 50–80 mg P/L for P2). Precipitation of sparingly soluble P phases was evidenced in the soils of the P2 effluent application area.