Preferential phosphorus leaching from an irrigated grassland soil

Abstract

Intact lysimeters (50 cm diameter, 70 cm deep) of silt loam soil under permanent grassland were used to investigate preferential transport of phosphorus (P) by leaching immediately after application of dairy effluent. Four treatments that received mineral P fertilizer alone (superphosphate at 45 kg P ha À1 year À1 ) or in combination with effluent (at $ 40-80 kg P ha À1 year À1 ) over 2 years were monitored. Losses of total P from the combined P fertilizer and effluent treatments were 1.6-2.3 kg ha À1 (60% of overall loss) during eight drainage events following effluent application. The rest of the P lost (40% of overall loss) occurred during 43 drainage events following a significant rainfall or irrigation compared with 0.30 kg ha À1 from mineral P fertilizer alone. Reactive forms of P (mainly dissolved reactive P: 38-76%) were the dominant fractions in effluent compared with unreactive P forms (mainly particulate unreactive P: 15-56%). In contrast, in leachate following effluent application, particulate unreactive P was the major fraction (71-79%) compared with dissolved reactive P (1-7%). The results were corroborated by 31 P nuclear magnetic resonance analysis, which showed that inorganic orthophosphate was the predominant P fraction present in the effluent (86%), while orthophosphate monoesters and diesters together comprised up to 88% of P in leachate. This shows that unreactive P forms were selectively transported through soil because of their greater mobility as monoesters (labile monoester P and inositol hexakisphosphate) and diesters. The short-term strategies for reducing loss of P after application of dairy effluent application should involve increasing the residence time of applied effluent in the soil profile. This can be achieved by applying effluent frequently in small amounts.

Impact of farm‐dairy effluent application on the amounts and forms of phosphorus loss by leaching from irrigated grassland

“…Therefore, FDE was preferentially transported through these channels resulting in higher P loss. The relative proportion of C species in leachate (total organic C: 93%, total inorganic C: 7%) was similar to the original FDE (total organic C: 92%, total inorganic C: 8%), which supports the preferential leaching of FDE through macropores (Toor et al 2004b). The corresponding C concentrations were three-to four-fold lower in leachate (total organic C: 191 mg litre ).…”

“…2) (Toor et al 2004b). On the other hand, in leachate, inorganic orthophosphate concentrations were only 12%, while organic P forms comprised up to 88% of P (orthophosphate monoesters: 67.4%, diesters: 20.2%) (Fig.…”

“…Of the P loss from the FDE treatments (T2 and T3) determined over the 2 years (3.3-4.4 kg ha -1 ), 60% of P loss (2.0-2.6 kg ha -1 ) occurred within 24 h after FDE applications (Toor et al 2004b). Using a dye tracer, Sinaj et al (2002) observed that most of the macropores in the Lismore soil were comprised ) of total P in leachate collected from different treatments (modified from Toor et al (2004c)).…”

“…Mean ± SEM (n = 5). Table 4 Concentrations of different P forms in farm-dairy effluent and leachate collected from lysimeters between August 1999 and May 2001 (modified from Toor et al (2004b)). Mean ± SEM (n = 8).…”

Effects of past and current management practices on crop yield and nitrogen leaching—a comparison of organic and conventional cropping systems

“…this limits our ability to extrapolate from the results and draw conclusions for sites with different soil types and environmental conditions, which means it is necessary to validate the conclusions for different environments including new Zealand (Condron et al 2000). Moreover, most lysimeter studies have examined leaching losses from grassland systems (e.g., Fraser et al 1994;di et al 1998;riley et al 2002;toor et al 2005) and only a limited number have studied losses from cropping soils (e.g., Eriksen et al 1999;Bergström & kirchmann 2004).…”