The effect of slurry processing on phosporus bioavailability and mobility

Livestock slurry is a valuable source of phosphorus (P) fertiliser for crops, but can also result in eutrophication following mobilisation and delivery to watercourses. The transfer of slurry-derived P to watercourses is likely to be strongly influenced by its particle size distribution and the chemical form of the P within the slurry. Advanced slurry processing (e.g. acidification, anaerobic digestion) is also likely to alter the forms and distribution of P within slurry particle size fractions. In response to this, this thesis explores the effects of slurry processing on P speciation within a range of slurry particle size fractions and on potential P bioavailability and movement through soil. The P characteristics of cattle slurry, acidified slurry, anaerobically digested slurry, in terms of P speciation of different particle size fractions, was studied in Chapter 3-5. The results showed that in the whole untreated cattle slurry, labile inorganic P (IP-H2O + IP-NaHCO3) and labile organic P (OP-H2O + OP-NaHCO3) dominated the total P (TP) of the whole slurry (45% and 24% of TP respectively). Acidification increased the labile inorganic P proportion to 55% of TP and decreased the stable P (IP-HCl + OP-HCl + TP-Residue) proportion from 21% to 15% of the TP of whole slurry. Anaerobic digestion increased the labile inorganic P and moderately labile P (IP-NaOH + OP-NaOH) proportions of whole slurry to 53% and 12%, but decreased the labile organic P proportion to 12% of TP. After physical separation, the proportion of TP in the liquid fractions ( 2000, >500, >63 μm), while solid fractions tended to show greater proportions of moderately labile and stable P than liquid fractions. After acidification, the P speciation of the solid fractions showed little change. However, the labile inorganic P and the moderately labile P proportions increased in TP in the acidified liquid fractions, while stable P proportion in the liquid fractions decreased. After anaerobic digestion, in the TP of both solid and liquid fractions, the labile inorganic P and moderately labile P proportions increased, while the labile organic P proportion was reduced. The colloidal fraction (0.45-63 μm) accounted for 62% of the TP of whole untreated cattle slurry. Acidification reduced this proportion, while anaerobic digestion did not change it. In contrast, anaerobic digestion increased the labile inorganic P content of the colloids. P availability in soil after slurry application was explored in an incubation experiment monitored by sequential fractionation method (Chapter 7), and in a pot experiment involving P uptake by ryegrass (Chapter 8). The vertical mobility of P in soil was studied in laboratory microcosms (Chapter 6) and a pot experiment (Chapter 8). The results showed that untreated and treated cattle slurry increased the labile P content of soil and the P lability decreased gradually over time. Ryegrass P uptake was also increased by addition of different slurries. However, the differences between untreated and treated slurry amended treatments was not necessarily significant for labile P content in the soil and P uptake by ryegrass. In Chapter 6, the results implied that colloid-associated P in cattle slurry caused greater potential P leaching and that anaerobic digestion even increased this mode of P loss. Although in the pot experiment, slurry amended treatments showed no difference in cumulative P leaching compared to the control, the colloidal fraction in untreated and treated slurries was still a dominant fraction associated with a large portion of P. Further work is necessary to demonstrate the effect of slurry processing on P bioavailability and mobility at the plot- and field-scale, and on recovering colloidal P prior to land spreading.