Biochemical characteristics of solid fractions from animal slurry separation and their effects on C and N mineralisation in soil

Solid fractions from separated animal slurry can be used as organic fertilisers on agricultural land. Solid fractions contain variable amounts of inorganic and organic N, so it is important to synchronise their application in the field with crop demand to ensure N availability in the growing season. This study quantified C and N mineralisation for a wide range of solid fractions from slurry separation applied to soil and examined potential correlations between chemical and biochemical characteristics of solid fractions and their C and N turnover. The solid fractions were mixed with soil and incubated at 14°C for 120 days, during which CO₂ evolution and inorganic N content of the solid fractions were determined. A two-parameter exponential function fitted to the individual solid fraction C mineralisation patterns explained 98% of the data variation, while a three-parameter Monod-type equation fitted to the net N mineralisation patterns explained 89% of the variation. Between 5% and 45% of initially added C was mineralised within the incubation period, with the largest proportion tending to be mineralised from simple mechanically separated solid fractions (MEC). Nitrogen was initially immobilised by the majority of solid fractions. Solid fractions from decanter centrifuged, anaerobically digested slurry (DEC) and chemically pre-treated and separated slurry (KEM) began to re-mineralise N after 20?C40 days, whereas N was continuously immobilised from MEC solid fractions. The carbon mineralisation rate constant was correlated with the C content in the neutral detergent soluble (NDS), hemicellulose and cellulose fractions and the N content in NDS. Net nitrogen mineralisation was correlated with the C/Norg ratio of solid fractions, the N content of NDS and the C content of hemicellulose and cellulose.