Direct chemical speciation of soil phosphorus in a Saskatchewan Chernozem after long and short-term manure amendments

Abstract

Chemical form of phosphorus (P) in soils influences both plant accessibility and solute transport of P. Soil P speciation receiving inorganic and organic fertilizers is extensively studied to address a range of agronomic and environmental concerns. It is known inorganic P sources react over long time scales in soils, but relatively few studies have focused upon long-term P speciation changes in soils receiving organic amendments. This study was conducted to address this gap by providing detailed information on the speciation of P in agricultural soils from short- (2 yr) and long-term (11 yr) applications of liquid hog manure (LHM) and solid cattle manure (SCM) made at a field research site. Chemical speciation was performed with a combination of laboratory-based soil test P extraction methods and solid-state synchrotron-based techniques. There was clear evidence that initial phosphate minerals in the manures rapidly transform into new phases after short-term soil application. The X-ray absorption near edge spectroscopy results for SCM soil samples were consistent with high solubility (likely Mg-substituted) dicalcium phosphate minerals and phosphate (PO4) adsorption species present after short-term period, whereas long-term SCM application resulted in transformation of soil phosphate into more crystalline calcium phosphate minerals. In contrast, phosphate speciation after short term of LHM application revealed predominantly magnesium phosphate and adsorbed P compounds. Soils under long-term LHM amendment still had phosphate speciation dominated by poorly crystalline dicalcium phosphate minerals. This suggests that the manure form plays a strong role not only in short-term plant availability but also in long-term P speciation.