The pH dependence of phosphate sorption and desorption in Swedish agricultural soils

Abstract

A number of previous studies have reported the existence of a minimum in phosphate solubility between pH 5.5 and 7 in non-calcareous soils. Different hypotheses have been forwarded to explain this phenomenon. In this study, ten soil samples with varying textures and phosphorus status were subjected to batch experiments in which dissolved phosphate was measured as a function of pH and phosphate load. Soil samples with more than 20 % clay all had a minimum phosphate solubility between pH 6 and 7, whereas for samples with < 10 % clay, no such minimum was observed. Further experiments involving additions of phosphate and arsenate showed an increasing adsorption of these anions with decreasing pH also below pH 6 in clay soils, suggesting that the pH dependence on adsorption and desorption in short-term experiments was not the same. Kinetic experiments showed that the increased phosphate desorption at lower pH values in noncalcareous clay soils was a quick process, which is consistent with adsorption/desorption being the most important mechanism governing the retention and release of inorganic P.Moreover, by comparing extraction results with batch experiment results for samples from a long-term fertility experiment, it was concluded that more than 60 % of the accumulated phosphate was occluded, i.e. not reactive within six days. Additional evidence for an important role of occluded phosphate comes from an analysis of the Freundlich sorption isotherms for the studied soils. It is hypothesized that interlayered hydroxy-Al and hydroxy-Fe polymers in clay minerals may be important for P dynamics in clay soils by trapping some of the P in an occluded form. The results also suggest that improved knowledge on the speciation and dynamics of phosphorus in soils is required for consistent mechanistically based modelling of phosphate sorption/desorption reactions.

Bone char effects on soil: sequential fractionations and XANES spectroscopy

“…In the BC treatment the proportions of AlPO 4 and Ca phytate did not change compared to the control, but the contribution of organic P increased by Mont-Al-IHP formation. The stability of different P fractions can be favored by the pH effect (Gustafsson et al, 2012) and likewise the dependence of BC particle solubility on the soil pH . In agreement with the incubation-leaching results (Table 4), Ca(H 2 PO 4 ) 2 was detected as a result of BC plus amendment even though similar proportions of AlPO 4 and Ca phytate were observed between the control and the BC plus treatment.…”

Spatial and seasonal distributions of soil phosphorus in a short-term flooding wetland of the Yellow River Estuary, China

“…The significant negative correlations between AP, TPS and pH value were observed since pH value was an important factor influencing phosphorus release (Sun et al, 2006). Gustafsson et al (2012) presented that the rise of pH would weaken the P adsorption in soil under alkaline conditions (pH N7). Additionally, higher salinity would exert negative effects on TP levels in wetland soils (Liu et al, 2012).TPS were significantly positively correlated with Al (p b 0.01), because high P retention in soil was mainly attributed to the active Al in organic and mineral fractions (Igwe et al, 2010;Satti et al, 2007).…”

Estimating the leakage contribution of phosphate dosed drinking water to environmental phosphorus pollution at the national-scale

“…Consequently, the pH of mains water is high relative to wastewater (Robertson et al, 1998), with 90% of European tap waters above pH 7 (Banks et al, 2015). Consequently conditions are much less favourable for P adsorption to clays and iron oxy-hydroxides (Gustafsson et al, 2012). While it is acknowledged that P may be retarded in the soil and unsaturated zone, an assessment of the sorption potential of sediments is beyond the scope of this present study.…”