Phosphorus fractionation and sorption in P-enriched soils of Norway

Phosphorus (P) enrichment can lead to imbalance in nutrient availability and pollution of terrestrial and aquatic ecosystems. Hence studies were carried out to investigate fractionation and sorption of P in eleven P-enriched soils collected from different agro-climatic sites in Norway. Different P fractions viz. total, organic, inorganic (easily soluble P, Fe-P, Al-P, Ca-P and occluded P), P(w) (water extractable), and NH4-lactate extractable P (P(AL)) at the beginning and after the completion of the experiments varied widely among the soils studied, indicating a wide variability of P supplying capacity of these soils. Soluble P was positively correlated to Ca-P (r = 0.94; P < 0.001), P(w) (r = 0.87; P < 0.001), pH (r = 0.79; P < 0.01) and P(AL) (r = 0.79; P < 0.01), whereas it was negatively correlated with ammonium oxalate-extractable Al (Al(ox)) (r = -0.68; P < 0.05). Iron-P was only moderately related to Al(ox)(r = 0.64; P < 0.05) and P(ox)(r = 0.70; P < 0.05), whereas it was not related to any of the other parameters tested. The alpha alpha = P(ox)/(Fe(ox) + Al(ox)) was highly correlated with P(AL) (r = 0.93; P < 0.001), pH (r = 0.87; P < 0.001), inorganic P (r = 0.80; P < 0.01) and P(w)(r = 0.77; P < 0.01) but moderately to total P (r = 0.71; P < 0.05). Adsorption data fitted well to the Langmuir equation for most soils. The P affinity constant (k), adsorption maximum (b) and thus maximum buffering capacity (mbc) and adsorption isotherm of P were highest in the sandy clay soil from Osaker, which also contained high amounts of Fe, Al and clay particles and the lowest in sandy soil from Vestralen, which contained very high initial P(AL)and the lowest content of Fe, Al, silt and clay among all the soils studied. The P affinity constant (k) was correlated positively and significantly to clay content (r = 0.66; P < 0.05), whereas mbc was correlated positively and significantly to clay content (r = 0.63; P < 0.05) and ammonium oxalate-extractable Fe (Fe(ox)) (r = 0.63; P < 0.05). Phosphorus desorption of the soils varied widely depending on the initial P status and texture of the soils. Phosphorus desorbed by NH4-lactate was many fold higher as compared to CaCl2 in most soils.