Phosphorus in light fraction organic matter separated from soils receiving long-term applications of superphosphate

The objectives of this study were to: (1) characterize the P composition of light fraction (LF) organic matter in soils with different P inputs, and (2) determine if LF-organic P (Po) is a useful indicator of labile Po. Soils (0–7.5 cm depth) were obtained from a long-term (1952–1999) experiment set up to examine the effects of single superphosphate (0, 188 or 376 kg ha-1 year-1) on productivity of an irrigated pasture grazed by sheep. About half the mass of LF (separated by flotation on sodium iodide solution with specific gravity of 1.7) consisted of fine-grained mineral material. Inorganic P (Pi) associated with this mineral material comprised 25–40% of the total P in LF. The organic matter in LF (average C content of LF was ~25%) appeared to consist of two distinct parts: a mineral-associated component (stable, humic material) and free organic matter comprising recently deposited material. Organic matter adsorbed onto mineral surfaces represented up to 20% of LF-C and 30% of LF-N. The LF was less enriched in Po than C or N (the average Po concentration in LF was only 1.5 times that of whole soil, compared with a factor of 5.6 for C and 3.4 for N). The majority (about 70%) of the Po in LF was allocated to the stable, mineral-associated fraction. The free organic matter in LF was very low in Po and it had C:Po ratios (>500:1) which were much wider than those of whole soil organic matter (<~80:1). Decomposition of this material would be expected to result in net immobilization of P and, thus, it is unlikely to contribute a significant amount of P for plant growth.