Soil phosphorus forms and their variations in depressional and riparian freshwater wetlands (Sanjiang Plain, Northeast China)

Little information exists on phosphorus (P) forms in Histosols or Spodosols, particularly wetlands situations. The objectives of this study were to: investigate concentrations and proportions of P forms in depressional (with Histosols) and riparian (with Spodosols) wetlands, correlate the content of P forms with basic soil chemical properties, and establish general differences in P forms influenced by land-use changes. Soil samples from selected five depressional and riparian wetlands (Sanjiang Plain, Northeast China) were obtained in winter, and soil P forms were determined by a modified Hedley fraction method. Results show that the total P (Pt) concentrations ranged from 240 to 1953 mg/kg and were enriched in the topsoil (A horizon) in most of the profiles. The depressional wetland topsoil contained more Pt (968–1953 mg/kg) than riparian wetland (482–755 mg/kg). Total organic P (Po) and Residual-P were the predominant forms and account for 61–84% of Pt in the depressional wetlands. Moreover, mean proportion of total Po ranged from 47% to 58% for depressional wetlands, and 15% to 45% for riparian wetlands. NaOH extractable Po was the predominant form of total extracted Po, which suggests that a relatively high proportion of Po (mean 38–73%) is in stable forms. The Pearson correlation matrix indicated that Resin-P did not show any significant correlations with soil geochemical properties; the majority of inorganic P (NaHCO3–Pi, NaOH–Pi, and Conc.HCl–Pi) were significantly positive correlated with Al and Fe; all forms of organic P (NaHCO3–Po + NaOH–Po + Conc.HCl–Po) and Residual-P were significantly positive correlated with the content of organic matter (Corg, Nt). The ratio of Corg to Po (C / Po) was used to estimate the mineralization potential of Po in the soils under investigation. The C / Po ratios of those degraded wetland soils ranged from 46 to 137, which means mineralization of Po could readily occur. The Pi in topsoil of cultivated fields (non-wetland: 678 mg/kg) was higher than in depressional (373 mg/kg) and riparian wetlands (284 mg/kg), but the Residual-P, Po, and Pt were lower. The reasons for these indicated that cultivated activities have decreased Po and Pt content in the topsoil.