3.2 Soil phosphorus forms and their proportions
Soil depth and marsh degradation degree also significantly affected the concentrations of other P forms, except for Fe-P (p < 0.05, Table 4). As the soil depth increased, soil organic P, Ca2-P, Ca8-P, Al-P, and occluded P concentrations generally decreased, whereas soil Ca10-P concentration slightly increased. The Fe-P concentration increased in LDM soil but exhibited no apparent trend in other soils (Figure 3a–d).
As illustrated in Figure 3a–d, the organic P concentration generally increased in degraded marshes. Over the entire soil profile, the average organic P concentrations in the MDM, HDM, and SDM were significantly higher (79.9, 78.1, and 82.1%, respectively) than those in the RPM (P <0.05). Along the marsh degradation gradient, the Ca2-P and occluded P concentrations increased, reaching a maximum in the MDM (Ca2-P) and LDM (occluded P) soils and subsequently decreased; Fe-P concentration decreased, reaching a minimum in the MDM soil and subsequently increased; Ca10-P concentration decreased gradually, whereas no apparent changes in Ca8-P and Al-P concentrations were observed. Compared with those in the RPM soil, the Ca2-P and occluded P concentrations from the entire profile of degraded marsh soils increased by 46.7%–137.7% and 31.7%–49.3%, respectively, whereas Fe-P and Ca10-P concentrations decreased by 17.2%–32.2% and 15.3%–27.8%, respectively.
Based on the proportion of each P form (ratio of various P forms to total P), Ca-P (including Ca2-P, Ca8-P, and Ca10-P) was the dominant P form for the whole soil profile. However, marsh degradation altered the proportion of the P form. Compared with those in the RPM soil, Ca2-P, occluded P, and organic P proportions in the 0–30 cm soil layer from degraded marshes increased by 21.9%–70.7%, 8.1%–15.8%, and 35.2%–64.0%, respectively, whereas Al-P, Fe-P, and Ca10-P proportions decreased by 10.7%–43.4%, 19.9%–60.5%, and 32.2%–50.7%, respectively (Figure 4a). The changes in soil Ca2-P, occluded P, organic P, Fe-P, and Ca10-P proportions at 30–100 cm depth were similar to those at 0–30 cm because of marsh degradation, whereas Al-P and Ca8-P proportions at 30–100 cm in degraded marsh soils irregularly changed compared with those in RPM (Figure 4b). Moreover, organic P in soils changed to the first dominant form at 0–30 cm depth, accounting for 41.2%–50.0% (Figure 4a).