The influence of land-use change on the forms of phosphorus in soil profiles from the Sanjiang Plain of China

Long-term intensive agricultural development since the 1950s has converted approximately 3,800,000ha of natural wetlands (NWL) and 1,200,000ha of natural forestland (NFL) to cultivated land (CL) in the Sanjiang Plain of China. However, it is not clear how the large-scale land-use shifts impact the chemical forms of phosphorus in the soil profile. Eleven soil cores were collected from NWL, NFL, and CL and analyzed for organic matter (OM), Fe, Al, Ca, pH, forms of phosphorus, and total phosphorus (TP). Results showed that Average TP contents were 672 to 2553mgkg ^-1, 355 to 858mgkg ^-1, 575 to 977mgkg ^-1, and 433 to 1017mgkg ^-1 for the soil profiles of the NWL, NFL, CL converted from NWL (CL-W), and CL converted from NFL (CL-F), respectively. Accumulation of OM in the surface layer (ca. 0-20cm depth) and Fe oxides in the deep layer (40 to 100cm depth) from the NWL led to higher TP contents in these layers than in the middle layer (20 to 40cm depth). TP content generally decreased with depth in the NFL and CL soil profiles. The conversion of the NWL to CL, and the ca. 40yr of cultivation thereafter, greatly decreased the TP (from 2065mgkg ^-1 to 866mgkg ^-1) and organic P (Po; from 1714mgkg ^-1 to 326mgkg ^-1) in the soil layer at depths of 0 to 20cm. However, the conversion of the NFL to CL moderately increased the TP (from 770mgkg ^-1 to 1014mgkg ^-1) and inorganic P (Pi; from about 261mgkg ^-1 to 482mgkg ^-1) in the soil layer at depths of 0 to 20cm. The contents of various forms of P were positively correlated with the levels of their corresponding solid phases. The P in the soils was mostly bound to OM or Fe oxide phases, depending on the geochemical composition of the soils. Together, about 77% to 98% of the TP was bound to OM and Fe oxides, thus these two solid phases are critical to the fate of P in the soils and ecosystem of the Sanjiang Plain.