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 3,800,000 ha of natural wetlands (NWL) and 1,200,000 ha 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 2553 mg kg− 1, 355 to 858 mg kg− 1, 575 to 977 mg kg− 1, and 433 to 1017 mg kg− 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. 0–20 cm depth) and Fe oxides in the deep layer (40 to 100 cm depth) from the NWL led to higher TP contents in these layers than in the middle layer (20 to 40 cm depth). TP content generally decreased with depth in the NFL and CL soil profiles. The conversion of the NWL to CL, and the ca. 40 yr of cultivation thereafter, greatly decreased the TP (from 2065 mg kg− 1 to 866 mg kg− 1) and organic P (Po; from 1714 mg kg− 1 to 326 mg kg− 1) in the soil layer at depths of 0 to 20 cm. However, the conversion of the NFL to CL moderately increased the TP (from 770 mg kg− 1 to 1014 mg kg− 1) and inorganic P (Pi; from about 261 mg kg− 1 to 482 mg kg− 1) in the soil layer at depths of 0 to 20 cm. 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.

► Impact of land-use shifts from natural to cultivated land on soil P was studied.
► Change from wetland to cultivated land decreased TP and organic P in topsoils.
► Change from forest to cultivated land increased TP and inorganic P in topsoils.
► P in the soil was mostly bound to OM or Fe oxide phases (about 77% to 98% of TP).