Long-term combined effects of tillage and rice cultivation with phosphogypsum or farmyard manure on the concentration of salts, minerals, and heavy metals of saline-sodic paddy fields in Northeast China

Soil sodicity is a major ecological problem in the western Songnen Plain of Northeast China and rice cultivation is the main approach used to mitigate saline-sodic soils. However, rice cultivation alone may not be the most effective practice. This study aimed to investigate the combined effects of annual tillage and rice cultivation with either phosphogypsum or farmyard manure on soil salinity, mineral status, and concentration of heavy metals in saline-sodic paddy fields. Treatments were: 1) untreated (no amendments), untilled, and uncultivated (no rice) saline-sodic native grasslands (UG); 2) untreated, tilled, rice-cultivated paddy fields (PFU); 3) tilled, ricecultivated, amended paddy fields with phosphogypsum (PFPG); and 4) tilled, rice-cultivated, amended paddy fields with farmyard manure (PFFM). The effectiveness of these treatments on soil improvement was evaluated after a 10-year field experiment. Compared to the UG control, the 0-20 cm topsoil layer of PFU, PFPG, and PFFM had respective decreases in Na+ concentrations of 42.9%, 61.5%, and 60.9%; in CO32- + HCO3- concentrations of 18.9%, 63.2%, and 57.9%; in Cl- concentration of 64.6%, 75.7%, and 79.9%; in pH units of 0.57, 1.05, and 1.30; in soil electrical conductivity (EC1:5) of 18.3%, 49.1%, and 48.3%; and in exchange sodium percentage (ESP) of 47.2%, 66.9%, and 72.5%. Also, the 0-20 cm topsoil layer of PFPG and PFFM had its concentrations of soil organic matter (SOM), available nitrogen (AN), and available phosphorus (AP) significantly (P < 0.05) increased compared to the UG control. However, the concentrations of five heavy metals (As, Pb, Cd, Cr, and Hg) were kept within a safe range in saline-sodic paddy fields amended with phosphogypsum or farmyard manure and were far below the environmental quality standard held for Chinese soils. Therefore, phosphogypsum and farmyard manure significantly decreased soil salinity and sodicity while increased soil fertility and SOM. Because these amendments are locally available and affordable to farmers, their use is deemed suitable for large-scale soil reclamation and the mitigation of salinity and sodicity in soils destined for rice cultivation in Northeast China.