Multi-element fingerprinting of soils can reveal conversion of wetlands to croplands

Little is known about the influence of conversion of wetlands to farmlands on concentrations and distribution of elements other than those most commonly studied, partly because of the lack of stratification in wetland soils. In this study, in the Sanjiang Plain in northeastern China, we determined the concentrations of 63 elements along soil profiles at three depths: 0-20 cm, 20-40 cm, and below the depth to which farmers would plow, at 40-60 cm, under four land uses: natural wetland, drained wetland, wetland converted to soybean field and subsequently to rice paddy field. Based on our previous work, we expected that changes in organic matter content would be an important factor affecting element concentrations, but that changes in land uses also led to decoupling of the influence of organic matter on elements. This would lead to other factors, such as changes in redox conditions, changes in hydrology and mixing of soils due to plowing, becoming more important factors affecting element distributions. Our study confirmed these expectations. Changes in organic matter content directly or indirectly affected many elements, explaining 67% of variation. Arsenic, cobalt, iron and nickel concentrations were dramatically higher and sulfur concentrations lower when wetland was converted to paddy field. Co and Ni were identified as potential chemical indicators of wetland conversion. Our research is the first to use multi-element fingerprinting to study effects of conversion of wetlands to croplands in China and showed that this relatively simple approach highlights the complexity of the many interacting factors in reclamation of wetland soils for agricultural uses.