Fate of Cd in Agricultural Soils: A Stable Isotope Approach to Anthropogenic Impact, Soil Formation, and Soil-Plant Cycling

Abstract

The application of mineral phosphate (P) fertilizers leads to an unintended Cd input into agricultural systems, which might affect soil fertility and quality of crops. The Cd fluxes at three arable sites in Switzerland were determined by a detailed analysis of all inputs (atmospheric deposition, mineral P fertilizers, manure, and weathering) and outputs (seepage water, wheat and barley harvest) during one hydrological year. The most important inputs were mineral P fertilizers (0.49 to 0.57 g Cd ha yr) and manure (0.20 to 0.91 g Cd ha yr). Mass balances revealed net Cd losses for cultivation of wheat (-0.01 to -0.49 g Cd ha yr) but net accumulations for that of barley (+0.18 to +0.71 g Cd ha yr). To trace Cd sources and redistribution processes in the soils, we used natural variations in the Cd stable isotope compositions. Cadmium in seepage water (δCd = 0.39 to 0.79‰) and plant harvest (0.27 to 0.94‰) was isotopically heavier than in soil (-0.21 to 0.14‰). Consequently, parent material weathering shifted bulk soil isotope compositions to lighter signals following a Rayleigh fractionation process (ε ≈ 0.16). Furthermore, soil-plant cycling extracted isotopically heavy Cd from the subsoil and moved it to the topsoil. These long-term processes and not anthropogenic inputs determined the Cd distribution in our soils.

Cadmium Isotope Ratios of Standard Solutions and Geological Reference Materials Measured by MC‐ICP‐MS

“…, Imseng et al . ) and recommended value (δ 114/110 Cd NIST SRM 3108 = −1.305‰) by Brand et al . ().…”

“…, Imseng et al . ) and was also used in this study to estimate the accuracy of Cd isotope ratio measurement results. However, SPEX Cd was not reported before, which is not the same as Spex‐1 Cd reported in previous studies (Cloquet et al .…”

High‐Precision Cd Isotope Measurements of Soil and Rock Reference Materials by MC‐ICP‐MS with Double Spike Correction

“…In recent decades, Cd mass fractions in world‐wide soils have dramatically increased due to contamination from mining, smelting, electroplating, wastewater irrigation, and use of chemical fertilisers and pesticides (e.g., Ghosh and Singh , Zhang et al , Wei et al ). Previous studies have found that polluted materials have different Cd isotope compositions and, therefore, Cd isotopes can be used to trace sources of contamination (e.g., Cloquet et al , Gao et al , Pallavicini et al , Wen et al , Imseng et al ).…”

“…With advances in instrumentation, increasingly high precision Cd isotope measurement results have been reported using different chemical purification procedures and different instrumental analysis methods (e.g., Wombacher et al , Cloquet et al , Ripperger and Rehkämper , Gao et al , Schmitt et al , Xue et al , Li et al ). The Cd isotopes of different types of samples with low Cd mass fractions including organic materials, plants, carbonates, silicate rocks, river sediments, seawater and meteorites have been determined precisely and accurately (e.g., Rosman and De Laeter , Cloquet et al , Ripperger and Rehkämper , Gao et al , Wombacher et al , Schmitt et al , Abouchami et al , Pallavicini et al , Georgiev et al , Wei et al , Wiggenhauser et al , Wen et al , Hohl et al , John et al , Imseng et al , Li et al , Viehman et al ). The mass bias produced by the instrument has been corrected by different methods including sample‐calibrator bracketing, and the Ag doping and double spike method (Wombacher et al , Cloquet et al , Ripperger and Rehkämper , Schmitt et al , Li et al ).…”

“…The double spike technique can effectively correct isotope fractionation from chemical separation and mass discrimination of the instrument (e.g., Compston and Oversby , Galer , Ripperger and Rehkämper , Rudge et al , Schmitt et al ). Recent studies have shown that the use of double spike is beneficial for determining the Cd isotope compositions in different types of samples with low Cd mass fractions (e.g., Ripperger and Rehkämper , Schmitt et al , Horner et al , Georgiev et al , Wiggenhauser et al , Hohl et al , John et al , Imseng et al ). Schmitt et al () and Rehkämper et al () have mentioned that when choosing a Cd double spike correction method, two factors should be considered: (a) isobaric and molecular interferences, and (b) limitations imposed by the collector configuration and the maximum mass dispersion that can be accommodated by the collectors.…”