Sun et al. 2023 - Biochar and cadmium uptake in rice

Sun and colleagues ran a three-year field trial in Cd-contaminated paddy soil near Shenyang, China, measuring cadmium in brown rice, rice organs, and soil Cd fractions after corn-stalk biochar application. The source is primary evidence for rice Cd mitigation and soil-to-rice transfer under a contaminated-field intervention. It is not a market-basket distribution: the field had total soil Cd of 3.17 mg/kg and was selected as a contaminated site, so the brown-rice values should be routed as agronomic intervention/context evidence rather than as a normal commercial-rice benchmark pool.

Key numbers

Trial and biochar setup

The study ran from 2014 to 2016. Treatments were CK (0 t/ha), Y (3 t/ha/year), C1 (7.5 t/ha once in year 1), C2 (15 t/ha once in year 1), and C3 (30 t/ha once in year 1). Each treatment had three randomized field plots. Each plot was 20 m by 6.5 m, and the total experimental area was about 2090 m2.

The field was described as a Cd-contaminated paddy in suburban Shenyang, Liaoning Province, polluted by sewage irrigation in the early 1990s. Baseline soil total Cd was 3.17 mg/kg, pH was 6.12, and soil organic carbon was 19.8 mg/kg. The biochar was made from corn stalks carbonized at 500 +/- 50 C for 90 min. Table 1 reports biochar Cd as not detectable, pH 8.98 +/- 0.11, CEC 21.59 +/- 0.63 cmol/kg, specific surface area 28.14 +/- 0.46 m2/g, and total carbon 43.58 +/- 1.09%.

Brown-rice Cd outcomes

Figure 2 reports Cd concentrations in roots, stem/sheath, leaf, husk, and brown rice for each treatment-year. The figure is graphical, so exact values are quoted only where the results text gives them directly.

The authors compared these values with Codex and Chinese food limits. In year 1, C1, C2, and C3 met the cited Codex limit of <0.4 mg/kg for Cd in food. In year 2, only C2 and C3 met that Codex comparator. In year 3, no treatment met the cited Codex comparator. None of the biochar-treated brown-rice Cd values met the cited Chinese national food-safety limit of 0.2 mg/kg.

The authors state that C3 significantly reduced Cd in brown rice, stem/sheath, and root in all three years, while C2 significantly reduced brown-rice Cd in the first two years and the yearly Y treatment significantly reduced brown-rice Cd in the third year. Dry matter accumulation was not significantly affected; Table 2 reports brown-rice dry matter of 4.60-6.09 t/ha across treatments and years.

Brown-rice transfer and distribution factors

Table 3 reports transfer factor (TF, percent of root Cd concentration) and distribution factor (DF, percent of aboveground Cd accumulation). Brown-rice rows:

The authors conclude that biochar reduced the concentration factor from soil to rice, but had little effect on within-plant transfer and distribution. Across rice organs, they report TF ranking as stem/sheath > leaf > husk > brown rice and CF ranking as brown rice > husk > leaf > stem/sheath > root.

Soil Cd fractions

Table 4 reports Cd fractions in soil in mg/kg, mean +/- SD, n = 3. F1 is exchangeable, F2 carbonate-bound, F3 Fe-Mn oxide-bound, F4 organic-bound, and F5 residual.

Biochar reduced exchangeable Cd and increased Fe-Mn oxide-bound and organic-bound Cd. The authors report no significant effect on F2. Their risk assessment code (RAC) was high risk for CK at 38-43%, high risk for Y at 31-38%, high risk for C1 at 33-38%, medium risk for C2 in years 2-3 at 28% and 30%, and medium risk for C3 across all three years at 24-30%.

Immobilization and RDA

Cd immobilization efficiency was above 10% for biochar treatments. C3 reached the highest reported immobilization efficiency, 39.05 +/- 5.49%, in the second year. C1 was reported as basically stable at 11.41-3.13% over the three years, while the yearly-addition Y treatment increased steadily and C2/C3 increased in year 2 before decreasing in year 3.

Redundancy analysis found that the first two axes explained 81.51% and 7.1% of the variance between rice Cd contents and soil environmental factors. Selected soil variables accounted for 93.5% of rice Cd variation. Soil pH and F3 Cd were the most important contributors: pH (F = 38.9, p = 0.002) accounted for 80.2% of variation, and F3 Cd (F = 4.6, p = 0.008) accounted for 7.4%.

Methods (brief)

The trial used japonica rice variety Yugeng65 under local production practices with alternate drying and wetting irrigation. Chemical fertilizer was applied annually at N 115.38 kg/ha, P2O5 61.54 kg/ha, and K2O 76.92 kg/ha. At physiological maturity each October, three representative plants were sampled from the middle of each plot, washed, oven dried, separated into root, stem/sheath, leaf, husk, and brown rice, and digested with HNO3/HCl/H2O2. Cd was measured by graphite furnace atomic absorption spectrometry using GB 5009.15-2014 quality-control framing.

Soils were sampled from the 0-20 cm arable layer at harvest each year using 10 points per plot. Soil Cd fractions were measured by modified Tessier sequential extraction, with Cd in extracts measured by graphite furnace atomic absorption spectrometry. Total soil Cd was measured by microwave digestion and graphite furnace atomic absorption spectrophotometry.

Implications

Certification: This source should not be pooled as a representative China-market or US-market rice occurrence distribution. It is a contaminated-field intervention study with dry, oven-prepared plant tissue. It is useful for rice Cd mitigation evidence: biochar reduced brown-rice Cd but did not bring any treatment below China’s 0.2 mg/kg food-safety comparator, and the Codex-comparator benefit declined over time.

Courses: Strong case study for separating agronomic mitigation from benchmark pooling. A source can be highly relevant to product risk management while still being unsuitable as a market-baseline distribution.

App: Route as rice and soil-to-rice transfer context for cadmium. Flag contaminated-site basis, China jurisdiction, brown-rice matrix, and biochar intervention before making any product-risk inference.

Wiki pages this source may touch

• rice
• rice-bulk-grain
• cadmium
• soil-to-plant-transfer
• agronomic

Verification notes

The PDF has author attribution and DOI 10.3390/agronomy13051335; no DOI conflict was observed. The paper reports total cadmium only, so no species substitution issue arises. The brown-rice values are from a field experiment in Cd-contaminated paddy soil, not from retail rice sampling. Values are reported on the study’s oven-dried plant-tissue basis. The biochar supplier name is omitted under the brand firewall. Exact brown-rice Cd concentrations are included only where the text reports numeric values; Figure 2 is graphical and is used for trend/significance statements rather than hand-read bar values.

Page history

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