Impact of Physical Interventions, Phosphorus Fertilization, and the Utilization of Soil Amendments on the Absorption of Cadmium by Lettuce Grown in a Solar-Powered Greenhouse

Overview

Zhang and colleagues measured cadmium uptake in lettuce grown for one cycle in a Cd-enriched solar greenhouse soil in Shaanxi, China. This is primary lettuce Cd evidence, but it is an intervention trial in contaminated greenhouse soil rather than a retail or ordinary-market occurrence survey. The routeable facts are the dry-sample aboveground lettuce Cd concentrations under deep plowing, soil covering, phosphorus fertilizers, biochar, attapulgite clay, and nano-hydroxyapatite treatments.

Key numbers

Experimental soil and materials

The experimental soil was a Lou-type heavy loam with pH 7.83 +/- 0.10, organic matter 36.12 +/- 1.2 g/kg, total nitrogen 1.72 +/- 0.06 g/kg, available phosphorus 302.6 +/- 8.52 mg/kg, available potassium 721.6 +/- 14.39 mg/kg, CEC 24.21 +/- 2 cmol+/kg, and Cd 1.75 +/- 0.41 mg/kg. The soil used for the 15 cm surface-covering treatment came from a nearby greenhouse not contaminated by Cd and had Cd 0.24 +/- 0.15 mg/kg.

The phosphorus fertilizers were diammonium phosphate (DAP), calcium magnesium phosphate (CMP), and calcium superphosphate (SSP), with P2O5 contents of 44%, 12.5%, and 46%, respectively. The paper states that Cd in the phosphorus fertilizers was less than 10 mg/kg. Soil conditioners were corn-straw biochar (pH 8.45), attapulgite clay, and analytical-grade nano-hydroxyapatite.

Physical measures

Figure 1 and the Results text report aboveground lettuce Cd values for the physical-measure experiment. The customary 15 cm plowing depth was the control (CK). Values are mg/kg in dried aboveground lettuce tissue based on the method’s drying and grinding protocol.

The authors describe the 40 cm plowing result as significantly lower than CK but significantly higher than the soil-covering treatment. Soil covering gave the largest Cd reduction, but Table 1 showed physical treatments also reduced lettuce biomass relative to CK.

Phosphorus fertilizer treatments

Figure 2 reports Cd in aboveground lettuce on a dry-weight basis under DAP, CMP, and SSP. All fertilizer treatments increased Cd relative to the no-fertilizer CK in this experiment, though CMP decreased with increasing rate.

The text reports that DAP at 150 and 225 kg/ha P2O5 increased lettuce Cd by 51.5% and 70.3% relative to the 75 kg/ha DAP treatment. For SSP, Cd increased with rate. For CMP, the 225 kg/ha P2O5 treatment was lower than the 75 and 150 kg/ha treatments.

Soil conditioners

The abstract and figures report conditioner-treatment Cd values as mean +/- SD in mg/kg for aboveground lettuce. Biochar increased Cd at low-to-mid rates and reduced it at the highest tested rates.

Attapulgite clay increased lettuce Cd at every tested rate relative to the experiment’s CK, with the highest mean at 120 kg/ha.

Nano-hydroxyapatite Cd values increased through 120 kg/ha, then fell at 160 kg/ha and partially rebounded at 200 kg/ha. The paper states there was no significant difference versus CK among nano-hydroxyapatite treatments.

Methods (brief)

The trial was conducted in a solar greenhouse in Nanying Village, Wuquan Town, Yangling District, Shaanxi Province, China. Lettuce cultivar Lisheng No. 2 was transplanted at the fourth-leaf stage and sampled 60 days after planting. Plots were arranged as randomized blocks with three replicates per treatment; each plot was 2 m by 1.5 m, and 30 lettuce plants were planted per plot. Aboveground lettuce tissue was washed, dried at 105 C for 1 hour and then 75 C to constant weight, ground, digested with HNO3-HClO4 (3:1), and analyzed for Cd by Hitachi Z-2000 atomic absorption spectrometry. Spinach standard material GBW-10015 was used for quality control with reported recovery of 95 +/- 5%.

Implications

Certification: This source should not be pooled as normal-market lettuce occurrence evidence. It is a Cd-contaminated greenhouse intervention trial with dry-tissue results and treatment-specific controls. It can support context on agronomic mitigation and on how alkaline greenhouse soil, phosphate fertilizer, and amendments can shift lettuce Cd uptake.

Courses: Strong example for teaching why soil-remediation interventions are not automatically protective. In this alkaline field, phosphorus fertilizer increased lettuce Cd, and low rates of biochar, attapulgite clay, and nano-hydroxyapatite tended to raise Cd before higher rates reduced it or plateaued.

App: Route as lettuce Cd context and as a soil-to-plant-transfer/mitigation source. Do not present the high dry-weight Cd values as representative of ordinary consumer lettuce without the contaminated-greenhouse caveat.

Wiki pages this source may touch

• Lettuce
• Leafy Vegetables
• Leafy / Green Vegetables, Other
• Cadmium
• Soil-to-plant transfer of heavy metals
• Mitigation

Verification notes

The PDF has author attribution and DOI 10.3390/biology13050332; no DOI conflict was observed. The article contains an internal contradiction for the physical-measure Cd results: the abstract reports 30 cm, 40 cm, 50 cm, and soil-covering values of 1.49 +/- 0.45, 1.26 +/- 0.02, 1.00 +/- 0.21, and 0.24 +/- 0.13 mg/kg, but Figure 1 and the Results text report 2.30, 1.84, 2.08, and 0.98 mg/kg against CK 2.57. This page follows the Results text and Figure 1 for the primary physical-measure values and preserves the contradiction here for audit. Brand/trade names for the cultivar and supplier companies are not used as routing targets under the brand firewall. The paper reports total Cd only; no Pb, As, Hg, Cr, Ni, or speciation data are reported for lettuce.

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