Yuwei Qin

In Ireland, natural geochemical enrichment in limestone-derived soils has produced some of the highest topsoil cadmium (Cd) concentrations in Europe. Consequently, Cd levels in Irish potato tubers from certain key horticultural regions have exceeded the European Commission’s maximum allowable concentration of 0.1 mg kg-1 (fresh weight). Cadmium is a toxic trace metal with no known biological function. It is classified as a Group 1 carcinogen because of its links to kidney damage, bone demineralization, and increased cancer risk. The application of soil amendments has been identified as a key strategy for mitigating Cd accumulation in Irish potatoes. Soil amendments such as lime, zinc (Zn), and organic composts have been studied in Ireland and worldwide for their potential to reduce plant Cd uptake. Although these amendments show considerable potential, their efficacy has often been inconsistent across soils and crop species, owing to the complex soil geochemical and plant physiological processes governing Cd transfer from soil to potato tubers.

The overall aim of this thesis was to investigate the key factors affecting Cd uptake by potato roots and to elucidate the mechanisms and effectiveness of soil amendments in reducing Cd uptake and accumulation in tubers. This was achieved through a combination of laboratory and greenhouse experiments, supported by a geochemical multi-surface modeling (MSM) approach. The specific objectives were:
o Objective 1: To validate and optimize an MSM for predicting the dissolved Cd concentration and speciation in amended and unamended soils
o Objective 2: To assess changes in soil chemical properties and subsequently in Cd availability over time in the presence of soil amendments
o Objective 3: To investigate factors influencing Cd uptake by potato roots from the soil solution
o Objective 4: To unravel the confounding mechanisms behind the effectiveness of soil amendments to reduce tuber Cd
o Objective 5: To identify soil chemical parameters that best predict Cd accumulation in potato tubers

Chapter 2 explored factors affecting Cd uptake by potato roots in a series of one-hour hydroponic experiments, thereby enabling independent investigation of root uptake processes relative to soil solid-phase interactions. The factors investigated were solution pH, competing cations (Ca2+ and Zn2+), natural dissolved organic matter (DOM; dissolved fulvic acid, DFA), initial Cd concentrations (10-9–10-3 M), and potato cultivar (Cara and Lady Rosetta). The results demonstrated that Cd content in root digests increased nearly log-linearly with initial Cd concentration. Cultivar differences in tuber Cd accumulation do not arise from short-term differences in root Cd uptake between the two Irish potato cultivars studied. Evidence of FA-enhanced uptake at lower free Cd2+ activities, together with proton and cation competition at higher Cd2+ activities, indicated that a transition from diffusion-limited to internalization-limited uptake occurred at Cd2+ activities of approximately 10-5.7 to 10-6.5 M. Diffusion-limited uptake implies that Cd acquisition by potato roots is governed primarily by soil geochemical processes rather than by root physiological mechanisms. Accordingly, variations in soil chemical properties (e.g., pH and ionic composition) affect Cd uptake mainly by regulating Cd equilibrium concentrations and replenishment in the soil solution. This has important implications for management practices: for example, Zn application is not expected to reduce Cd uptake through direct competition for root internalization in natural soils. Similarly, Ca addition via calcitic lime is unlikely to affect Cd uptake via direct competition for uptake sites during root internalization.

Chapters 3, 4, and 5 were based on a pot experiment in which three amendments, lime, Zn, and spent mushroom compost (SMC), were evaluated for their effects on reducing tuber Cd concentrations. Each amendment is characterized by distinct soil- and/or plant-driven mechanisms for reducing tuber Cd concentrations in potato plants grown in two Irish tillage soils previously identified as inducing high and low tuber Cd contents. Soil properties, including pH; concentrations and composition of soil organic matter (SOM) and DOM; clay and metal oxide contents; and reactive and dissolved Cd pools, were monitored at multiple time points throughout the growing season. Dissolved Cd concentrations were measured in three solution media: soil pore water and two chemical extractants (1 mM Ca(NO3)2 and 0.1 M CaCl2), and their relationships with soil Cd availability were examined. Comparisons between paired planted and unplanted pots enabled the assessment of soil-plant interactions and their effects on soil chemistry.

Chapter 3 validated the MSM-predicted dissolved Cd concentrations against measured values in soil pore water, 1 mM Ca(NO3)2, and 0.1 M CaCl2 extracts from two Irish tillage soils. Reactive SOM, i.e., humic acids (HA) and fulvic acids (FA), accounted for over 90% of Cd binding in pore water and Ca(NO3)2 extracts, and approximately 65% in CaCl2 extracts. Improving estimates of reactive SOM by repeating NaOH extractions three times instead of once substantially reduced model error in predicting dissolved Cd concentrations. Additionally, using isotopically determined reactive Cd, rather than conventional 0.43 M HNO3 extraction, improved predictions in Mn oxide-rich soils, indicating that HNO3 extraction may overestimate reactive Cd by dissolving slowly desorbing oxide-bound fractions. Model residuals increased at higher pH and lower Cd concentrations, likely reflecting uncertainties in high-affinity binding parameters or kinetic limitations. Overall, the MSM was shown to be an effective tool for predicting changes in dissolved Cd concentrations in amended soils, with improved performance at higher Cd concentrations and in solution media with elevated Ca and/or Cl.

Chapter 4 evaluated treatment-, plant-, and time-dependent changes in soil chemistry and resulting Cd availability during the potato growing season. Amendment application altered soil pH, concentrations and composition of SOM and DOM, and dissolved Ca concentrations, each exerting either mobilizing or immobilizing effects on Cd. Liming reduced soil Cd availability across all solution media despite simultaneous increases in dissolved Ca and DOM. Zn application mobilized Cd due to competition for soil sorption sites. SMC effects varied, increasing Cd availability in pore water due to elevated dissolved Ca and DOM, but decreasing Cd availability in CaCl2 extracts due to increases in pH and SOM. Plant nitrate uptake induced root-mediated cation imbalances, resulting in increased soil pH and reduced Cd availability over time. Overall, soil pH emerged as the dominant factor controlling Cd availability across all solution media, while DOM became more influential for pore water Cd at higher pH. Contrasting SMC effects between pore water and CaCl2 extracts were primarily attributed to differences in dissolved Ca and DOM effects under varying solid-to-solution ratios.

Chapter 5 examined the effects of amendments on tuber yield and tuber Cd and Zn concentrations and investigated the mechanisms underlying their effectiveness. Lime application increased soil pH and reduced measured soil Cd availability but unexpectedly increased tuber Cd concentrations, which were hypothesized to result from plant physiological responses to alkaline stress, such as upregulated Cd/Zn transporter activity. Zn treatment had minimal impact on Cd competition at the root level (as shown in Chapter 2) but modestly reduced tuber Cd concentrations, likely by limiting Cd translocation within the plant. SMC was the most effective amendment in geogenic-risk soil, reducing tuber Cd concentrations by up to 47% while also increasing yield. This effect was attributed to SOM- and pH-driven Cd immobilization in non-limed soils and to improved nutrient balance under limed conditions. A “dilution by growth” effect was ruled out. Relationships between measured and modelled soil Cd fractions and tuber Cd concentrations showed that 0.1 M CaCl2 best reflected Cd availability by integrating both Cd intensity and Cd quantity. Across treatments, modelled electrostatically bound Cd associated with clay and SOM showed the strongest correlation with tuber Cd concentrations, outperforming measured dissolved Cd. Cadmium specifically bound to low-affinity SOM sites also contributed to uptake, whereas Cd bound to high-affinity SOM sites contributed primarily to soil retention. While Chapter 2 demonstrated that Cd-DFA complexes dissociate rapidly enough to contribute to uptake in hydroponic systems, Chapter 5 showed that in soil-plant systems, Cd-DOM complexes were weakly correlated with tuber Cd and contributed minimally to uptake. Together, these results indicate that Cd uptake in soils is dominated by desorption from the soil solid phase rather than by Cd-DOM complex dissociation.

Key findings, limitations, implications, and future outlook were discussed in Chapter 6. Cadmium availability in amended soils was concluded to be primarily governed by soil pH, with both Cd intensity (solution-phase Cd concentration) and Cd quantity (the soil’s capacity to replenish Cd in solution) identified as critical determinants of plant uptake. The modelled electrostatically bound Cd fraction associated with clay and SOM emerged as a promising indicator of Cd uptake by potato plants. Among the chemical extractants evaluated, 0.1 M CaCl2 proved to be the most effective predictor of Cd availability, as it captures both Cd intensity and Cd quantity. Several study limitations were acknowledged, including constraints in the design of the hydroponic and pot experiments. Recommendations were made to refine MSM parameters and extend its application as a predictive tool for tuber Cd concentrations without direct measurement of soil inputs. Based on the overall findings, Cd remediation strategies in potato cultivation should prioritize increasing soil pH within crop-appropriate limits to reduce Cd intensity and enhancing SOM through amendments such as SMC to increase Cd quantity. Finally, stricter regulatory limits for Cd in potato tubers and other food crops should be carefully evaluated for feasibility, particularly in regions with geogenically elevated soil Cd concentrations, such as Ireland.