Source-tracking cadmium in New Zealand agricultural soils: a stable isotope approach

Cadmium (Cd) is a toxic heavy metal, which is accumulated by plants and animals and therefore enters the human food chain. In New Zealand (NZ), where Cd mainly originates from the application of phosphate fertilisers, stable isotopes can be used to trace the fate of Cd in soils and potentially the wider environment due to the limited number of sources in this setting. Prior to 1997, extraneous Cd added to soils in P fertilisers was essentially limited to a single source, the small pacific island of Nauru. Analysis of Cd isotope ratios (ɛ114/110Cd) in Nauru rock phosphate, pre-1997 superphosphate fertilisers, and Canterbury (Lismore Stony Silt Loam) topsoils (Winchmore Research Farm) has demonstrated their close similarity with respect to ɛ114/110Cd. We report a consistent ɛ114/110Cd signature in fertiliser-derived Cd throughout the latter twentieth century. This finding is useful because it allows the application of mixing models to determine the proportions of fertiliser-derived Cd in the wider environment. We believe this approach has good potential because we also found the ɛ114/110Cd in fertilisers to be distinct from unfertilised Canterbury subsoils. In our analysis of the Winchmore topsoil series (1949-2015), the ɛ114/110Cd remained quite constant following the change from Nauru to other rock phosphate sources in 1997, despite a corresponding shift in fertiliser ɛ114/110Cd at this time. We can conclude that to the present day, the Cd in topsoil at Winchmore still mainly originates from historical phosphate fertilisers. One implication of this finding is that the current applications of P fertiliser are not resulting in further Cd accumulation. We aim to continue our research into Cd fate, mobility and transformations in the NZ environment by applying Cd isotopes in soils and aquatic environments across the country

Tracing sources of cadmium in agricultural soils: a stable isotope approach

Cadmium (Cd) is a biotoxic heavy metal, which is accumulated by plants and animals and thereby enters the human food chain (Gray et al. 2003). The application of phosphate fertilisers has also resulted in the long-term accumulation of Cd in agricultural soils around the world, including New Zealand (NZ). In 1997, the main source of NZ phosphate fertilisers was changed from Nauru island phosphate rocks (450 mg Cd kg-1 P) to a variety of phosphate rocks with lower Cd concentrations, in order to meet more stringent Cd limits in P fertiliser. Following this change, the accumulation of Cd in topsoil samples from the Winchmore research farm (South Island, NZ) was evaluated and was found to have plateaued post-2000 (McDowell, 2012). In this study, stable isotope analysis was used to trace the fate of Cd in Winchmore farm soils in order to determine the cause of the plateau. The isotope ratio of Cd (δ114/110Cd) was measured in pre-2000 and post-2000 phosphate fertilisers, phosphate rocks, topsoil (0-7.5 cm) and control (unfertilised) subsoil (25-30 cm) samples from the Winchmore site. The analysed topsoil samples were archived samples collected over the period 1959-2015. The isotopic compositions of fertilised topsoils ranged from δ114/110Cd = 0.08 ± 0.03 to δ114/110Cd = 0.27 ± 0.04, which were comparable to pre-2000 fertilisers (δ114/110Cd = 0.10 ± 0.05 to 0.25 ± 0.04) but distinct from the post-2000 fertilisers (δ114/110Cd range of -0.17 ± 0.03 to 0.01 ± 0.05) and control subsoil (δ114/110Cd = -0.33 ± 0.04) (Salmanzadeh et al., 2017). We combined this stable isotope data with Bayesian modelling to estimate the contribution of different sources of Cd. An open source Bayesian isotope mixing model implemented in Matlab (Arendt et al., 2015) was used here with some modifications to estimate the fractional contribution of different sources of Cd through time including pre- and post-2000 fertilisers, and the control soil. The Matlab code of Arendt et al., 2015 was modified to consider only one isotope system (rather than two), and fewer sources. This modelling confirmed the dominant contribution (about 80%) of Nauru-derived (i.e. pre-2000) fertilisers in increasing the Cd concentration in Winchmore soils. To help constrain the soil Cd mass balance we used an existing model (CadBal) (Roberts and Longhurst, 2005), to estimate residual soil Cd and output fluxes based on known P fertiliser application rates, the initial Cd concentration, farm and soil type, and soil dry bulk density. We incorporated the isotope data into the mass balance expression in order to evaluate the performance of CadBal in estimating the past topsoil Cd accumulation and predicting the future concentrations and isotope ratios of Cd (up to 2030 AD). The results of mass balance modelling confirm that recent applications of phosphate fertilisers have not resulted in an accumulation of Cd during the most recent period, thus Cd removal by either leaching or crop uptake has increased, which is consistent with the modelled isotope data (Figure 1). We can conclude that it becomes possible to distinguish the sources of Cd within the soil using stable Cd isotopes (Imseng et al., 2018) and that the residual Cd in topsoil at Winchmore still mainly originates from historical phosphate fertilisers (Salmanzadeh et al., 2017). One implication of this finding is that the contemporary applications of phosphate fertiliser are not resulting in further Cd accumulation. We aim to continue our research into Cd fate, mobility, and transformations in the NZ environment by applying Cd isotopes in soils and aquatic environments across the country. Figure 1. Results of Cd mass balance modelling in CadBal for the period of topsoil fertilisation including a prediction up to the year 2030 AD. (a) Mean concentration of Cd in the dryland treatment of Winchmore long-term irrigation trial (symbols) and the CadBal model (lines) outputs (red symbols = this study- plot 15 of Winchmore site; grey symbols = McDowell study-average of all plots; solid black line = dryland optimized CadBal from McDowell (2012) for all irrigation plots; black dashed line = Plot 15 dryland optimized CadBal-this study, first scenario; blue line = Plot 15 dryland optimized CadBal-this study, second scenario; red line = Plot 15 dryland optimized CadBal-this study, third scenario; red dashed line = Plot 15 dryland optimized CadBal-this study, fourth scenario); (b) Measured and modelled Cd isotope ratios based on CadBal outputs, isotope ratios measured in fertilisers and the fractionation factors of Wiggenhauser, et al. (2016); lines designate modelling scenarios as in (a), red dots are the third scenario with no fractionation (α factor not applied); (c) modeled scenario 3 (solid) and scenario 4 (dashed) isotope ratios in topsoil (red lines), leachate (blue lines) and pasture (green lines)

Source-tracking cadmium in New Zealand agricultural soils: A stable isotope approach

Cadmium (Cd) is a toxic heavy metal, which is accumulated by plants and animals and therefore enters the human food chain. In New Zealand (NZ), where Cd mainly originates from the application of phosphate fertilisers, stable isotopes can be used to trace the fate of Cd in soils and potentially the wider environment due to the limited number of sources in this setting. Prior to 1997, extraneous Cd added to soils in P fertilisers was essentially limited to a single source, the small pacific island of Nauru. Analysis of Cd isotope ratios (ɛ114/110Cd) in Nauru rock phosphate, pre-1997 superphosphate fertilisers, and Canterbury (Lismore Stony Silt Loam) topsoils (Winchmore Research Farm) has demonstrated their close similarity with respect to ɛ114/110Cd. We report a consistent ɛ114/110Cd signature in fertiliser-derived Cd throughout the latter twentieth century. This finding is useful because it allows the application of mixing models to determine the proportions of fertiliser-derived Cd in the wider environment. We believe this approach has good potential because we also found the ɛ114/110Cd in fertilisers to be distinct from unfertilised Canterbury subsoils. In our analysis of the Winchmore topsoil series (1949-2015), the ɛ114/110Cd remained quite constant following the change from Nauru to other rock phosphate sources in 1997, despite a corresponding shift in fertiliser ɛ114/110Cd at this time. We can conclude that to the present day, the Cd in topsoil at Winchmore still mainly originates from historical phosphate fertilisers. One implication of this finding is that the current applications of P fertiliser are not resulting in further Cd accumulation. We aim to continue our research into Cd fate, mobility and transformations in the NZ environment by applying Cd isotopes in soils and aquatic environments across the country

Use of cadmium isotopes to distinguish sources of cadmium in New Zealand agricultural soil: Preliminary results

In New Zealand’s agricultural soils, phosphate fertiliser applications are the main source of cadmium (Cd). In 1997, the NZ fertiliser industry discontinued sourcing rock phosphate from Nauru (about 450 mg Cd/ Kg P) and began producing superphosphate from other rock phosphate sources (such as Morocco), which have generally lower concentrations of Cd. Research on the concentration of Cd in soils from the long-term irrigation trials at the Winchmore research farm (Canterbury) indicates that Cd accumulation rates have started to slow in the period since 1997 (Fig. 1) (McDowell 2012)

Tracing sources of cadmium in agricultural soils using cadmium stable isotopes

The application of phosphate fertilizers has, on a global basis, resulted in long-term accumulation of cadmium (Cd) in agricultural soils [1]. While this accumulation has led to concern over potential environmental consequences, we currently lack a viable tool to track fertilizer- derived Cd in terrestrial environments. In 1997, the main source of phosphate fertilizers in New Zealand (NZ) was changed from Nauru to a mixed product sourced from other phosphorites with lower concentrations of Cd. Around the same time, Cd accumulation in a 66-year-long field trial (Winchmore Farm, South Island, NZ) showed an apparent plateau [2]. In this study, Cd isotope ratios (ɛ114/110Cd) were used to trace Cd sources in Winchmore soil and determine the cause of this plateau. The ɛ114/110Cd was measured in archived phosphate fertilizer, phosphorite and topsoil (0-7.5 cm) samples from Winchmore. The ɛ114/110Cd of fertilized topsoils and fertilizers was distinct from control (unfertilized) subsoils by around +0.6‰. Bayesian isotope modelling using pre- and post-2000 fertilizers and control soil as the endmembers, confirmed the dominant contribution of Cd is from pre-2000 fertilizers (ɛ114/110Cd=2.48±0.37) with signature comparable to source rocks (ɛ114/110Cd=2.19± 0.39) but distinct from control subsoil (ɛ114/110Cd=-3.33 ±0.41). The decline in Cd concentration after 2000 followed the reduction in fertilizer Cd concentration. The ɛ114/110Cd of soil remained quite constant following the source change, confirming that soil Cd represents the historical burden of Cd (originating from Nauru phosphorites) and concurrent applications of fertilizer are not resulting in further accumulation of Cd

Cadmium accumulation in three contrasting New Zealand soils with the same phosphate fertilizer history

Cadmium (Cd) concentration in New Zealand (NZ) agricultural soils has increased due to phosphate fertilizer application, but it is not clear whether soils with different properties accumulate Cd at similar rates for given P loadings. Here, the distribution of Cd was measured in three soils: the well-drained Horotiu series (Orthic Allophanic Soil in NZ soil classification, Typic Hapludand in US soil taxonomy), poorly-drained Te Kowhai series (Orthic Gley Soil in NZ classification, Typic Humaquept in US soil taxonomy) and an intergrade between them, Bruntwood series (Impeded Allophanic Soil in NZ soil classification, Aquic Hapludand in US soil taxonomy). All three soils often occur in the same paddock with the same fertilizer history, but have differing drainage and mineralogical characteristics, permitting an assessment of the potential for varying accumulation/translocation of Cd in contrasting soil conditions. Thirty soil profiles from ten paddocks on a dairy farm near Hamilton, NZ, with a uniform fertilizer history were sampled to depth of 60 cm. The Cd concentration in topsoil (0–7.5 cm) samples (mean of 0.79 mg kg−1 ) was about 7–8 times greater than in deeper horizons (P b 0.001). No significant differences in Cd concentration or fractionation among the soil series were detected. Cluster analysis showed that Cd, phosphorus (P) and uranium (U) were highly correlated, consistent with a common source, most likely phosphate fertilizer. The absence of a difference in the Cd depth profiles in the three soils indicates that Cd was preferentially adsorbed to the topsoil and was not significantly mobilized by drainage in the soils. The lack of difference in Cd distribution between contrasting soil series supports the use of one Cd management system tool for all of these soils

The effect of irrigation on cadmium, uranium, and phosphorus contents in agricultural soils

Cadmium (Cd) is a toxic metal which has accumulated in New Zealand agricultural soils due to phosphate fertilizer application. Understanding the contribution of plant uptake or leaching of Cd to observed Cd losses from soil is important. The concentration and distribution of Cd in irrigated and unirrigated soils with the same phosphate fertilizer history were investigated. Twenty-two pairs of soil samples from four depths (0–0.1, 0.1–0.2, 0.2–0.3 and 0.3–0.4 m) were taken from irrigated and unirrigated areas in the same field on dairy farms in three regions of New Zealand. The mean concentration of Cd at depths of 0–0.1 m and 0.1–0.2 m, as well as the cumulative masses of Cd (0–0.2, 0–0.3 and 0–0.4 m) in unirrigated soils were significantly higher (P < 0.05) than in irrigated soils. The concentration of phosphorus (P) at all depths (except for 0.2–0.3 m), as well as the cumulative mass of P in all depths of unirrigated soils, was also significantly higher (P < 0.05) than irrigated soils. However, no significant difference was detected in the concentrations of uranium (U) between irrigated and unirrigated soils. Irrigation induced a ∼7% Cd loss from topsoil (0–0.1 m), with the average rate of Cd loss from the top 0.1 m (due to irrigation) being 2.3 g ha⁻¹ yr⁻¹. This study therefore confirms that irrigation can enhance Cd mobilization, however Cd is mainly adsorbed to the surface soil

Cadmium accumulation in agricultural soils

Cadmium (Cd) has accumulated in New Zealand (NZ) soils as a result of phosphate fertiliser application. Cadmium is a biotoxic heavy metal and can be adsorbed by soil and enter the human food chain. Three objectives were investigated in this thesis: 1. Determine if the distribution of Cd varies between soils with contrasting mineralogy and drainage characteristics, but the same phosphate fertiliser history, 2. Evaluate the utility of Cd stable isotope ratios (δ¹¹⁴/¹¹⁰Cd) to trace the sources of Cd in NZ soils through time and distinguish the contribution of different sources of Cd in NZ soils, and, 3. Determine whether there is a difference in the concentration of Cd in irrigated and unirrigated soils within the same paddock. The concentration of Cd was measured in three soils, with contrasting mineralogy and drainage characteristics within the same paddock, and thus same fertiliser history. The mean concentration of Cd in topsoil (0-7.5 cm) samples was 0.77 mg kg⁻¹ (range 0.56-0.99) in the Horotiu soil (Orthic Allophanic Soil in NZ soil classification, Typic Hapludand in US soil taxonomy), 0.83 mg kg⁻¹ (range 0.60-1.11) in the Bruntwood soil (Impeded Allophanic Soil in NZ soil classification, Aquic Hapludand in US soil taxonomy) and 0.78mg kg⁻¹ (range 0.46-0.96) in the Te Kowhai soil (Orthic Gley Soil in NZ classification, Typic Humaquept in US soil taxonomy). There were no significant differences in the concentration, and/or the total mass of Cd between the three soils. Cadmium was mainly adsorbed to the near surface soil regardless of soil mineralogy and drainage characteristics. Thus, it was concluded that it is appropriate to apply the same Cd management approach (The Tiered Fertiliser Management System) to the investigated soil types. Isotope ratios of Cd (δ¹¹⁴/¹¹⁰Cd) were used to trace the sources of Cd in a longterm irrigation and fertiliser trial at Winchmore, Canterbury, New Zealand. The isotopic composition of pre-2000 fertilisers (δ¹¹⁴/¹¹⁰Cd = 0.10 ± 0.05 to 0.25 ±0.04) was comparable to the Nauru source rocks used in fertiliser manufacture (δ¹¹⁴/¹¹⁰Cd = 0.22 ± 0.04), but distinct from the control subsoil (δ¹¹⁴/¹¹⁰Cd = -0.33± 0.04) and post-2000 fertilisers (δ¹¹⁴/¹¹⁰Cd range of -0.17 ± 0.03 to 0.01 ± 0.05). The isotopic compositions of fertilised soil samples ranged from δ¹¹⁴/¹¹⁰Cd = 0.08± 0.03 to δ¹¹⁴/¹¹⁰Cd = 0.27 ± 0.04, which were comparable to pre-2000 fertilisers. Thus, it becomes possible to distinguish the sources of Cd within the soil using isotopes. The fractional distribution of Cd sources confirmed the main contribution of Nauru-derived phosphate fertilisers (pre-2000 fertilisers) in increasing the amount of Cd in soils at the Winchmore research farm. The concentration and distribution of Cd in adjacent irrigated and unirrigated soils with the same phosphate fertiliser history were investigated. Twenty-two pairs of soil samples from 4 depths (0-10, 10-20, 20-30 and 30-40 cm) were taken from irrigated and unirrigated areas in the same paddocks on different dairy farms from three regions of New Zealand (Bay of Plenty, Manawatu-Wanganui, and Canterbury). The mean concentration of Cd (depth of 0-10 cm and 10-20 cm) and the cumulative mass of Cd (depths of 0-20, 0-30, and 0-40 cm) were higher (P < 0.05) in unirrigated soil than in irrigated soil. Total Cd was about 10% less abundant in the 0-40 cm depth range in irrigated soil (mean of 0.63 kg ha⁻¹) than unirrigated soil (mean of 0.73 kg ha⁻¹), with the average difference of 7.2 g ha⁻¹yr⁻¹ for the 0-40 cm depth. The significant difference (P < 0.05) in the cumulative mass of Cd between irrigated and unirrigated soils demonstrated that irrigation may have enhanced the mobility of Cd. However, overall the results demonstrate that Cd was generally immobile and mainly absorbed to the near surface of the soils studied

Is there any difference between distributions of cadmium in different soils with the same fertiliser history?

Cadmium (Cd) is a potentially biotoxic metal that can be absorbed by soils and plants. The amount of Cd in NZ soils has previously increased following application of phosphate fertilisers. Cd concentration in three soil types: Te Kowhai, Horotiu and Bruntwood is being investigated. All three soils often occur in the same paddock, so have the same fertiliser history. However, these soils have contrasting drainage and mineralogical characteristics. Two paddocks with all three soils were sampled (depth of 60 cm) from a dairy farm near Hamilton and Cd concentration was determined. Initial results suggested that total Cd in the poorly drained Te Kowhai was higher than in Bruntwood/Horotiu. In some topsoils, Cd concentrations were greater than 1 mgkg-1, which according to Tiered Fertiliser Management System, would require fertiliser management by a “balanced programme” to ensure that Cd will not exceed an acceptable threshold (1.8 mgkg-1) in the next 50 years