Tracing the legacy of fertilizer use: understanding old soil cadmium using new tricks

The Pacific island of Nauru was a major source of the phosphate rocks used by New Zealand agriculture in the mid-to-late 20th century. During the 20th century, cadmium (an element found in these rocks) accumulated in New Zealand’s soils as a direct result of fertilizer use. This increase in cadmium has led to concerns over its biotoxic effects because it can enter the human food chain via plants and grazing animals. A major unknown in current cadmium management practice is the extent to which cadmium presently in New Zealand soils originates from older fertilizer applications, or from ones that are more recent.

GA revised
Graphical abstract of the research paper

A new study published in the prestigious journal Environmental Science & Technology addresses this question by using a new isotope technique. PhD candidate Mahdiyeh Salmanzadeh, describes her study into the accumulation of soil cadmium in the following blog post. 

Welcome to my first blog post for Waikato Environmental Geochemistry, which is about one of the most exciting projects I’ve been involved in: an isotope evaluation of soil cadmium accumulation. Firstly, thanks to the Fertiliser Association of New Zealand for generously providing funding to support this PhD research.

The idea of this research came from this figure, which shows the accumulation history of cadmium (Cd) in an agricultural soil:

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McDowell (2012), The rate of accumulation of cadmium and uranium in a long-term grazed pasture: implications for soil quality. New Zealand Journal of Agricultural Research, 55, 133-146.

The data shows a plateau in soil Cd accumulation at the Winchmore research farm in New Zealand after 1997. The historical phosphate fertilisers in New Zealand originated from Nauru rock phosphate, which contains high amounts of Cd. In 1997, the main source of phosphate fertilisers in New Zealand (NZ) was changed from Nauru rock phosphate to a mixed product sourced from other phosphorites (high phosphorus rocks used in fertiliser manufacture) with lower concentrations of Cd. So the plateau in Cd accumulation at Winchmore coincided with a source change in phosphorus rocks- leading to the idea that we could differentiate the sources of Cd in this soil system using isotope ratios (which are basically measures of the proportion of marginally heavier to lighter version of the same element, e.g. 114/110Cd).

To answer this question we used stable isotope ratios of Cd (δ114/110Cd) to trace the fate of Cd in Winchmore soils. Soil samples from 1959, 1961, 1967, 1974, 1979, 1993, 1999, 2002, 2004, 2009 and 2015 – plot 15, irrigation trial (Winchmore research farm), rock phosphate samples from Nauru and Christmas Island, and fertilisers from the mid-1980s, 1998, 2000, 2001, 2005, 2007, 2009, 2011, 2013 and 2015 were used for Cd concentration and isotope analysis.

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Fertilizer and rock samples

I digested the samples in the geochemistry lab at the Waikato University and the concentration of Cd in samples was measured using mass spectrometry (ICP-MS). Then I travelled to Dunedin in New Zealand’s south island, to analyse the isotope ratios of Cd using a form of high precision mass spectrometry (MC-ICPMS) at the Centre for Trace Element Analysis, University of Otago.

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Travel to Dunedin

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Once we had the data, we used Bayesian mixing modelling to calculate the fractional contribution of different sources of Cd. CadBal (A cadmium mass balance model used to study the accumulation of Cd in New Zealand soils), was also used to estimate the future concentration and isotope ratios of Cd.

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Fractional contribution of different sources of Cd in Winchmore soil samples based on a Bayesian Cd isotope mixing model for pre-2000 AD fertilisers (black circle), post-2000 AD fertilisers (red circle), and control soil (open circle)

Long story short, the isotope ratios of fertilised soils were distinguishable from the native soil and comparable to the source rocks, meaning that the approach worked and we could calculate the proportions of the Cd sources in the soil through time.

The results showed that although the main sources of fertilisers did change, Winchmore soils are still mainly affected by pre-2000 fertilisers (fractional contribution of about 80%), with post-2000 fertilisers (fractional contribution of 17%) and native soils (fractional contribution of 10%) making up the remainder.

The results of modelling confirmed that Cd from recent fertilisers contributed less to the total Cd isotope compositions than predicted by the model. Therefore, this study has shown that Cd isotopic analysis is a promising tool to trace the fate of fertiliser-drive Cd in the environment.

The most exciting part of this research is this paper, which is a paper I am very proud to have been a part of.

Thanks to Adam and all of the co-authors.

Cheers for reading

Mahdiyeh

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