For the first time in geological history, the imprint and geological signal of humans is being felt across the planet in aspects as diverse as radioactive elements dispersed by nuclear bomb tests, carbon dioxide accumulation in the atmosphere, global contamination with lead, and pollution of the oceans by trillions of plastic microbeads, so many that they will soon outnumber fish.
The Anthropocene is a by-product of the huge expansion in the global human population. According to some ecologists, the Earth is now at about 30 times its natural human carrying capacity and in danger of ecological collapse.
Whether or not that occurs, expansion of the human population was made possible through the development and global rollout of industrialised agriculture. This in turn rested on development of a science of agricultural chemistry from the mid-19th century and accompanying miracles of chemical engineering. Two of these involved the chemical elements phosphorus and nitrogen.
Phosphorus and nitrogen are essential ‘nutrient’ elements required for soil fertility, plant growth and agriculture. As crops grow, they take these elements up from the soil. When their supply in soil becomes depleted, or soil stocks are naturally low, plants and animals do not thrive.
The problem of supplying large amounts of phosphorus in a plant-available form was solved first. In the 1840s, agricultural chemist John Bennet Lawes (1814–1900) patented and trialled a process for treating phosphate minerals with sulphuric acid. The resulting product, which also supplies sulphur and calcium, was called ‘superphosphate’.
About a century after its discovery, widespread application of superphosphate to New Zealand pastures began in earnest, with the invention of aerial top-dressing in the late 1940s, partly facilitated by a surplus of aircraft and trained pilots in the years after World War II. The practice proved successful, and since then most productive farms have been top-dressed with annual applications of superphosphate to boost and maintain soil fertility.
Animal and human manure has always been a good source of nitrogen, but expansion of farming required a more abundant and reliable source. Developed in the first half of the 20th century, the revolutionary Haber-Bosch process allowed us to take unreactive nitrogen gas out of thin air to make a usefully reactive form of nitrogen (ammonia), which found widespread use in both explosives and fertiliser. In agriculture, this development meant that nitrogen-poor soils, and soils depleted by farming, could be pressed into service — making the Green Revolution possible. Synthetic nitrogen fertilisers now supply over half the world’s food crops and are essential for the nourishment of at least two billion people. Through making it possible to mass-produce high explosives, the same breakthrough also opened the way for humans to wage war with industrial efficiency.
From the perspective of their capacity to cause environmental contamination, there is one fundamental difference between nitrogen and phosphate fertilisers. Since they are made from nitrogen in air, synthetic nitrogen fertilisers are inherently ‘clean’ — they can supply too much nitrogen, but they do not carry other chemical contaminants. The environmental downsides of nitrogen fertilisers are all linked to nitrogen as a pollutant in its various forms. As a form called nitrate (which in chemical symbol form is written NO3-), excess nitrogen which dissolves in surface run-off or ground water fertilises waterways and coastal areas worldwide, causing eutrophication. Another by-product, nitrous oxide (N2O), is a powerful greenhouse gas.
By contrast, since it is made from mixed phosphate rock and mineral deposits that contain natural contaminants, superphosphate fertiliser is inherently ‘dirty’. In addition to the phosphorus and sulphur, it contains several trace elements as impurities which would be difficult to remove without significant expense. The most notable of these are the heavy metal cadmium, the highly reactive element fluorine, and isotopes of uranium, thorium and radium, along with a certain amount of natural radioactivity. Cadmium, fluorine and interesting isotopes also exist naturally as constituents of soils, but at much lower concentrations than in phosphate rock, or fertiliser made from it. When superphosphate is added to soil, these contaminant stowaways tend to be retained in the surface soil layers by becoming adsorbed to organic matter and specific inorganic minerals such as clays and iron oxides.
How cadmium enters the food chain
The amount of contamination added in any one year is minor, but the process is incremental. When superphosphate is applied year after year, concentrations of the contaminant elements in surface soils progressively increase.
The long-standing dairy farming regions of Waikato, Bay of Plenty and Taranaki have the longest history of superphosphate use. Up until the last 10–15 years, each hectare of an established dairy farm received about 500–580 kilograms of superphosphate annually. This figure has decreased by over 40 per cent in recent years with moves to use fertiliser more efficiently, guided by results of soil-fertility testing. Nonetheless, for a farm that has received annual applications of superphosphate since the advent of aerial top-dressing, the superphosphate history translates to each hectare of soil having received about 900 grams of additional cadmium, 520 kilograms of fluorine, and 2–3 kilograms of uranium.
To hypthetical geologists in a distant future, the onset of a sudden widespread increase in concentrations of phosphorus, cadmium, fluorine, uranium, thorium and radium in agricultural soils in New Zealand and many other countries would represent a further geological imprint of the Anthropocene. As with radioactive elements dispersed through the environment by nuclear bomb tests, the approximate starting date when these elements first started to accumulate in agricultural soils on a wide scale was 1950.
Several possible problems can be caused by the accumulation of an unwanted trace element in soils. The initial focus here will be on cadmium.
To humans, cadmium is a highly toxic element, on a par with mercury. In agricultural surface soils of the long-standing dairy regions, the average concentration of cadmium is currently about 700 parts per billion, or five times its natural level.
This figure hides three subtleties. The first is that a sizeable proportion of properties have significantly more cadmium than the average. The second is that as total cadmium concentrations in soil increase, the ‘plant-available’ proportion increases at a higher relative rate. However, a third important factor is that the soil chemistry of cadmium is on something of a knife-edge.
After superphosphate is applied, about 90 per cent of the cadmium becomes bound to surface soils — from which it could theoretically be taken up in produce — and 10 per cent is leached to deeper layers. We think of this as a complex type of chemical equilibrium system where, at any point of time, most (usually over 99 per cent) of the cadmium that was left in the surface soil will be found bound to solid phases of the soil, but a small proportion (usually less than 1 per cent) will be found dissolved in the soil pore-water.
This minor proportion that exists in a dissolved form is important, because this is the form most readily taken up in crops, pastures and animals, or leached through the soil in the direction of underlying ground water. At the moment, the surface soils are doing us a service by locking up most of the retained cadmium. However, many factors can alter the balance of this type of equilibrium, making the proportion of cadmium released to pore-water vulnerable to changing environmental conditions.
As two examples, if soil pH becomes more acidic, more of the previously accumulated cadmium will be released from the soil minerals and back into pore-water. Similarly, if a competing trace metal which shares a similar chemistry to cadmium were added, that would be likely to promote release of some cadmium to soil pore-water.
Fortunately, at current concentrations and soil chemistry conditions, cadmium is not particularly toxic to soil organisms. Instead, the main problem it triggers is linked to the small but measureable proportion of cadmium that is taken up into food crops and produce. It is thought that the widespread use of superphosphate is likely to have significantly increased the overall exposure to cadmium in Western populations. On occasion — and in some foods such as animal kidneys, some leafy green vegetables, potatoes and some wheat grain — cadmium uptake is sufficient to exceed food standards.
A secondary related impact is felt when a city expands and longstanding agricultural land on an expanding urban fringe is converted to residential use. Some agricultural land has accumulated so much cadmium that it exceeds standards for this heavy metal in residential soils. Under rules of a national environmental standard, soils like this have to be remediated before the subdivision can proceed. These rules are to protect human health, and allow for the fact that many home gardens have a vegetable patch.
Problems caused by the presence of cadmium in agriculture have been recognised at a national level in both New Zealand and Australia. In 2011, New Zealand’s Ministry of Agriculture and Forestry (now Ministry of Primary Industries) released a national Cadmium Management Strategy. Some solid progress has been made in working through the initial aspects of this strategy.
In some cases, the causes of environmental contamination can be traced back to a single person. A notable example is the American mechanical engineer and chemist Thomas Midgley Jr (1889–1944). In the course of his tireless career, Midgley invented both chlorofluorocarbons (CFCs), responsible for destruction of the ozone layer, and tetraethyl-lead, which was added to petrol. Worldwide uptake of the latter invention caused global dispersal of lead particles from vehicle exhausts, with widespread contamination of urban environments in particular. In the view of environmental historian J. R. McNeil, in his personal contribution Midgley ‘had more impact on the atmosphere than any other single organism in Earth’s history’. New Zealand banned use of lead additives in petrol in the 1990s, but lead contamination of our urban soils is a lasting legacy, because as a heavy metal lead does not break down.
When leaded petrol was in use, it was possible to measure the weekly accumulation of lead on surfaces such as the leaves of deciduous trees. An example from our environmental research showing lead accumulating on leaves of horse chestnut trees in Christchurch parks is shown in Figure 1.
Progressive contamination of New Zealand soils with the heavy metal cadmium involves entire regions of agricultural land across the country and might be regarded as New Zealand’s largest and slowest land contamination event. If we were to (perhaps unfairly) trace its origin back to a single person, a Midgley of agriculture, we would have to nominate agricultural chemist John Bennet Lawes, the inventor of superphosphate. Other contenders could be Alexander von Humboldt (1769–1859), who studied guano and its fertilising properties at Callao in Peru, and whose subsequent writings made the subject known in Europe; and Justus von Liebig (1803–1873), whose work was foundational to the modern understanding of plant nutrition. However, Lawes receives the coveted title in this case, because superphosphate has always dominated as the main phosphorus fertiliser used on New Zealand farms.
Figure 1. Lead concentrations in leaves of horse chestnut trees in Christchurch parks during the era when New Zealand used leaded petrol. (Each concentration shown is the geometric mean of results for six parks, and concentrations are on an ash-weight basis.) As soon as the new leaves opened, lead particles emitted from car exhausts were being deposited on their surfaces. This carried on until autumn, when the leaves dropped. During this era, underlying soils in each park, urban soils in general and urban residents all received a continual loading of lead from this source, all year round.
Two types of evidence have been used to measure and project cadmium accumulation in soils. The first involved trials at experimental research farms or trial plots, where the amounts and types of fertiliser applied to each paddock is known. Small-scale trials of this type allow accumulation models for the particular soil type to be developed. The second, more recent approach involves large-scale ‘State of the Environment’ soil sampling by regional councils, which started in the mid-1990s. Results of wide-scale sampling of this type allow scientists to calculate summary statistics comparing levels of cadmium in different agricultural soils with those at native ‘background’ sites that have not received superphosphate.
The Waikato Regional Council currently samples 120 farms across the region, on a rolling cycle. Each farm is visited once every five years. (Cadmium and other contaminant accumulation in soils is slow enough that it would be pointless returning every year, because the signal of additional cadmium would be lost in the expected variability involved with the sampling and analysis.)
Based on results of both approaches, it has been assumed that cadmium will continue to accumulate in most agricultural surface soils, while superphosphate fertilisers continue to be used, as illustrated in Figure 2.
Until recently. Lately, the soil sampling record has become long enough to start making comparisons of changes going on at the same farm over time. The results have been a surprise. While fluorine (also from superphosphate) appears to be still accumulating in surface soils, cadmium concentrations appear to have reached a plateau, and may even be declining slightly.
Based on experimental farm trials, this should not be happening. Cadmium is still being applied to soil with superphosphate fertiliser, and from our knowledge of contaminated industrial sites in other countries we know that soil itself has plenty of capacity to retain cadmium, lead, mercury and many other heavy metals up to very high concentrations (often hundreds of times higher than current levels). However, the new real-world results for Waikato agricultural soils suggest that cadmium
Figure 2. Conceptual reconstruction of the assumed gradual increase in cadmium concentrations in Waikato soils inferred solely by comparison of modern results for farmed soils with background soils. (The modest decrease in the accumulation rate shown from the mid-1990s corresponded to a fertiliser industry decision at that point to set an upper limit on the cadmium concentrations in superphosphate.) being applied with superphosphate may no longer be becoming as strongly fixed to the surface soil layers. Some of it is going missing. Presumably it is being leached into the deeper soil, in the direction of ground water.
Consistent with this picture, cadmium has been detected in some ground waters below farms that use large amounts of fertiliser, and in lake and coastal sediments surrounded by farmland. If true, what could have caused this reversal?
Here the story turns to zinc. Facial eczema is a devastating disease of livestock. We now know that the underlying cause is a toxin (called sporidesmin) produced by spores of a fungus that grows in pasture grasses in warm, humid weather. When animals eat the grass, they eat the toxin. This toxin can seriously damage the animal’s liver. Light-absorbing chlorophyll from the grass and other waste products normally broken down in the liver then build up in the animal’s bloodstream, causing photosensitivity and severe sunburn in exposed skin. In their distress, animals rub themselves on available objects, causing peeling and wounding. Facial eczema can occur throughout the world, but conditions in New Zealand’s North Island are particularly favourable for growth of the ryegrass fungus from spring through summer. In the 2016 season, South Island cases were also prominent.
The underlying cause of facial eczema was unknown in the 1950s, when Te Aroha farmer Gladys Reid (1914–2006) started searching for a solution. Based on research into another photosensitive disease (pellagra), she initially treated her stock with vitamin B3. Later, on learning that zinc was an essential cofactor of this vitamin, she began administering zinc oxide to calves. Zinc appeared to make a real difference. By 1969, she was carrying out a controlled experiment on a ‘zinc hypothesis’ involving her own farm and the farm next door, which she had purchased. Her ideas were initially controversial, but results were obvious and by the mid-1970s her work was gaining widespread acceptance in farming circles. In 1982, zinc treatment of animals was officially recommended for facial eczema prevention.
Until the mid-1970s Gladys Reid thought that the reason zinc worked so well was that the animals were zinc-deficient. We now know that this was not the case. Instead, she had inadvertently discovered a means to disrupt the chemical structure of the toxin sporidesmin, preventing the toxin from damaging the liver.
In the past three decades, a small industry has sprung up around facial eczema treatments. Products range from soluble zinc salts to add to stock water, to zinc ‘bullets’ that slowly dissolve inside the animal’s stomach, to zinc-enriched molasses that cows can enjoy while being milked. Zinc salts are inexpensive, and the industry has been inventive. The concentrations of zinc being administered are so high that they are regarded as sub-toxic in themselves, and they can induce copper deficiency; but overall zinc treatment is the dominant method now used for control of facial eczema.
The only hitch is that in order for treatment to be effective, animals need to be fed large doses of zinc daily for several months of the year, so that zinc is continually circulating in their bloodstreams. The same animals are continually attempting to clear and excrete the excess zinc, most of which therefore ends up being deposited on the farm fields.
Through surveying it has been found that most sheep, beef and dairy farmers now use zinc treatments every year. It is estimated that 5000–8000 tonnes of zinc is now being applied (with animal waste) to Waikato pasturelands each year. In only three decades, the average concentration of zinc in Waikato soils has doubled, from 30 to 60 parts per million. On about 10 per cent of central North Island properties, soil zinc levels now exceed 100 parts per million.
We will return to some implications of all this excess zinc later, but in the meantime focus on another apparent positive.
Quite by accident, Gladys Reid’s discovery of zinc as a facial eczema preventative appears to have provided a partial answer to a key legacy effect of John Bennet Lawes’ superphosphate invention. All of this extra zinc may have been working to cleanse North Island surface soils of cadmium.
How might this have worked? A method that has occasionally been used for remediating contaminated surface soils is to add high concentrations of another benign chemical that works to dislodge the adsorbed contaminant and move it downwards through the soil profile. For example, surface soils contaminated by copper from fungicide sprays have been treated by irrigation with water containing the binding agent EDTA. This extracts some of the bound copper from the soil as a copper-EDTA complex and moves it downwards through the soil profile.
Similarly, an experimental approach of decontaminating surface soils of arsenic is to irrigate the area with water containing high levels of dissolved phosphate. The usual chemical form of arsenic (arsenate, or AsO43-) is chemically similar to phosphate (PO43-) and both compete to be adsorbed by the same soil mineral phases. High concentrations of dissolved phosphate therefore displace some bound arsenic from soil minerals, and also reduce the remaining number of soil binding sites. On an arsenic-contaminated site, irrigation with water containing high levels of phosphate has the effect of dislodging some of the arsenic from the surface soil and pushing it further down, deeper into the soil profile.
We speculate that a similar event is occurring here. Although zinc is essential, and cadmium is both non-essential and highly toxic, the two metals fall in the same group within the periodic table of the elements. When it comes to their broad soil chemistry, they are similar: both take on a ‘positive 2’ charge, symbolically written as Zn2+ for zinc and Cd2+ for cadmium. To a first approximation, zinc and cadmium share preferences in terms of what soil sites they prefer to bind to. Cadmium does bind more strongly than zinc, particularly to sulphur atoms present in soil organic matter, but when high levels of zinc are added to soil, some cadmium is likely to be released from its binding sites into the soil pore-water.
The impact of zinc on cadmium adsorption to soil can be shown in a laboratory. Figure 3 shows how cadmium adsorption to a particular soil decreases when zinc is also present.
On the real-world farms where zinc-based facial eczema treatments are being used, significant competition for soil adsorption sites is likely to be occurring because zinc is being added to soils at about 1000 times the loading of cadmium. On an annual per-hectare basis, superphosphate top-dressing adds 4–5 grams of cadmium, whereas facial eczema treatment adds 5000–6000 grams of zinc to the same soils. When there is a lot more zinc than cadmium available to bind, its ability to be adsorbed by soil will inevitably drop. Zinc will occupy many of cadmium’s preferred soil adsorption sites. The high levels of zinc will work to shunt some of the surface cadmium down, deeper into the soil profile.
Figure 3. Adsorption graph showing the strength of binding of cadmium to a silt loam under controlled conditions, both with and without the presence of zinc in solution. In the presence of 40 mg/L (ppm) zinc, the strength of cadmium binding drops by a factor of five. This means that when more zinc is present, more cadmium remains in solution instead of being adsorbed to the soil. The result also implies that high levels of zinc being deposited on farm soil will cause release of some of the previously fixed cadmium, because the two metals compete for many of the same soil adsorption sites.
Assuming this interpretation is correct and there has been a genuine plateauing or marginal decline in cadmium concentrations in surface soils, this has been a short-term benefit. The new field data suggests that from the mid-1980s cadmium accumulation in many surface soils may have hit a speed-bump, presumably caused by the sudden unanticipated arrival of large amounts of zinc. Three decades later, cadmium is still being inhibited from binding to surface soils by the presence of large amounts of zinc.
Zinc has therefore been a short-term remedy for both facial eczema and cadmium contamination of agricultural surface soils. No one had foreseen this second beneficial impact of Gladys Reid’s discovery.
Long-term impacts are being seen now
However, there is no such thing as a free lunch. To switch metaphors, in an apparent inversion of the adage ‘every cloud has a silver lining’, the zinc breakthrough for facial eczema treatment could be considered to be a silvery lining with storm clouds attached. Gladys Reid received an OBE for her work in 1983, but like many good things taken to excess — Thomas Midgley’s tetraethyl-lead and CFCs included — there are some serious downsides.
Over the next decades, the negative impacts of too much zinc in surface soils will begin to manifest, if zinc treatment for facial eczema remains the norm. These problems may include the induction of widespread copper deficiency (zinc also competes with copper), direct zinc toxicity to soil microbial function and plants, and the promotion of antibiotic resistance in soil bacteria. The last effect can occur when concentrations of a metal reach levels that cause partial die-off of soil microbial populations. Those microbes that survive and reproduce by developing resistance to the metal also tend to become resistant to antibiotics.
These potential on-farm issues can be added to a potentially serious ‘off-site’ problem that appears to be growing.
Zinc is a relatively soluble metal. Whenever it rains, a proportion of the zinc makes its way from farms to freshwaters, and on to rural lake beds or coastal marine areas (this also occurs with cadmium, but at much lower concentrations). There the zinc can become adsorbed to the sediment. As shown in Figure 4, sampling of 21 freshwater lakes in the Waikato region has revealed that most show elevated zinc levels in their sediments, indicating that it has been accumulating. In fact, zinc is the most consistently enriched of over 30 trace elements that were tested for. Coastal marine sediments bordering farming areas also show signs of zinc enrichment.
This outcome raises a new concern as to whether zinc in bed sediments might eventually reach levels that become toxic to sediment-dwelling organisms which sit at the base of aquatic food webs.
Although zinc is essential for the health of most animals, at higher concentrations it is toxic to aquatic organisms. By comparing our results to sediment quality guidelines, we can tell that zinc toxicity to sediment-dwelling organisms is unlikely to be occurring at the moment, and has not in fact reached levels already seen in some urban lakes (for which two sources of zinc are rainwater run-off from galvanised iron roofing, and particles of car-tyre rubber). However, with ongoing accumulation in the decades to come, high levels of zinc have the potential to partly ‘sterilise’ lake bed sediments and compromise some lake ecosystems.
It might be noted that facial eczema treatments are not the only source of zinc in New Zealand farming, but are overwhelmingly dominant. They are used in pastoral farming (particularly in the North Island), and pastoral farming dominates our agricultural land use. Though horticulture is an important land use, less than 0.5 per cent of our land area is devoted to it. Since plants are not treated for facial eczema, it might be assumed that zinc is not likely to accumulate in horticultural soils.
However, it does — because a separate source exists in horticulture. A widely used class of fungicide sprays contain zinc as part of their chemical structure. These compounds, called thiocarbamates, do break down in the soil, but in doing so leave the zinc behind as an indestructible inorganic payload. Horticultural soils also commonly accumulate the metal copper, through use of fungicides such as copper oxychloride.
Figure 4. Relative zinc enrichment in sediments of 21 rural Waikato lakes. For three lakes, evidence of zinc enrichment is only marginal. The others show the varying levels of zinc in sediments have significantly increased over their expected natural concentrations by factors of 2.4 to 7.9 times.
In the past, old orchards accumulated lead and arsenic from the use of the toxic compound lead arsenate as an insecticide until approximately the mid-1980s. This spray is no longer used, but many old orchards still contain arsenic at levels significantly higher than those now allowed in new residential subdivisions. Several city and district councils around New Zealand have had to grapple with what to do about residential areas that were built over the footprints of old orchards before residential standards were developed, and which have subsequently been found to contain high arsenic levels. Under a national environmental standard, assessment and remediation of such sites is now required whenever an old orchard is subdivided for residential use.
Soil chemistry can be inaccessible and hard to comprehend, but the history behind why we do what we do now often comes down to a mixture of good intentions, wrong assumptions, serendipitous accidents and enthusiastic application. New Zealand’s early attempts to control pest animal species provide a useful comparison. The Department of Conservation records that stoats, ferrets and weasels were introduced in the 1880s to control rabbits and hares. However, they preferred to feast on bird eggs and chicks. For native birds, stoats are now considered to be ‘public enemy number one’. When we look back at the reasons behind the mass use of zinc in pastoral agriculture, we see a similar set of factors at play. Missing to date are a widespread appreciation that things have now gone too far, and a motivation to change current practice before we hit various choke-points caused by excess zinc.
These considerations suggest that it is time for us to find a better way of treating or preventing facial eczema in grazing animals. A lot of innovative scientific research has actually been carried out in this area, but most has been forgotten or sidelined because zinc is so (comparatively) convenient and inexpensive.
Retrospectively, we can see that zinc treatment was a wonderful discovery for its time. However, now we are clear about the cause of facial eczema, the approach seems almost medieval. Essentially, we allow grazing animals to ingest large concentrations of a toxin, and then administer extremely high (sub-toxic) concentrations of zinc to counter the toxin, over weeks and months. Our grazing animals are having a rough time under our peculiar stewardship — it is impossible to identify any other area where we would knowingly tolerate humans or any other non-pest species ingesting high levels of a toxin in the first place.
Even this extreme approach does not always appear to work: 2016 was an extreme season for facial eczema, with fungal spore counts on some properties reaching the millions. Many animals became ill despite zinc supplementation, and a debate now exists over whether high spore counts can overwhelm zinc. It does not seem unreasonable to suggest that as the climate warms, the incidence of seasons with extreme spore counts may progressively increase.
We should not drop the ball on cadmium, either. Some of the cadmium being shunted downwards may reach regional ground water. Currently there is no evidence that the amount of cadmium reaching ground water is significant — when testing is undertaken, the limit for cadmium drinking water (4 parts per billion) has so far always been met. However, a large-scale breakthrough to ground water could occur in the future, because a large mass of cadmium has been set free from surface soils and it was last seen heading in a downwards direction.
Measures will still need to be taken to minimise the amount of cadmium in the food chain and human diet, both as part of the national Cadmium Management Strategy and more widely. In New Zealand and internationally, regulators would not like to see people eating any more dietary cadmium than they are at the moment, and the problem still exists that food standards for cadmium are occasionally being broken.
Nick Kim is a senior lecturer in chemistry and toxicology at Massey University's School of Public Health. Matthew Taylor is a soil scientist at the Waikato Regional Council. This article is an essay in Massey University's 2017 New Zealand Land & Food Annual - No free lunch. It is reposted here with permission. The New Zealand Land & Food Annual 2017, Edited by Claire Massey, published by Massey University Press, RRP: $39.99, available in bookshops nationwide.