How Copper Oxide Affects the Soil on Farmlands
Research suggests that CuO NPs affect soils by reducing the degree of nitrification, denitrification, and soil respiration that happens in them. This has consequences for the plants that are dependent on the products of these processes for their thriving.
Nitrification
In nitrification, soil ammonia is oxidized into nitrates (NO2- ) and then into nitrites (NO3-), in a two-step conversion involving different kinds of microorganisms. The conversion of ammonia into nitrates is carried out by autotrophic bacteria and Achaea, and the transformation into nitrites is done by Nitrobacter and nitrospira.
These two processes are necessary for the eventual release of nitrogen, with which plants produce proteins essential for their growth.
When CuO NPs are applied to the soil in significant quantities, they could inhibit this process and shrink the production of vital nitrates. This appears to be more severe in coarse and medium-textured soils, where the nanoparticles prove most toxic for nitrifying bacteria.
Denitrification
Heterotrophic bacteria also breaks down nitrates into molecular nitrogen in a process known as denitrification. As already explained abovelants rely on nitrogen to make their cell proteins. When bacteria that make nitrogen available is adversely affected by CuO NPs, the plants that benefit from their nitrogen yielding activity may also suffer.
Some researchers have established that CuO NPs have an even more significant inhibiting effect on denitrification compared to nitrification. One 90 day study reports reductions in soil denitrification rates of between 20 and 40 percent in the presence of 100 mg/g of CuO NP. Of the five kinds of soils examined in the study, silty clay soils were found to be the most sensitive to this inhibiting effect.
Soil Respiration
When soil microorganisms respire, they produce carbon dioxide (CO2). The CO2 is released into the air, captured by plants, and converted into organic compounds through photosynthesis. Plants either use the compounds to build their structure or release energy from them during respiration.
Respiring soil organisms are also affected by toxic quantities of CuO NP. With their actions forced into the decline by this compound, there’s less CO2 being released into the air from the soil, a situation which may affect the agricultural ecosystem over the medium to long term.
How Plants are Affected
Available evidence suggests that crops are impacted differently by similar amounts of copper nanoparticles, at least to an extent. But consequences arising from exposure to significant amounts of the compound which seem relatively common to a lot of crops surveyed by researchers.
Plants exposed to CuO NPs could suffer stunting, cell death, and the loss of leaf coloration. Overexposure could also result in a broadening of the roots and stems of the crops.
Increased Toxicity
One study examined the possible outcomes from a direct exposure scenario. About 10 or 250 milligrams of Copper Oxide were applied on leaves of lettuce and cabbage plants on a daily basis, for between 5 and 15 days.
At the end of 15 days, it was found that the lettuce had absorbed more than 300 times the amount of the compound than unexposed control plants, and cabbage plants had taken in over 400 times the normal amounts found in unexposed crops. Some of the Copper Oxide did remain on the surface of the leaves, but a lot of it was absorbed into the plants through their stomata.
These large amounts may have consequences for the people who consume crops that have been overexposed to CuO NPs. Long term copper poisoning is known to damage the human liver and kidneys.
Stunting
There are noticeable growth impairments to plants exposed to copper oxide. Copper nanoparticles were said to have reduced crop growth in the study of cabbage and lettuce referred to earlier. Plants lost up to 60 percent of their water content.
In another study carried out on rice in paddy fields, it was discovered that exposure to CuO NP caused roots to thicken and shorten. The authors of that report suggested that this was more frequently observed for crops planted in clay-rich soils.
Seedlings growing in soils with a fair amount of CuO NPs may grow at a much slower pace than normal. Rates of growth retardation approaching 20 percent have been reported in at least one published paper.
However, small amounts of copper nanoparticles may contribute to plant growth. This too is well accounted for. And it’s not surprising; copper is an essential micronutrient for plants.
Changes in Leaf Pigmentation
This change isn’t always prominent, but it’s important nonetheless. For example, potato crops grown in soils with excess copper may exhibit pale or white bleaching around their veins. Some areas around the veins may be sunken, but the greater part of the affected leaves retain their normal colour.
Reduced chlorophyll levels and alterations to plants’ chloroplast structure are known to affect plants grown in such conditions (crops like rice, spinach, and wheat are particularly impacted by this). Because the excess copper restricts the photosynthetic process, it modifies the pigment of the plant’s cell membranes and also changes its protein composition. Some of these transformations can be observed in leaves that assume an unusual colour after prolonged exposure to copper nanoparticles.
Shortage of Other Essential Micronutrients
One crucial effect of excess copper nanoparticles in the soil is the accompanying deficiency of iron (Fe). There’s a tendency for copper to ‘compete’ with iron, especially in the soil around crops. In a study involving bean plants, a five-fold increase in the amounts of copper in the soil was followed by a four-fold decrease in the quantity of iron around the same soil region.
This depletion of iron may cause leaves to turn yellow. This happens because of the absence of chlorophyll, which can’t be produced without iron.
There’s also evidence that the presence of considerable deposits of copper in paddy fields may prevent rice planted in such fields from absorbing zinc (Zn). Manganese (Mn) and Aluminum (Al) are similarly impacted by large concentrations of copper nanoparticles.
Differences in Tolerance for Copper Nanoparticles Among Plants
Some plants are more sensitive to CuO NPs than others and may exhibit symptoms of overexposure at very different degrees of contamination.
Lettuce may collect more copper nanoparticles than spinach. Alfalfa has a higher tolerance for it than, say, tomatoes. But there’s also some variance in the amounts accumulated in different plant organs. In lettuce, up to 80 percent of the copper accumulated stays in the roots. But it’s almost evenly distributed across the plant in spinach.
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