Quinoa Production in the Urban Environment

Introduction

With urban agriculture becoming more prevalent in cities comes the need for health assessments of crops grown in urban environments. Many urban farms are located on “brownfields,” lots that have become contaminated with trace metals. Plants grown in brownfields can take up trace metals from contaminated soil, irrigation water or dust that settles on leaf surfaces, posing serious human health risks if these metals accumulate at high levels in edible plant parts. Heavy metal uptake patterns vary greatly between plant species and are influenced by the conditions and history of growing locations. 

Quinoa (Chenopodum quinoa) is a specialty crop that is well suited to grow in Vancouver’s climate. It has a deep taproot and extensive fibrous root system which allows it to grow in poor soils; however this also increases its ability to uptake heavy metals. Researchers investigated the pattern of heavy metal uptake in quinoa in order to determine the feasibility of growing quinoa in urban farms for consumption in Vancouver. More specifically, researchers looked at differences in heavy metal accumulation in different plant organs and at different life stages in quinoa.

Research Process

Urban soil was collected to a 30 cm depth from a Vancouver brownfield site and used to grow potted quinoa plants in a greenhouse at the University of British Columbia. The experiment was split into two groups: Group A had quinoa plants growing in the original brownfield soil, while Group B was grown in soil spiked with Zinc, Cadmium, Lead, Nickel and Copper. Each group had 8 identical pots, making a total of 16 pots for the experiment. Two seedlings were transplanted into each pot. One of these seedlings was removed for assessment 30 days after transplant (DAT), and the other was removed and assessed 110 DAT. 

Four parts of the quinoa plants were analyzed for heavy metal contamination: the roots, stems, foliage and seeds. Quinoa seed contamination levels were assessed using guidelines developed by the European Scientific Committee for the World Health Organization to determine if they were fit for human consumption. The metals assessed in this experiment were cadmium, lead, nickel, copper and zinc.

Results

In young plants (30 DAT), cadmium, lead and nickel concentrations were highest in stems, and copper and zinc concentrations were highest in the foliage. Copper and zinc were found at even higher levels in roots. Cadmium, nickel, lead and zinc were found in much lower concentrations in the foliage tissue of mature (110 DAT) plants. Nickel, lead and cadmium concentrations were highest in the seeds of mature plants, while zinc and copper concentrations were highest in foliage. All metals assessed were more localized in the roots of mature plants than above-ground plant parts. 

Seed concentrations of cadmium, lead, nickel and copper are lower when more seed is produced per plant. This suggests that more productive plants grown in similar soils may show less contamination in their seeds. When the average concentrations of all trace metals were compared to the European Scientific Commission’s Health Standards, it was found that the quinoa seeds had higher levels of copper and lead than recommended for safe human consumption.

Implications

 Metal concentrations found in plant tissues were higher than their original concentrations in soil, showing that quinoa is very sensitive to heavy metal contaminated soil. All metals tested were found at higher concentrations in the roots than in above-ground plant tissues, however, seeds may be unfit for human consumption due to unhealthy levels of copper and lead. Due to the patterns of translocation seen in quinoa, the plant may be useful in the context of bioremediation by taking up and removing heavy metals from soil, but should not be grown for consumption on contaminated sites due to its high level of heavy metal bioaccumulation. 

Urban farmers should be careful in assessing the history of potential farm sites prior to choosing heavy metal-sensitive plants such as quinoa, and if possible, select sites away from roadways to reduce contamination from traffic. Certain agronomic practices such as planting hedges or barriers near roadways, and using hoop houses or row covers can manually protect plants from contamination. Finally, since metals are more mobile in acidic soil, maintaining a neutral soil pH level may decrease the extent of heavy metal uptake in plants.