Growing Opuntia (cactus) and Brassica species for the long-term remediation of selenium-contaminated soil under field conditions

Authors: Banuelos, Gary; SCHULIN, R - University Of Zurich

Submitted to: Biomedical Research on Trace Elements
Publication Type: Book / Chapter
Publication Acceptance Date: 10/20/2009
Publication Date: 12/28/2009
Citation: Banuelos, G.S.and R. Schulin.2009.Growing Opuntia (cactus) and Brassica species for the long-term remediation of selenium-contaminated soil under field conditions.Biomedical Research on Trace Elements.Chihuahua Press, Chihuahua, Mexico. pp:427-429.

Interpretive Summary: Mustard and cactus species were evaluated as potential crops for low quality soils in the Westside of central California. These plants may possess the ability to survive in the arid and semiarid region and more importantly survive in high salt and boron conditions. We evaluated their abilities to survive, volatilize and accumulate selenium under adverse field conditions. Mustard was directly planted by seed, and cactus was propagated by planting cut leaves (cladodes) directly into the saline soil high in boron and selenium. As the plants matured, measurements were taken for gaseous selenium expelled by both plant species. Mustard produced at least 80% more gaseous selenium than cactus during the growing season. Mustard seed was harvested, and the selenium-enriched seed was eventually processed for its oil. This oil was used as a blend for making biofuel, while the residual seed meal was tested as a biofumigant for preventing weed emergence. Cladodes and fruit were subsampled annually and showed an accumulation of selenium; selenium-enriched cactus parts. Growing mustard and cactus as alternative crops in poor-quality soils of the westside of central California may gradually remove Se from the soil via volatilization and plant accumulation, but importantly, plant products (e.g., biofuel, biofumigant, Se-enriched cladodes and fruit) from both species may have economic and nutritional value as alternative crops for the westside of central California.

Technical Abstract: Identifying alternative crops for planting in Se-containing agricultural soils of western central California will depend upon the plants’ ability to tolerate high salt and boron (B) conditions. Multi-year field studies were conducted on Se-laden soils with different cactus clones (Opuntia-ficus indica) as perennial crops, and with mustard (Brassica juncea) as an annual crop. Eight-week-old root-bearing cladodes and 4-week old mustard plants were transplanted into a Se-laden drainage sediment soil, with an electrical conductivity (EC) of 6.2 dS/m, extractable B of 6.5 mg/L and a total Se of 2.7 mg/kg DM in top 50 cm of soil, and irrigated by surface drip. Using volatilization chambers, measurements for Se volatilization were made in the mustard field at each growing season and on cactus clones at 2nd year of growth. Mustard was harvested annually for seed, while cladodes and prickly-pear fruit from cacti were harvested in 3rd year of growth. Plant tissues were analyzed for Se. Results show rates of Se volatilization as high as 65 and 105 µg Se/m2/d in the Opuntia clones and in the mustard fields, respectively, while mean plant Se concentrations were 7.8 and 6.1 mg/kg DM in cladodes and fruit, respectively. Mustard seed yields were at least 1 ton/acre and yielded about 570 L/acre of 100% potential biodiesel. After transesterifcation, the mustard oil was then used as a blend for making BD 20 biodiesels (80% diesel and 20% mustard oil), while the residual seed meal was tested as a biofumigant for preventing weed emergence. Preliminary testing rates of 2 to 8 tons/acre show that weed emergence was inhibited in different weed species at the high rates of application. Successfully growing Opuntia and mustard as alternative crops in poor quality soils not only accumulates Se gradually from the soil, but importantly, plant products from the two species may have economic values for irrigated agriculture in the westside of central California.