Reproduced from Journal of Environmental Quality. Published by ASA, CSSA, and SSSA. All copyrights reserved. The Effect of pH on Metal Accumulation in Two Alyssum Species Urszula Kukier,* Carinne A. Peters, Rufus L. Chaney, J. Scott Angle, and Richard J. Roseberg ABSTRACT tion and larval feeding on leaves, giving these species an advantage on metal-rich soils (Martens and Boyd, Nickel phytoextraction using hyperaccumulator plants offers a po- 1994). A large number of species have developed the tential for profit while decontaminating soils. Although soil pH is considered a key factor in metal uptake by crops, little is known about ability to accumulate Zn, Cd, Ni, Co, Se, and Cu, and are soil pH effects on metal uptake by hyperaccumulator plants. Two Ni usually endemic to various metalliferous soils (Baker and Co hyperaccumulators, Alyssum murale and A. corsicum, were et al., 2000). The greatest diversity of hyperaccumulator grown in Quarry muck (Terric Haplohemist) and Welland (Typic species evolved in Ni-rich serpentine soils. The largest Epiaquoll) soils contaminated by a Ni refinery in Port Colborne, number of Ni-accumulating species belongs to the Alys- Ontario, Canada, and in the serpentine Brockman soil (Typic Xero- sum genus (Baker et al., 2000). The unique flora en- chrepts) from Oregon, USA. Soils were acidified and limed to cover demic to serpentine areas had been studied for decades pH from strongly acidic to mildly alkaline. Alyssum grown in both but the idea of using hyperaccumulator species for metal industrially contaminated soils exhibited increased Ni concentration phytoextraction from soil is relatively new. Growing in shoots as soil pH increased despite a decrease in water-soluble soil hyperaccumulator plants on Ni-rich soils and ashing har- Ni, opposite to that seen with agricultural crop plants. A small decrease in Alyssum shoot Ni concentration as soil pH increased was observed vested biomass is an economically sound alternative way in the serpentine soil. The highest fraction of total soil Ni was phyto- of producing Ni ore (Chaney, 1983; Nicks and Cham- extracted from Quarry muck (6.3%), followed by Welland (4.7%), bers, 1995; Robinson et al., 1997; Brooks et al., 1998; and Brockman (0.84%). Maximum Ni phytoextraction was achieved Chaney et al., 2000; Li et al., 2003b). This new way of re- at pH 7.3, 7.7, and 6.4 in the Quarry, Welland, and Brockman soils, moving metals is termed “phytomining.” There are two respectively. Cobalt concentrations in shoots increased with soil pH categories of soils suitable for production of Ni crops: increase in the Quarry muck, but decreased in the Welland soil. Plants serpentine soils naturally rich in Ni, which occur all over extracted 1.71, 0.83, and 0.05% of the total soil Co from Welland, the world, and industrially contaminated soils. Nickel Quarry, and Brockman, respectively. The differences in uptake pat- phytomining from industrially contaminated soils can be tern of Ni and Co by Alyssum from different soils and pH were also viewed as environmental cleanup. This dual function probably related to the differences in organic matter and iron contents of the soils. makes hyperaccumulator plants an especially promising, cost-effective alternative for decontamination of large areas of Ni-rich land. The unique ability of hyperaccumulator plants to accu- H yperaccumulator plants are species able to accu- mulate and tolerate extraordinarily high concentrations mulate very high amounts of trace metals in their of metals stimulated basic research toward a better under- tissues, at concentrations 10 to 100 times higher than standing of hyperaccumulator physiology and biochemis- tolerated by crop plants. For Ni, Co, Cu, and many other try. Less attention was paid to the effect of soil properties, metals the threshold for hyperaccumulation was set at other than total or exchangeable Ni concentrations, on 1000 mg kg -1 of shoot dry matter (Brooks et al., 1977). the Ni uptake by hyperaccumulator plants. For Zn, this threshold was established at 10 000 mg kg -1 A very important question raised regarding plant–soil– (Reeves and Brooks, 1983). Higher concentrations of metal interactions is how much soil pH affects uptake of accumulated metals are found in the shoots than the Ni and other metals by hyperaccumulator plants. This roots of hyperaccumulators (Chaney et al., 1997). This question was addressed in the recently published study is opposite to the strategy used by nonaccumulator spe- by Li et al. (2003a), who investigated the effect of chang- cies, which accumulate metals in their roots when ex- ing soil pH on Ni and Co concentrations in the shoots of posed to high soil metal concentrations. For nonaccu- A. murale and A. corsicum. Plants were grown in soils mulators, exclusion of metals from shoots and/or roots contaminated with Ni and Co by industrial emissions. and retention of metals in root cell walls and vacuoles The study resulted in the remarkable observation that has been found to be a defense mechanism minimizing an increase in soil pH was associated with increase in metal phytotoxicity. In contrast, metal accumulation in shoot Ni concentration. This observation is in contrast shoots of hyperaccumulator plants is believed to provide to an overwhelming number of published scientific data a unique method of self-defense against microbial infec- demonstrating that increased soil pH causes a decrease of Ni concentrations in various nonaccumulator plant U. Kukier and R.L. Chaney, USDA-ARS, Animal Manure and By- species grown in Ni-contaminated or serpentine soils rich Product Laboratory, BARC-W, Building 007, Beltsville, MD 20705. in Ni of geogenic origin (Crooke, 1956; L’Huillier and C.A. Peters and J.S. Angle, Department of Natural Resource Sciences, University of Maryland, College Park, MD 20742. R.J. Roseberg, Edighoffer, 1996; Kukier and Chaney, 2004). Cobalt in Oregon State University, Southern Oregon Experimental Station, this study followed the usual pattern: decrease of shoot Central Point, OR 97502. Received 4 Dec. 2003. *Corresponding concentration with pH increase. However, in another author (kukieru@ba.ars.usda.gov). study there was an indication that Ni and Co hyper- Published in J. Environ. Qual. 33:2090–2102 (2004).  ASA, CSSA, SSSA Abbreviations: DTPA, diethylenetriaminepentaacetic acid; HFO, hy- drous ferric oxide. 677 S. Segoe Rd., Madison, WI 53711 USA 2090