Changes in soil biological activities under reduced soil pH during Thlaspi caerulescens phytoextraction Autumn S. Wang a, * , J. Scott Angle a , Rufus L. Chaney b , Thierry A. Delorme c , Marla McIntosh a a Department of Natural Resources and Landscape Architecture, University of Maryland, 2109 Plant Science Building, College Park, MD 20742, USA b USDA-ARS Animal Manure and By-products Laboratory, Beltsville, MD 20705, USA c Kent State University, Ashtabula, OH 44004, USA Received 2 May 2005; received in revised form 21 October 2005; accepted 1 November 2005 Available online 10 January 2006 Abstract Phytoextraction of soil Cd and Zn may require reduction in soil pH in order to achieve high metal uptake. Reducing the pH of high metal soil, however, could negatively affect soil ecosystem function and health. The objectives of this study were to characterize the quantitative causal relationship between pH and soil biological activities in two Zn and Cd contaminated soils and to investigate the relationship between metals and soil biological activities under low pH. Soils were adjusted to five or six different pH levels by sulfur addition, followed by salt leaching. Thlaspi caerulescens was grown for 6 months, and both the rhizosphere and non-rhizosphere soil biological activities were tested after harvest. Reducing pH significantly lowered soil alkaline phosphatase activity, arylsulphatase activity, nitrification potential, and respiration. However, acid phosphatase activity was increased with decreasing pH. The relationship between soil biological activities and pH was well characterized by linear or quadratic regression models with R 2 values ranging from 0.57 to 0.99. In general, the three enzyme activities, nitrification potential, and the ratio of alkaline phosphatase to acid phosphatase activity were very sensitive indicators of soil pH status while soil respiration was not sensitive to pH change. The rhizosphere soil had higher biological activities than non-rhizosphere soil. The negative effects observed in the non-rhizosphere soil were alleviated by the rhizosphere influence. However, rhizosphere soil after 6 months phytoextraction showed lower nitrification potential than non-rhizosphere soil, probably due to substrate limitation in our study. q 2006 Elsevier Ltd. All rights reserved. Keywords: Soil pH; Biological activity; Soil enzyme; Soil respiration; Nitrification; Thlaspi caerulescens; Rhizosphere 1. Introduction Phytoextraction uses unusual hyperaccumulator plants to accumulate high quantities of metals in plant biomass. It offers a low cost strategy to clean up contaminated soils and the plant ash may also have economic value (Baker et al., 1994; Chaney et al., 2000). Thlaspi caerulescens is an accumulator of Cd and Zn and is one of the most studied of all hyperaccumulators. The hyperaccumulation process of T. caerulescens involves rapid uptake, high rates of translocation from roots to shoots, and huge storage capacity by vacuolar compartmentalization (Chaney et al., 1997). However, the first step is uptake rate- limiting and thus critical to the effectiveness of the phytoextraction success. Plant uptake of Cd and Zn is generally limited by metal solubility. Increasing metal solubility usually results in enhanced uptake and higher shoot metal concen- tration (Brown et al., 1995a). The success of phytoextraction depends on appropriate soil management practices to make metals more available to plants. Among the diverse strategies to enhance phytoextraction, pH adjustment has received the most attention, because bioavail- ability of Cd and Zn is largely controlled by soil pH. Theoretically, lowering pH will increase solubility of most metals. Studies conducted on other crops have shown a negative correlation between soil pH and metal transferred to plants (Narwal et al., 1983; Castilho and Chardon, 1995). Only a few studies have examined the effect of soil pH on T. caerulescens hyperaccumulation (Brown et al., 1994, 1995b). Although reducing soil pH appears to be an effective strategy to enhance Cd and Zn phytoextraction, precaution is needed because low pH and elevated metal concentrations may cause negative impacts to already vulnerable soil ecological systems. How do we increase phytoextraction efficiency Soil Biology & Biochemistry 38 (2006) 1451–1461 www.elsevier.com/locate/soilbio 0038-0717/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.soilbio.2005.11.001 * Corresponding author. Tel.: C1 301 405 1346; fax: C1 301 314 9089. E-mail address: wspring@wam.umd.edu (A.S. Wang).