Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/21/2005
Publication Date: 1/10/2006
Citation: Wang, A.S., Angle, J.S., Chaney, R.L., Delorme, T.A. 2006. Changes in soil biological activities under reduced soil ph during thlaspi caerulescens phytoextraction. Soil Biology and Biochemistry. 38(6):1451-1461. Interpretive Summary: Phytoextraction is a new method to remove contaminant metals from soils which is likely to be used on soils which comprise risk to human health. Mine-waste contaminated rice soils have caused adverse cadmium health effects on subsistence rice farmers in several nations. In order to maximize annual phytoextraction, soil pH is lowered. The present experiment tested if lowering pH to improve annual Cd phytoextraction caused an adverse effect on soil microbial populations or enzymatic activities. Soils were collected at two sites near a Zn smelter at Palmerton, PA, USA. Soils were acidified using sulfur addition and incubation until constant pH was reached, with daily mixing to speed the process; then soils were leached to remove soluble salts formed during acidification. Thlaspi caerulescens from southern France was grown for six months and removed large amounts of Cd from both soils. After the cropping soils were examined for effect of pH adjustment and presence of plants (rhizosphere) on activity of enzymes and density of classes of microbes. For both soils the lowest pH levels caused reduced yield of the phytoextraction crop, and caused moderate to strong reduction of biological activities. Interestingly, the reduction in activities at acidic pH was greater for the lower metal concentration soil, perhaps supporting the “metalloregion” model where contamination over time causes selection of more metal resistant organisms. For both soils, an intermediate pH gave maximum annual Cd phytoextraction with only small effect on soil biological activities, indicating that careful management can prevent adverse effects of phytoextraction on soil ecology.
Technical 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 are 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 5 or 6 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 R2 values ranging from 0.57-0.99. 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.