Cadmium toxicity and soil biological index under potato (Solanum tuberosum L.) cultivation Waseem Hassan A,B,F , Rizwana Bano C , Safdar Bashir D , and Zubair Aslam E A Landwirtschaftlich-Gärtnerischen, Humboldt-Universität zu Berlin, 14195 Berlin, Germany. B Department of Soil and Environmental Sciences, Muhammad Nawaz Shareef University of Agriculture, Multan, Pakistan. C On Farm Water Management, Multan, Pakistan. D Institute of Soil and Environmental Sciences, University of Agriculture Faisalabad, 38000, Pakistan. E Department of Agronomy, University of Agriculture Faisalabad, 38000, Pakistan. F Corresponding author. Emails: wasagr@yahoo.com; h.wasagr@hu-berlin.de Abstract. Increasing cadmium (Cd) pollution in soil is of great concern. A pot experiment was conducted with the aim of assessing the effect of Cd on soil biological indices under potato cultivation. Cadmium was added to 10 kg soil in each pot (6 seeds pot –1 ) as Cd(NO 3 ) 2 at 0, 15, 30, 45 and 60 mg kg –1 with three replications. All soil and plant parameters decreased with all Cd treatments; however, high levels of Cd had a significant (P < 0.05) suppressive effect. The highest Cd level significantly (P < 0.05) decreased microbial biomass carbon (2.16-fold), nitrogen (11.37-fold) and phosphorus (10.3-fold), as well as enzyme activities of dehydrogenase (4.36-fold), phosphatase (9.23-fold), and urease (9.61-fold). The highest Cd level also decreased pH (1.46-fold), potato shoot (3.55-fold) and root (7.43-fold) length, root (10.9-fold) and shoot (6.04-fold) fresh weight, root (7.51-fold) and shoot (13.7-fold) dry weight, chlorophyll content (27.0-fold), carotenoid content (4.08-fold), and plant macronutrient and micronutrient uptake in potato root and shoots. Conversely, the highest level of Cd significantly (P < 0.05) increased the biomass C : N (5.27-fold) and C : P (4.77-fold) ratios, soil extractable Cd (5.38-fold), and Cd uptake in potato root (5.05-fold) and shoot (4.82-fold) at the end of the experiment (day 60). Cadmium contamination substantially affected soil biological indices and growth of potato, and the Cd threshold was strongly associated with the extent of Cd concentration and duration to accumulate. Soil microbial biomass, enzymatic activities, pH and potato physiological parameters could be used as a sensitive indicators to reflect environmental stresses in soil ecosystems. Additional keywords: Cd toxicity, incubation time, potato plant, soil microbiological index. Received 11 July 2014, accepted 4 September 2015, published online 20 June 2016 Introduction Contamination of agricultural soils with heavy metals is a global problem and a major environmental threat for the past several decades (Azevedo et al. 2012; Hassan and David 2014). Heavy metals are not subjected to degradation, and they therefore remain almost indefinitely in the environment and accumulate in different parts of the food chain (Majid et al. 2012; Hassan et al. 2013a). Heavy metals at high concentrations generally affect the growth morphology and metabolism of soil biota (Hassan et al. 2014a). Extreme accumulation of heavy metals in agricultural soils has long-term hazardous impacts on soil biological processes. It also elevates the uptake of heavy metals by crops and affects food quality and safety (Wang and Yanli 2013; Hassan and David 2014). Among the heavy metals, cadmium (Cd) is ubiquitous. It plays a major role in the contamination of soil and atmosphere, due to its relatively high mobility in the soil–plant system (Hassan et al. 2014a). Cadmium has been mined and used by humans ever since metals were first produced from ores; however, the toxicity of Cd was only recognised in the 1970s (Nordberg et al. 2007). The average annual production of Cd throughout the world is reported to have increased from 20 t (in the 1920s) to ~20 000 t owing to emerging industrial activities (Wang and Yanli 2013). According to an estimate, ~25 000 t Cd year –1 is released into the environment. Of this Cd emission, ~85–90% is estimated to be due to anthropogenic activities such as smelting and refining of nonferrous metals, fossil fuel combustion and municipal waste incineration (Azevedo et al. 2012). Cadmium-affected soils contain Cd in the range of 15–35 mg kg –1 , whereas the industrialised areas contain Cd up to 80 mg kg –1 (Azevedo et al. 2012; Wang and Yanli 2013). High concentrations of Cd have negative effects on many ecological and microbiological parameters and ultimately on soil quality and health (Hassan et al. 2013a). Similarly, Cd toxicity affects the morphology, growth and photosynthetic processes of plants. It causes Journal compilation Ó CSIRO 2016 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2016, 54, 460–468 http://dx.doi.org/10.1071/SR14360