Response of soil microbial biomass and enzymes activity to cadmium (Cd) toxicity under different soil textures and incubation times

摘要

The contamination of agricultural soils by heavy metals is a global problem. Soil texture as a biotic factor represents one of the most important factors that influences the distribution of organic matter and ultimately play decisive role in retention of heavy metals in soil ecosystems. An incubation experiment was conducted to estimate the response of soil microbial biomass and enzymes activity to Cd toxicity and how this toxicity behaves under different textures. Three different textured soils (sandy loam, clay loam and loam) were collected from three diverse areas across Potohar region of Pakistan and were incubated at five different Cd levels (0.00, 50, 100, 150 and 200 mg kg-1). Microbial biomass carbon (Cmic), nitrogen (Nmic), phosphorous (Pmic), and enzymes activity (dehydrogenase, urease and phosphatase) were quantified at 10, 20 and 30 days interval after Cd-contamination. The application of Cd had strong negative effect on the size of soil microbial biomass and enzyme activities. The highest Cmic, Nmic, Pmic, dehydrogenase, urease and phosphatase (137.30, 16.30, 9.56, 60.93, 5.30 and 33.63 mg kg-1 soil) activities were observed at control in loamy texture, whereas the lowest values (21.40, 0.50, 0.20, 6.66, 0.20 and 0.70 mg kg-1 soil) were observed at 200 mg Cd kg-1 in the sandy loam soil. Moreover, texture and incubation time had profound effect on Cd toxicity to microbial biomass and enzymes activities. The soil texture was positively linked with Cd toxicity, among textural classes, clay loam and loam were more resistive and having maximum quantities of soil microbial biomass and enzyme activities than sandy loam. Cd toxicity increased with the incubation time, therefore the lowest microbial biomass and enzymes activity was observed at thirty days of incubation. Our results revealed that soil microbial biomass and enzyme activities were strongly inhibited by Cd. Furthermore, Cd toxicity changed under different textures deducing that Cd threshold is strongly associated with texture of the soils.