Copper availability and selective microbiological properties of an intensively cultivated ultisol in Nuwara Eliya

Abstract

A study was undertaken to assess the effect of copper on microbiological properties of intensively cultivated vegetable fields at Nuwara Eliya. Soil samples were collected from six cultivated fields and an undisturbed forest were assessed for pH, total and DTPA extractable Cu, biomass nitrogen (BN) and substrate induced respiration (SIR). Total and Cu-resistant bacteria were enumerated using four agar media. Soil pH of experimental soils ranged from 4.44 to 5.44. Organic C content in cultivated soils varied from 1.8 to 3.3 % and it was 6.8% in forest soil. Total and DTPA extractable Cu contents varied from 14.4 to 25.6 mg kg-1 and 1.2 to 4.5 mg kg-1, respectively. Forest soil showed the highest SIR (46 μg CO2 9-1 soil h-1) and BN (267 μg N g-1 soil). The highest Cu-resistant bacterial population was 0.43% of the total population reported for the forest. Building up of Cu and increasing of population of Cu-resistant bacteria was not evident due to cultivation. The percentage of Cu-resistant bacteria correlated positively with DTPA extractable Cu (R = 0.49) suggesting that threshold Cu levels for bacterial growth in experimental soils remain within the range of extractable Cu concentrations reported. This relationship was influenced by soil organic C content and pH. Tryptic soy agar (TSA) medium produced higher percentages of Cu-resistant bacteria for both forest and cultivated soils. Those percentages showed linear relationships with total Cu (r2 = 0.95) and percentage of DTPA extractable Cu (r2 = 0.94) indicating suitability of the TSA medium to enumerate Cu-resistant bacteria.