Symbiotic effectiveness of Rhizobium trifolii and mineralization of legume nitrogen in response to past amendment of a soil with sewage sludge

• Published in: Journal of sustainable agriculture 1997-01, Vol.11 (1), p.23-37
• Main Authors: Munn, K J, Evans, J, Chalk, P M, Morris, S G, Whatmuff, M
• Format: Article
• Language: English
• Subjects: Rhizobium trifolii
• ISSN: 1044-0046

The effects of sewage sludge contaminated with heavy metals on the symbiotic effectiveness of R. trifolii, white clover N sub(2) fixation and N mineralization were investigated in surface (0-10 cm) soil from a 10 year-old field experiment at Glenfield, NSW. The soil had been amended with nil, low (40 dry t ha super(-1)) or high (120 dry t ha super(-1)) rates of sludge, applied annually for 5 consecutive years to an unlimed (pH 4.6) and limed (pH 5.2) red podsolic soil. The total heavy metal content of the soil collected from the site 5 years after the last sludge application ranged (mg kg super(-1) soil) 114-122 (nil sludge), 286-295 (low sludge) and 532-583 (high sludge). Air-dried soil was sieved (< 2 mm), then placed in pots which were sown to white clover and ryegrass, or to ryegrass only. The amount and proportion of clover N derived from air after 7 weeks growth in a glasshouse were determined using super(15)N isotope dilution; and the intrinsic symbiotic effectiveness (ISE) of R. trifolii isolated from the clover nodules was estimated from the N yield of clover seedlings grown on N-free seedling agar inoculated with the isolates. Treatment effects on N mineralisation were determined from the concentrations of inorganic N released on soil incubation with or without incorporated lucerne. Amending soil with sludge did not reduce the ISE of R. trifolii in either unlimed or limed soil, the between-isolate variance in ISE, nor the proportion of clover N derived from air, as compared to unamended soil. The amount of N fixed by clover was increased in the sludged soils. The concentrations of inorganic N in the N mineralisation study were greater in the sludge amended soils and with incorporation of lucerne; and the differences in inorganic N between plus and minus lucerne incorporation were similar in both the sludged and unamended soils. The sludge rates and associated heavy metal loads historically imposed on this soil have not proven detrimental to N sub(2) fixation by R. trifolii, or to the net mineralisation of soil or incorporated organic N.