Effects of potentially toxic metals in soil derived from past applications of sewage sludge on nitrogen fixation by trifolium repens L

TLDR: In this article, the growth and N2-fixation of Trifolium repens were measured in pots, using 15N techniques, on either metal-contaminated or uncontaminated soils which came from field plots which last received sewage sludge (Contaminated) or farmyard manure (uncontaminate) more than 20 years ago.

Abstract: The growth and N2-fixation of Trifolium repens was measured in pots, using 15N techniques, on either metal-contaminated or uncontaminated soils which came from field plots which last received sewage sludge (contaminated) or farmyard manure (uncontaminated) more than 20 years ago. Plants grown on sludged soil had lower concentrations of N and smaller yields; the root nodules were small, white, numerous and ineffective in N2-fixation. By contrast, nodules on plants grown on farmyard manure-treated soil were pink (indicating the presence of leghaemoglobin), large and few in number, characteristic of effective nodules. The depressive effect on yield was overcome, at least in the short-term, by adding nitrogen fertilizer to the soil. Concentrations of metals in the leaf material were low and direct phytotoxic effects unlikely. The two soils were mixed in various proportions to test the effects of increasing concentrations of extractable metals in the soil on N2-fixation. A 50% reduction in fixation was observed in soil containing approximately the following concentrations (mg kg−1) of metals extracted by EDTA: 165Zn, 60 Cu, 7.3 Ni and 5.3 Cd. Corresponding “total” concentrations (mg kg−1) were 334 Zn, 99 Cu, 27 Ni and 10 Cd. These results are discussed in relation to the current guidelines designed to limit the accumulation of metals in agricultural soils receiving sewage sludge.