Nitrogen fixation characteristics of Rhizobium surviving in soils 'equilibrated' with sewage biosolids

Abstract

To determine the effects of urban sewage biosolids on the symbiotic effectiveness of Rhizobium leguminosarum bv. trifolii and N₂ fixation, glasshouse and laboratory studies were carried out with several soils, biosolids, and biosolid application levels. Symbiotic effectiveness of R. l. trifolii was estimated as the dry weight or N content of seedlings of subterranean clover grown with only N₂ fixation and seed N as the available nitrogen sources. The N fixed by legumes in unamended and biosolid-amended soils was determined using the ¹⁵N isotope dilution method. Six soils were represented in the experiments. Each of these was equilibrated over a period of 12 months with dried, finely ground biosolids (DWS) from the Malabar sewage treatment plant, at biosolids levels ranging from the equivalent of 60 to 240 t DWS/ha. One of the soils was also equilibrated with each of 4 other biosolids. The maximal concentration of heavy metals in soil amended with biosolids was 1026 mg/kg.

The effect of biosolids on symbiotic effectiveness depended on the soil type and biosolid applications level. Thus, biosolids reduced the symbiotic effectiveness of R. l. trifolii in 2 of the 6 soils, although at different levels of biosolid. In most soil treatments N₂ fixation was detected in subterranean clover, confirming the persistence of symbiotically effective rhizobia in most biosolids-amended soils. In addition, in strongly acidic soils plant N and N₂ fixation increased significantly with biosolids addition, probably in response to higher soil pH, exchangeable Ca, and available P. In the treatments in which the symbiotic effectiveness of R. l. trifolii was reduced by biosolids, this was reflected in poor N₂ fixation. However, symbiotic effectiveness did not correlate well with N₂ fixation, probably because increases in soil nitrate at higher biosolids levels inhibited N₂ fixation. Nevertheless, there were instances at 240 t DWS/ha where this was unlikely to explain the decrease in N₂ fixation. It was concluded that adverse effects of biosolids on symbiotic effectiveness depend first on soil type, and then on biosolid type and application level; and the response in symbiotic effectiveness to adding biosolids to soil needs to be determined for each distinctively different site of biosolids reuse.