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Residual Effects of Sewage Sludge on Soybean: II. Accumulation of Soil and Symbiotically Fixed Nitrogen¹

ABSTRACT

Nitrogen and heavy metals derived from sewage sludge may potentially affect N₂ fixation and soybean [Glycine max (L.) Merr.] growth. Field studies were conducted at two locations in 1983 and 1984 to examine the residual effects of sewage sludge on N₂ fixation in soybean. The Fairland plots (Typic Hapludults) were amended with two rates of anaerobically digested sludge in 1975. The Beltsville plots (Typic Paleudults) were established in 1976 with limed-anaerobically digested, limed-raw, limed-compost, or heat-treated sludge, and in 1978 with Nu-Earth (Chicago, IL) anaerobically digested sludge. Nonnodulating and nodulating isolines of ‘Clark’ soybean were grown on the sludge-treated plots, which consisted of various sludge rates and pH regimes. Dry matter and total N accumulation were determined in the shoots at the R4 stage of maturity. Dry matter and N accumulation by nonnodulating soybean generally increased due to greater availability of soil N on the sludge plots. In nodulating soybean, dry matter production and N₂ fixation were enhanced by most sludges with the greatest increase occurring under moisture stress. In 1983, a dry year, dry matter of nodulating soybean and N₂ fixation (estimated by the difference method) were increased by all sludges at Beltsville. During this year, dry matter and amount of N fixed increased from 16.5 g plant^–1 and 285 mg N plant^–1 (control) to 31.1 g plant^–1 and 706 mg N plant^–1 at the 672 Mg ha^–1 limed-compost sludge rate, respectively. In 1984, a normal rainfall year, dry matter and N₂ fixation was moderately higher on sludge plots compared to control levels. Dry matter and N₂ fixation showed a quadratic response due to overliming by high applications of limed-digested and limed-raw sludges. Evidence for toxicity of sludge metals on N₂ fixation was not found at Beltsville sludge plots. However, a toxic effect may be suggested by high metal sludge at Fairland sludge plots. In addition to a decrease in the amount of N₂ fixed, nonnodulating plant dry matter increased with sludge, whereas nodulating plant dry matter decreased. Results suggest that while sludge amended soil may enhance soybean growth, such beneficial results may be limited to sludges low in heavy metals.

Key Words: yield • rhizobia • Mn • Zn • Cd • Cu • Ni • lime • pH • heavy metals • Glycine max (L.) Merr.

¹ Scientific Article no. 7295 and Contribution no. A-4306 of the Maryland Agric. Exp. Stn., Dep. of Agronomy, College Park, MD 20742.

² Graduate Research Assistand and Assistant Professor, Dep. of Agronomy, Univ. of Maryland, College Park, MD 20742; and Research Agronomist, USDA-ARS, Beltsville, MD 20705.

Received for publication December 9, 1985.