Biosolids application to soybeans and effects on input and output of nitrogen

A significant portion of biosolids applied to land in Maryland are incorporated into soil planted to soybeans (Glycine max (L.) Merr.). It is generally assumed that nitrogen in biosolids reduces the process of biological Nfixation, although recent evidence suggests that this may not always be true. The objectives of this study were: (i) to determine the fate, plant uptake and cycling within the rooting zone of N derived from biosolids applied to soybeans; (ii) to assess the potential impact of biosolids application to soybeans on groundwater quality; and (iii) to determine the effects of biosolids application on soybean yield. Six field plots (0.3 ha each) were established and one-half of the plots were amended with biosolids to supply 120 kg ha^-1 available N. Plots were cropped to full season soybeans and N content of the soil periodically measured. The application of biosolids to soybean was not shown to significantly repress N-fixation. Plants continued to fix nearly normal amounts of N even in the presence of N from biosolids. This resulted in the presence of excessive quantities of N in soil following harvest. When soil was sampled to a depth of 2 m, the NO3-N concentration in the plots amended with biosolids was found to be slightly higher than under the unamended controls. Groundwater NO3-N concentrations were elevated under the biosolids amended plots; however, differences between treatments were rarely significant. It is hypothesized that the excess N under soybeans amended with biosolids may be lost to the atmosphere due to the process of denitrification.