Discrepancy in Fertilizer Nitrogen Equivalency Estimates of Dry Sewage Sludge Application in Soil

Abstract

Fertilizer nitrogen (N) equivalency estimates of sewage sludge are essential to assess benefits from its use in agricultural fields and minimize agronomic failure and environmental risks. Using an outdoor pot experiment, where sludge was applied to the upper layer of the soil in pots, this study aimed to estimate the rate of dried sewage sludge application needed to replace a given amount of inorganic fertilizer and to investigate eventual variations of this estimate depending on whether N replacement value referred to grain yield of barley, biomass production, or overall plant N uptake. Equivalencies were obtained from the response curves of these crop characteristics in relation to the rate of sludge application. It was shown that the patterns of carbon (C) and N allocation to grain and straw were different between the inorganic fertilizer and the sewage sludge treatment, presumably due to the timing of mineral N provision. When the same grain yield was obtained at these two treatments, straw yield and grain N content were greater at the sludge fertilized crop. It was concluded that fertilizer N equivalency value of sewage sludge should inevitably refer to a single crop yield component determined by a producer.

Keywords:

• Biosolids
• fertilizer equivalency
• fertilizer N replacement value
• sewage sludge
• soil amendment

Acknowledgments

The author thanks S. Anastasiou and M. Elia for technical assistance and the staff of Central Chemistry Laboratory of ARI.