Long-Term Biosolids Application on Land: Beneficial Recycling of Nutrients or Eutrophication of Agroecosystems?

Murray B Mcbride

doi:10.3390/soilsystems6010009

Abstract

The impact of repeated application of alkaline biosolids (sewage sludge) products over more than a decade on soil concentrations of nutrients and trace metals, and potential for uptake of these elements by crops was investigated by analyzing soils from farm fields near Oklahoma City. Total, extractable (by the Modified Morgan test), and water-soluble elements, including macronutrients and trace metals, were measured in biosolids-amended soils and, for comparison, in soils that had received little or no biosolids. Soil testing showed that the biosolids-amended soils had higher pH and contained greater concentrations of organic carbon, N, S, P, and Ca than the control soils. Soil extractable P concentrations in the biosolids-amended soils averaged at least 10 times the recommended upper limit for agricultural soils, with P in the amended soils more labile and soluble than the P in control soils. Several trace elements (most notably Zn, Cu, and Mo) had higher total and extractable concentrations in the amended soils compared to the controls. A radish plant assay revealed greater phytoavailability of Zn, P, Mo, and S (but not Cu) in the amended soils. The excess extractable and soluble P in these biosolids-amended soils has created a long-term source of slow-release P that may contribute to the eutrophication of adjacent surface waters and contamination of groundwater. While the beneficial effects of increased soil organic carbon on measures of “soil health” have been emphasized in past studies of long-term biosolids application, the present study reveals that these benefits may be offset by negative impacts on soils, crops, and the environment from excessive nutrient loading.

Highlights

The use of biosolids as agricultural fertilizers is common practice inNorth America and considered by its proponents to be a beneficial recycling of nutrients and organic matter from human excrement [1,2,3,4]
The potentially toxic trace metals (Pb, Cd, Cr, and Ni) were all at relatively low concentrations in the alkaline biosolids sampled in 2016 and 2019, levels that would not present a concern for accumulation in agricultural soils over the time frame of several decades assuming agronomic application rates
The very high Ca levels in the biosolids reflect the lime product (CaO or Ca(OH)2 with a pH of 11. 7 measured in water) that was mixed into the sewage sludges at the wastewater treatment plant to inactivate pathogens and stabilize the material [30,31]

Summary

Introduction

The use of biosolids (sewage sludges) as agricultural fertilizers is common practice inNorth America and considered by its proponents to be a beneficial recycling of nutrients and organic matter from human excrement [1,2,3,4]. The issue of soil contamination by a very large number of chemical contaminants in these waste materials, including toxic metals and persistent organic pollutants such as pharmaceuticals, dioxins, per- and polyfluoroalkyl substances (PFAS), and brominated flame retardants, has been raised by several scientists [5,6,7,8]. P concentrations in biosolids have generally not decreased and may have increased over this same time period, as tertiary treatment of wastewater to lower dissolved phosphate in treated water has become more commonplace at municipal treatment plants [13]. This process retains a larger fraction of wastewater P in the biosolids. Because farms following state and federal guidelines generally apply biosolids based upon crop N requirements, the typical ratio of

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Conclusion