Long‐Term Application of Biosolids on Apricot Production

Abstract

The use and disposal of biosolids, or wastewater treatment sludge, as a fertilizer and soil amendment is becoming increasingly widespread. We evaluated the multiyear use of biosolids in apricot (Prunus armeniaca L.) production, grown on productive agricultural soils. Class A biosolids were initially applied annually at rates of 0, 1.9, 5.8, and 11.7 Mg · ha⁻¹ (dry basis) to a 2‐year‐old apricot orchard on the USDA‐ARS research site on the eastern side of the San Joaquin Valley, CA. These application rates provided estimated rates of 0 (control), 57, 170, and 340 kg total N · ha⁻¹ yr⁻¹, respectively. Compared to the control treatment, the applications of biosolids significantly increased soil salinity (electrical conductivity from 1:1 soil–water extract) and total concentrations of nutrients [e.g., calcium (Ca), magnesium (Mg), sulfur (S), phosphorus (P), zinc (Zn), and copper (Cu)] after 7 years but did not increase the concentrations of selected metals [cadmium (Cd), chromium (Cr), cobalt (Co), nickel (Ni), and lead (Pb)] between 0‐ and 60‐cm soil depths. Mean concentrations of total nitrogen (N) and carbon (C) in soils (0‐ to 15‐cm depth) ranged from a low of 1.3 g kg⁻¹ to a high of 5.2 g · kg⁻¹ and from 14.1 g · kg⁻¹ to 45.7 g · kg⁻¹ for the control and high biosolids treated soils, respectively.

Biosolids applications did not lead to fruit yield reductions, although fruit maturation was generally delayed and more fruits appeared at picking times at the high rate of application. Yellow fruits collected from all biosolids applications were significantly firmer than were fruit collected from control trees, and they had higher concentrations of Ca, potassium (K), S, iron (Fe), and Zn in the fruit. Among the fruit quality parameters tested, the juice pH, total acidity, and fruit skin color were not significantly affected by biosolids applications. Malic acid concentrations decreased most of the time, while citric acid concentrations increased with increasing rates of biosolids applications. Overall, our results suggest that nonindustrial biosolids applied at an annual rate at or less than 11.7 Mg N · ha⁻¹ (340 kg N · ha⁻¹) can be safely used for apricot production on sandy loam soils.

Keywords:

• Apricot
• biosolids
• fruit quality
• nutrients
• sewage sludge
• trace elements

Acknowledgments

We acknowledge the financial support from Agriculture Research Initiative, California State University, Fresno (CSUF), and USDA‐ARS‐WMR. We thank Sanliang Gu at Viticulture and Enology Department CSUF, for his useful comments, and Irvin Arroyo and Tom Pflaum at USDA‐ARS‐WMR for their analytical assistance.