Nitrogen and Phosphorus Movement in Compost‐Amended Soils

Abstract

AbstractThe application of high rates of sewage sludge can pollute ground water if significant levels of nutrients move through the soil. A field study was conducted to investigate the effect of composted sewage sludge on NO3‐N, PO4‐P, and electrical conductivity (EC) levels in soil water collected at three depths. Composted municipal sewage sludge was disked into a Chester silt loam (Typic Hapludult) at rates of 0, 150 and 300 dry t/ha. White pine (Pinus strobus L.) seedlings and hybrid poplar (Populus generosa Henri) cuttings were grown on the plots, which had been seeded with ‘Kentucky 31’ tall fescue (Festuca arundinacea Schreb.). For 30 months, beginning the spring after compost application, NO3‐N, PO4‐P, and EC of the soil‐water solution were monitored in subsurface water samples taken from the Ap, B, and C horizons, which corresponded to depths of 15, 45, and 100 cm, respectively.During the year following the compost application, concentrations of NO3‐N were highest, 70–80 µg NO3‐N/mL, in the C horizon in the 300‐t/ha treatment. However, the corresponding concentrations decreased to <10 µg NO3‐N/mL by 30 months after compost addition. Total Kjeldahl N (TKN) concentrations of the soils treated with 150 and 300 t/ha of compost were approximately two and three times higher than those in control soils, respectively. The TKN concentrations in compost‐treated soils showed <7% decrease over the 30 months. The PO4‐P concentrations of water samples in compost‐treated soils were generally below 0.08 µg PO4‐P/mL, and never exceeded those concentrations found in control soils. The EC of water samples increased with the compost additions. After 267 d, EC values averaged 1.0 mmhos/cm in the 300‐t/ha treatment, and 0.7 mmhos/cm in the 150‐t/ha treatment, as compared with 0.2 mmhos/cm in the control. The EC values of the sludge compost treatments declined to <0.4 mmhos/cm by the second spring after the compost application, and EC levels were below levels toxic to plants.