Surface-Applied Biosolids Enhance Soil Organic Carbon and Nitrogen Stocks but Have Contrasting Effects on Soil Physical Quality

Table 2

Means (se) for selected soil properties in biosolids-amended soils (0–30 cm).

Application rate (Mg ha^−1 yr^−1)	20	40	60	20
Application duration (yr)	8	8	8	20^a
Sand^b (%)	21.3 (2.3)	22.2 (2.9)	24.4 (1.6)	25.5 (2.1)
Silt (%)	55.0 (1.0)	56.1 (1.3)	53.5 (1.7)	52.7 (0.6)
Clay (%)	23.7 (2.0)	21.7 (2.1)	22.1 (1.3)	21.8 (2.0)
pH	7.50 (0.03)	7.60 (0.04)	7.62 (0.03)	7.49 (0.03)
Electrical conductivity (mS cm^−1)	0.69 (0.03)	0.70 (0.04)	0.74 (0.12)	0.54 (0.05)
Extractable (mg N kg^−1)	25.4 (6.8)	31.8 (3.6)	39.5 (8.5)	14.3 (1.2)
Water-soluble (mg P kg^−1)	55.0 (21.1)	22.2 (2.3)	28.6 (2.2)	90.4 (7.5)
Particulate organic matter (mg g^−1)	10.9 (1.0)	8.6 (0.4)	9.2 (1.1)	12.9 (1.0)

Annual applications in this treatment reduced to biennial applications of 5 Mg ha⁻¹ yr⁻¹ in 2007. ^bParticle size distributions (sand, silt, and clay) were determined by hydrometer [18]; pH and electrical conductivity were determined by electrode in 1 : 1 water solution [19], EPA 9045D; 2M KCl-extractable and water-extractable were measured in 1 : 10 soil extracts using continuous flow colorimetry (EPA 353.2, EPA 365.5); and particulate organic matter (0.5–2.0 mm) was measured by weight loss on ignition [20].