Landscape Plant Response to Surface-Applied and Incorporated Biosolids and Compost Products on a Compacted Urban Subsoil.

Highway roadside soils are often highly degraded, as construction removes topsoil and subjects soil to extreme compaction. We conducted a 3-year field study to evaluate effects of yard-debris compost, biosolids-bark blend, and vermicompost, and the effects of surface application vs. incorporation on landscape plants and soil on a heavily-compacted highway roadside gravelly sand subsoil. Treatments included: unamended control, woody yard debris compost (C:N 35:1), and Class A biosolids-bark blend (C:N 33:1) applied 7.5 cm deep and either incorporated or left on the surface. Surface-applied vermicompost (C:N 20:1) was also compared. Amendments were applied, appropriate treatments incorporated, and all treatments covered with 7.5 cm bark mulch in February 2007, followed by transplanting coralberry (Symphoricarpos orbiculatus), fragrant sumac (Rhus aromatica 'Gro-Low'), and Oregon grape (Mahonia aquifolium). Measurements included plant vigor and shoot growth index (each year), plant biomass (January 2010), and soil bulk density and carbon (sampled 2008). Plant growth and vigor generally increased in the order: control, yard-debris compost, biosolids-blend, vermicompost. Treatment effects were greatest for fragrant sumac and least for Oregon grape. Incorporated treatments had generally lower plant vigor and shoot-growth index. Incorporation did not affect biomass. Plant vigor averaged across all treatments declined in 2008, but increased again in 2009. Soil bulk density averaged 2.19 g cm-3 beneath the surface amendment treatments, unchanged from the extremely compact control. Bulk density in the incorporated treatments averaged 0.93 g cm-3. Soil C averaged 4 g kg-1 for the control and surface applied treatments compared with 44 g kg-1 for yard debris compost and 57 g kg-1 for biosolids-blend. Plant roots were still confined to the amended layer (surface or incorporated) after three years. Although plant survival was high (85% in amended treatments), the shallow root zone could limit plant growth and health in the long run.