Biosolids as a Roadside Soil Amendment

Vegetation is a critical component of the roadside environment. It supplies services that include stabilizing soil, filtering runoff, trapping debris before it enters drains and water supplies, and providing visually appealing scenery for roadway users. These services are threatened when soil is unable to provide an environment suitable to the needs of perennial vegetation, leading to annual, often invasive, species that outcompete and thrive in dry, low nutrient and disturbed habitats dominating the environment. As a means of improving the availability of the macronutrient nitrogen, a limiting nutrient for many plants, we amended roadside soil in Saunderstown, Rhode Island with seven different products: five stabilized biosolids (products whose organic material consisted only of sewage sludge) and two composts (products whose organic material consisted either partially or completely of yard waste). The biosolids were applied at rates of 48 kg N/ha, 144 kg N/ha and 288 kg N/ha (nitrogen available in the first year) while the composts were applied at rates of 15%, 30% and 45% v:v of the first 15 cm of the soil. Control plots received no fertilization beyond the 95.2 kg/ha of 19-19-19 fertilizer included in the hydroseeding mixture. All plots were hydroseeded in September 2012 with a seed mixture of Kentucky bluegrass (Poa pratensis), perennial ryegrass (Lolium perenne) and red fescue (Festuca rubra). Quality of the vegetation within the plots amended with biosolids one year after planting indicated that soil would provide sufficient nitrogen without posing a significant leaching risk. However, results two years after planting indicated that the composts were a better source of long-term nitrogen and soil organic matter compared to biosolids. The vegetation on biosolids amended plots demonstrated a vulnerability to droughtinduced senescence due to low soil moisture associated with insufficient soil organic matter. By the end of the second year, no biosolids-amended plots had turfgrass quality significantly differing from the control plots, while all compost-amended plots had significantly better quality than the control. A combination of composts and biosolids products applied in a manner that increases soil organic matter levels to the already recommended 5% while providing 144 kg N/ha of first-year available nitrogen should provide sufficient short-term and long-term nitrogen to support persistent growth of planted perennial species. Two surveys of the study area, conducted in June and September of 2014, measured how the relative coverage of planted species versus weed species and perennial species versus annual species varied based on amendment and, rate of amendment application, and distance from the road. With the exception of the WRB (anaerobically digested biosolids) amended plots monitored in September 2014 (33% relative planted coverage), all other amendments and rates assessed in June 2014 (39% 61% relative planted coverage) and September 2014 (35% 45% relative planted coverage) had significantly greater relative coverage of planted species than the control (June 2014 – 28%, September 2014 24% relative planted coverage). As for the relative coverage of perennial species, no individual rate of application in either June 2014 (72% 79% relative perennial coverage) or September 2014 (55% 60% relative perennial coverage) produced a relative perennial coverage significantly different than that of the control (June 2014 – 68%, September 2014 – 50% relative perennial coverage). In June 2014, individual biosolids amendments had significantly greater relative coverage of perennial species (RMI [wood-ash stabilized biosolids] – 83%, WW [aerobically composted biosolids] – 82% relative perennial coverage) than the control (68%). No other amendments in either June 2014 (79% 67% relative perennial coverage) or September 2014 (46% 64% relative perennial coverage) had relative coverage percentages significantly different from the control (June 2014 – 68%, September 2014 – 50% relative perennial coverage). Distance from the road had a significant impact on the relative coverage of both planted and perennial species. The area within 1 meter of the road had significantly less planted coverage than the areas of the plots further from the road in all plots except the BBCC (biosolid cocompost) and control. Relative perennial coverage within the first meter from the road was significantly less than the rest of the area within the plots for both composts and biosolids. Digitaria sp. dominated that first meter, with the effect being especially pronounced on the side of the road where traffic was closer to the vegetation. In order to promote the establishment and persistence of perennial species within Rhode Island highway roadsides, the use of composts that have both sufficient first-year plant-available nitrogen (144 kg n/ha) and high levels of organic matter (sufficient to raise soil levels to a minimum of 5% organic matter) are recommended. Regions of the roadside within one meter of the paved surface require the development and implementation of specific management techniques beyond amending the soil with organic materials.