Alternate: Brownfield Restoration with Biosolids in Metropolitan Chicago: Soil Health Reconstruction

Industrial activities have created many brownfields throughout the Chicago metropolitan area. Brownfields are generally associated with poor soil health, inferior nutrient cycling, and attenuated soil ecosystem services, as demonstrated in low primary production and carbon (C) sequestration, low diversity of soil biota and plants, and high runoff with impaired water quality. Thus, restoration of brownfield sites not only increases the economic value of the sites, but also increases the ecosystem services the land provides. A research and demonstration site for brownfield restoration was established on Metropolitan Water Reclamation District of Greater Chicago (MWRD) land along the Chicago Sanitary and Ship Canal, which has been classified as a brownfield site due to prior use of the site for storage of petroleum and chemicals and destruction of soil horizons. Due to their physical, chemical, and biological properties, biosolids, which contain high amounts of organic matter, are an ideal soil amendment for degraded and contaminated soils. In addition to C and nutrients, biosolids contribute to an active microbial community, which is a critical component of soil ecosystem function, including nutrient cycling, C sequestration, plant symbiosis, and contaminant degradation. Research plots were established in October-November 2021 using Split Plot design with four replications. The main treatments were 1) control (no amendments to the existing soil), 2) air-dried biosolids at one-half rate (air-dried biosolids applied at 2.5 cm depth or 130 Mg ha-1), 3) air-dried biosolids (air-dried biosolids applied at 5 cm or 260 Mg ha-1), 4) composted biosolids (woodchips composted with biosolids-3:1 at 5 cm depth or 185 Mg ha-1), and 5) a blend of biochar and air-dried biosolids (pyrolyzed biosolids at 1.25 cm depth and air-dried biosolids at 3.75 cm depth or 105 Mg ha-1 pyrolyzed biosolids mixed with 195 Mg ha-1 air-dried biosolids). For the same thickness (5 cm) of soil amendment applied, the loading rate in weight basis was lower in composted than air-dried biosolids due to the lower bulk density for composted biosolids. Since composted biosolids contained more organic C (25.8%) than air-dried biosolids (19.8%) by mass, the C input from the two amendments applied in 5-cm treatments were comparable. The biochar/air-dried biosolids blend also leads to a similar C input to the 5 cm air-dried biosolids treatment, as biochar contained 21.5% of organic C, just slightly higher than that for air-dried biosolids. Subtreatments are two prairie plant communities with C4 grasses for subtreatment 1 and C3 grasses and forbs for subtreatment 2. The subplot size was 4.5 m x 5 m. The plots were hydro-seeded with native plants in November 2021. Additional seeding by drilling is still needed due to the poor germination during the first year, which is common for establishing native plants. This report will focus on assessing the impacts of biosolids on soil health in degraded urban soil, which supports plant establishment. We sampled soil in April 2022 at 010 cm depth and analyzed the key parameters for soil health, including soil organic C (SOC), nitrogen (N) mineralization potential (NMP), microbial basal respiration, microbial biomass C (MBC), and water-holding capacity (WHC). Due to the high gravel content of the brownfield soil, all results except for soil WHC are presented as 'stock' in 010 cm soil depth by using bulk density of soil materials (< 2mm). Results for two subtreatments within a main plot are pooled for this report of the soil measurement. All soil amendments drastically increased soil WHC from 53% to 100121% (dry soil weight basis), which is associated with the increase in SOC (Table 1). Soil organic C in the control plots had a mean of 17.4 kg ha-1 in the top 10 cm (Table 1) with a range of 10.026.0 kg ha-1). The C-rich soil amendments considerably increased SOC, leading to higher than 30 kg ha-1 in soil for all four amendments. Microbial biomass C measurement indicated that microbial populations were more abundant in plots with the soil amendments than the control: however, it varied depending on the amendment type (Table 1). Microbial biomass C was highest under the air-dried biosolids, followed by air-dried biosolids at one-half rate, composted biosolids, and the biochar-biosolids blend, and lowest in the unamended control soil. Microbial respiration, the CO2 output of the microbial community, reflects microbial quantity and activities and the utilization of substrates by microbes. We found that the soil microbial basal respiration in the control plots was low relative to the treatment plots (Table 1). The high biosolids rate had a higher basal respiration rate than the other treatments. The microbial metabolic quotient (qCO2), the respiration rate per day per unit microbial biomass, helps us understand the environmental stress level for microbes in the soil. High qCO2 indicates greater environmental stress for soil microbial communities. The full rate of air-dried biosolids reduced the qCO2 compared to the control, though the other soil amendment treatments had a similar qCO2 to the control (Fig. 1), indicating that air-dried biosolids can provide a less stressful habitat for microbial communities. Nitrogen mineralizing potential is important for evaluating N accessibility to soil microorganisms and plants. For the sampling period, approximately six months after plot establishment, control plots reflecting the degraded conditions of the site had a very low NMP of only 0.01 kg ha-1, compared to a range of 2.584.2 kg ha-1 in the treatment plots (Table 1). All three treatments that received air-dried biosolids had a higher NMP than composted biosolids. The results presented here showed that the use of biosolids as a soil amendment gave rise to significant improvement in the health of degraded urban soil, which will facilitate plant establishment and possibly drive interactions among microorganisms, plants, and soil components for restoring ecosystem services of the brownfield.