Biosolids as a Soil Amendment: Implications for Soil Health, Carbon Sequestration, and Sustainable Crop Productivity

Madeline Pierce Desjardins

Biosolids are recycled wastewater products (sewage sludge) that have been treated to meet EPA standards for agricultural land application. They are primarily applied as alternatives to nitrogen (N) fertilizers but also contain organic matter (OM; 60-80% dry weight), carbon (C; 35-40%), and other plant essential micro- and macro-nutrients. Biosolids provide an avenue for waste recycling that returns nutrients and organic materials from food back into agricultural systems, thus, closing nutrient cycles, and potentially offsetting energy used in the production of synthetic N fertilizers. Biosolids also have the potential as OM and C sources to improve soil health, store additional soil C, and help farmers integrate additional soil health practices into rotation.The research presented in this dissertation was conducted in semi-arid dryland (rainfed) agricultural systems, which face specific challenges relating to moisture limitation and erosion. Thus, many of the research questions and objectives of this work are focused on how biosolids might positively impact soil health and C accrual within these systems. Chapter two of this dissertation investigated the effects of biosolids on critical soil health functions for semi-arid systems in two long-term biosolids trials, one located in a Washington (WA) state grain-fallow system which compared three biosolids rates to a synthetic N fertilizer and unfertilized control, and the other in a Colorado (CO) system, comparing biosolids applied based on soil N availability to synthetic N fertilizer in two cropping rotations (wheat-corn-fallow and wheat-fallow). Application rate and timing impacted the effects of biosolids at the two sites. In WA increasing biosolids applications increased soil C and N pools, N-acetyl β-glucosaminidase, phosphomonoesterase enzyme activity, available water holding capacity, and decreased bulk density. At both sites, biosolids increased microbial biomass as measured by phospholipid fatty acids and Mehlich-3 extractable soil P. Treatment effects with biosolids at the CO site were within the wheat-corn-fallow rotation, which received the highest biosolids application of the two rotations. This study shows that biosolids can improve key soil health functions of OM and nutrient cycling, and water storage in semi-arid dryland cropping systems. Chapter three of this dissertation investigated how long-term (28 year) applications of biosolids influenced soil organic C (OC) accumulation and stability by measuring soil OC stocks and fractions (water extractable OC, free particulate OC, occluded particulate OC, and mineral associated OC). The trial was located on a long-term WA biosolids trial, with three biosolids application rates (4.9, 7.0 and 10.0 Mg ha-1), applied every four years, compared to synthetic fertilizer and an unfertilized control. C stocks were measured on soil cores collected as deep as possible (typically ~60 cm), and cut in 15 cm segments (0-15, 15-30, 30-45, 45-60 cm). This study found that in semi-arid dryland systems with low clay content, the 7.0 and 10.0 Mg ha-1 rates of biosolids increase soil OC stocks and free and occluded particulate OC but not water extractable or mineral associated OC, indicating that the more stable fraction of C may not change with additional C inputs under current site conditions. However, this study does show that additional C can be stored in this system in particulate OC fractions with biosolids compared to synthetic N fertilizers. Chapter four of this dissertation investigated the short-term (two applications, three years apart) impacts of biosolids applications and cover crop grazing on soil health, soil moisture, and yield, along with the influence of biosolids on grazable biomass in semi-arid systems. The experiment was a split plot design with cropping system as the main plot (cover cropping + grazing or chemical fallow) and fertility as the sub plot (biosolids, synthetic fertilizer, or an unfertilized control). This study found that short-term biosolids applications are effective at increasing grazable biomass and nutrient availability in semi-arid dryland systems and highlights the need for continued applications at agronomic rates for soil health benefits observed in long-term trials. Biosolids were also found to have the potential to reduce soil moisture due to increased biomass growth in biosolids amended plots but cover cropping vs fallowing did not seem to effect soil moisture availability.

Biosolids as a Soil Amendment: Implications for Soil Health, Carbon Sequestration, and Sustainable Crop Productivity

Files and links (1)

Abstract

Metrics

Details