The Impact of Biosolids on Soil Quality and Native Grasses
This brief is based on research that has been published in a peer-reviewed journal.
Biosolids are organic amendments resulting from municipal wastewater treatment processes. They contain appreciable amounts of nitrogen, phosphorus and organic matter, and are regulated to meet safe pathogen levels and metal contents. In British Columbia (BC), 38,000 dry tonnes of biosolids are produced at wastewater treatment facilities every year, the majority of which are destined for mine reclamation and to lesser extent for rangeland fertilization.
Potential benefits of using biosolids as fertilizer include:
Alternative to chemical fertilizers and manure
Increasing organic matter and soil water retention
Improving soil structure
Sustainable method to manage sewage and wastewater
Semiarid environments tend to be the best for biosolids use since their dry conditions limit the microbial activity, in turn reducing the mobility of nutrients such as nitrogen and phosphorus and their movement into ground or surface water.
The objective of this study was to determine the effects of a single, surface biosolids application on the soil structure in relation to changes in plant species composition in a degraded semiarid grassland in the southern interior of BC.
The study took place near Cache Creek, BC on a long-term field experiment established in 2001. Four treatments were evaluated: (1) surface biosolids application at 20 and (2) 60 megagrams per hectare, (3) mineral fertilizer, and a (4) control with no biosolids application all replicated in four blocks.
Soil samples were collected in spring, summer and fall four, five, and eight years after treatment establishment. Samples were taken at 0-5 cm, and used to assess for soil structure, carbon and nitrogen concentration.
Plant composition was evaluated eight years after treatment establishment. The amount of plant cover was estimated through visual observations and by collecting clippings of individual species at ground level.
Soil structure, which can be representative of overall soil health or quality, was measured in this study. Stable soil structure indicates greater organic matter, biological activity, and resistance to erosion. The data showed that soil quality did improve where biosolids were applied. The better stability of soil structure was in soils that received biosolids compared to the control and chemical fertilizer plots.
The authors also reported that soil carbon and nitrogen concentrations were greater in spring on plots with biosolids application. This early growing season nutrient pulse was still being observed eight years after the treatments were applied.
Plant Species Composition
The plant community assessment revealed that the greatest difference was for perennial grass cover. The control and biosolids rate of 20 megagrams per hectare (Mg/ha) had 30% and 32% perennial grass cover, respectively, while biosolids rate of 60 Mg/ha had only 7% perennial grass cover. Perennial forb biomass was also influenced by treatment, where forb biomass increased with increasing fertilization rate, from 176 kg/ha in the plots with biosolids rate of 20 Mg/ha to 228 kg/ha in the plots with biosolids rate of 60 Mg/ha.
About this research
This brief is based on the following journal article:
Wallace, B. M., Krzic, M., Newman, R. F., Forge, T. A., Broersma, K., & Neilsen, G. (2016). Soil aggregate dynamics and plant community response after biosolids application in a semiarid grassland. Journal of Environment Quality, 45(5), 1663.
Evaluating the soil and plant data together, the authors suggest that forbs were able to take advantage of the increase in nitrogen early in the growing season, but not the perennial grasses. They concluded that while the soil quality showed improvement when amended with biosolids, improvements to plant species composition were not observed. Considering the slow nutrient turnover and seasonal cycles in this region, the authors express concern that biosolids application could favor non-native species that can better utilize available soil N before native grasses are actively taking up nutrients and producing biomass.