Lasting effects of biosolids in agroecosystems

Photo courtesy of Flickr/cityofgeneva Wastewater treatment plants collect everything that goes down the drain and return treated water for future use. During this process, many by-products are collected (e.g., debris, solid waste) that can contain pathogens, heavy metals, and other contaminants. These by-products need to be disposed of, and the most common options for doing so include burying in a landfill or incineration. However, these disposal options have disadvantages—putting anything in a landfill takes up space while contaminating water and incineration of waste can produce air pollution. One solution to the disposal problem is to recycle the wastewater by-products. With additional processing (e.g., anaerobic digestion and heating), pathogens can be removed or killed. The resulting material, typically called biosolids, can then be used as fertilizer on agricultural fields, in backyard gardens, or in landscaping. One of the oldest examples of biosolids being sold as fertilizer is Milorganite, a product that has been produced and sold by the city of Milwaukee, WI since 1926. The benefits of using biosolids as fertilizer include increasing nitrogen, phosphate, and potassium without chemical inputs, providing fertilizer in a slow-release form, which minimizes runoff, increasing soil organic matter and soil water retention, and improving soil structure. There are economic benefits, too. Wastewater treatment facilities can reduce the disposal costs and may even generate revenue by selling biosolids. However, the use of biosolids on agricultural land is controversial. There are concerns over human exposure to pathogens, heavy metals, and other emerging contaminants either directly or through uptake by crops grown in fields fertilized by biosolids. Additionally, heavy metals can accumulate in soils where biosolids are used frequently. In general, regulations focus on minimizing risks to human health and avoiding the increase of heavy metals in soils, but the specific details vary from one country to another. Two recent papers in the Journal of Environmental Quality examine some of the environmental impacts of biosolid applications. One study reports on the effects of biosolid application on semi-arid grassland soils and vegetation in Canada. The other, from the United Kingdom, focuses on the response of soil microbes to the metals in biosolids. In both studies, sites were sampled for more than five years after the last application of biosolids and provide insight into the lasting effects of biosolid application. In Canada, up to half of the biosolids produced are applied to agricultural fields, reclamation sites, and rangelands. Dr. Brian Wallace, an SSSA member and range soil ecologist with the British Columbia Ministry of Forests, Lands, and Natural Resource Operations, is a co-author on an article titled, “Soil Aggregate Dynamics and Plant Community Response after Biosolids Application in A Semi-Arid Grassland” (see http://bit.ly/2dJH4Se). The objective of this study was to determine the effects of a single biosolid application on the stability and turnover of soil aggregates in relation to changes in plant species composition. Wallace first worked at this site, a semi-arid grassland in British Columbia, as a master's student. Resampling the site and looking at these data years later, he says, “We were still detecting changes in our measured properties five, six, eight years after a single application to the surface. That was really quite surprising.” Experimental treatments were applied once, in 2001, and included N and P chemical fertilizer, 20 dry Mg/ha biosolids (Bio-20), and 60 dry Mg/ha biosolids (Bio-60). Researchers also established control plots to compare treated and unamended soil over time. Soil samples were collected in spring, summer, and fall in 2005, 2006, and 2009 and analyzed for aggregate stability and total C and N. Soil aggregate dynamics, which can be representative of overall soil health, were measured in this study. High aggregate stability indicates greater organic matter, biological activity, and resistance to erosion. “The other great thing about [aggregates] is that you can detect changes. [Aggregates are] responsive to management,” Wallace says. The aggregate data showed that soil quality did improve where biosolids were applied. The soil aggregate mean weight diameter (MWD) and proportion of stable aggregates (in the 2- to 6-mm size class) were greater in soils that received biosolids compared with the control and chemical fertilizer plots. The authors also report that where biosolids were applied, C and N concentrations within stable aggregates were greater in spring and declined into the summer, which suggests enhanced N mineralization. This early growing season nutrient turnover was still being observed eight years after the treatments were applied. When the plant community was surveyed in 2009, the greatest difference in vegetation was observed for perennial grass cover. The control and Bio-20 plots had 30 and 32% cover, respectively, while Bio-60 plots had only 7% cover. Perennial forb biomass was also influenced by treatment, where forb biomass increased with increasing fertilization rate, from 176 kg/ha in the Bio-20 plots to 228 kg/ha in the Bio-60 plots. Evaluating the soil and plant data together, the authors suggest that unlike the perennial grasses, the forbs were able to take advantage of the increase in N early in the growing season. 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 biosolid application could favor non-native species that can better utilize available soil N before native grasses are actively taking up nutrients and producing biomass. In the United Kingdom, finding local sustainable sources of phosphorus is important since in Europe, there is only one mine. Biosolids help to fill this need, and up to 80% of wastewater solids are processed and applied to land in the UK. Dr. Ruben Sakrabani, an SSSA member and Senior Lecturer in Soil Chemistry at Cranfield Soil and Agrifood Institute, Cranfield University, is a co-author of the recent article titled “Long-term Impact of Sewage Sludge Application on Rhizobium leguminosarum biovar trifolii: An Evaluation Using Meta-analysis” (see http://bit.ly/2dtsUpS). Sakrabani says, “We didn't collect any new data,” indicating that they instead used existing data from the Long-Term Sludge Experiment (LTSE). Data from the LTSE have been analyzed before, but Sakrabani and his colleagues took a different approach, using a meta-analysis to look at the magnitude and direction of effects. The LTSE was established in 1994 with treatments applied annually from 1994 through 1997. Treatments included biosolids with elevated levels of zinc (Zn), copper (Cu), or cadmium (Cd) and biosolids with no metal additions. Treatments and data collection were the same across all sites, making the data ideal for a meta-analysis approach. One of the measurements collected throughout the experiment is the number of Rhizobium leguminosarum biovar trifolii, a nitrogen-fixing bacteria that nodulates with clover. Rhizobium levels were sampled every two years starting in 1997 and ending in 2005. When anticipating the impact on soil microbes, Sakrabani says, “[We] thought cadmium would be the problem, but zinc may be more harmful.” The observed negative impact of Zn was unexpected because zinc is a micronutrient that microbes, plants, and animals need. Cadmium, on the other hand, is toxic with no known biological function. Images from left to right courtesy of Flickr/Steve Jurvetson, Wikimedia/Red58bill, and Tracy Yager (USGS). The authors report that Zn had a negative effect on Rhizobium in all soil types, but the greatest decrease in Rhizobium was reported in sandy clay loam soils. Sakrabani says that “soil characteristics are key” in determining the accumulation of metals. For example, the sandy clay loam soils, where Rhizobium showed the greatest reduction, had low soil pH, which increases the bioavailability of metals. Both Cu and Cd had negative impacts on Rhizobium too, but of a lesser magnitude. While this article only evaluates Rhizobium data, Sakrabani and his colleagues did a similar meta-analysis using data for microbial biomass in the LTSE. The microbial biomass results, published in Environmental Pollution (http://bit.ly/2dladVy), report a similar pattern where Zn had the greatest negative effect. Sakrabani comments that these articles may generate new hypotheses as to the effect of biosolids on soil microbial communities. And, given the advances in analytical methods in biological analysis since the final data collection in 2005, he would like to re-sample these sites to determine if Rhizobium levels are still being impacted by the metal treatments. Source: Adobe Stock. These papers suggest that regulations and guidelines for the use of biosolids may need to be adjusted to avoid negative impacts to agroecosystems. The semi-arid grassland results provide an example of how soil processes can be slower in some environments compared with others. Wallace suggests that data from a wide range of sites should be considered when setting limits for biosolid use. “If we're applying an amendment based on plant uptake, we should start there and understand what the potentials are for plant uptake in [semi-arid] systems,” and recognize how they differ from other grasslands and row-crop agriculture. Sakrabani points out that biosolids are a valuable fertilizer resource for the UK, but scientists and regulators need to consider the long-term sustainability of biosolid application. Determining how metals like Zn impact the microbial community will be very important when deciding where and how frequently biosolids can be applied.

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