Abstract:
Soil microbial biomass was used in the last two years of a five-year, randomized replicated field trial as a bioassay for potentially harmful substances in biosolids-amended soil. The purpose of using a bioassay was that it was not necessary to know the identity of harmful substances a priori. It answered the question “what about all the chemicals you do not monitor?”
Anaerobically digested biosolids were spread at operational rates (about 8 to 10 tDS/ha) as lagooned liquid and as dewatered cake. The trial was farmed as an arable rotation. The impact on soil biomass was studied by collecting soil samples at 2 or 3 monthly intervals. The size of the biomass and its metabolic quotient were measured.
Soil microbial respiration quotient proved very sensitive. As expected it showed the stress of seasonal climatic variation. One microbial stress event was detected across all the plots, irrespective of whether biosolids had been applied or not. The field activity diary revealed the stressor was an application of selective herbicide that had been applied two days before sampling. The metabolic quotient had recovered by the next sampling. The size of the biomass increased after biosolids were applied and then decreased again as the organic matter and nutrients were used. The metabolic quotient was unaffected by biosolids showing that no constituent or cocktail of constituents in the biosolids stressed the microbial biomass. Crop yield data enabled the construction of a reliable model for predicting fertiliser replacement value.
Key words
Agronomy, bioassay, biosolids, cocktail, fertiliser value, potential pollutant, soil microbial respiration quotient, substance of concern
Introduction
It is possible to find in sewage sludge and biosolids measurable amounts of most of the chemicals used in society. However, mere presence is not the question; the question should be whether there is unacceptable risk. As Paracelsus said 500 years ago “the dose makes the poison”. Sewage sludge might be a source but is the concentration sufficient and is there a pathway to deliver a harmful dose to a receptor? In the case of sewage sludge produced in developed countries and used or disposed in accordance with today’s rules, the consensus of informed scientific opinion is that the answer to this question is “no”. Through a combination of hazardous substances regulations, which have eliminated some chemicals, changes in industrial practices and restricting discharges from factories, the concentrations of hazardous substances in sewage are dramatically less then they were a few decades ago. Despite this the “urban myth” of “heavy metals” in sewage sludge persists.
A report that a “substance of concern” has been detected in biosolids has on occasions led to devastating consequences such as the collapse in confidence by farmers’ organisations in Sweden following a report by the Environmental Protection Agency (de Wit, 2000) that brominated flame retardants had been found in biosolids. However they did not take the trouble or intellectual rigour to estimate risk. When risk was estimated subsequently, biosolids were found to be of no concern, but the harm to land application had been done.
In addition to the question of single substances or families there is the question of the cocktail effect, i.e. might a mixture be more potent than the sum of its components? There can be many motives for fanning such controversies, such as not liking the smell when biosolids are spread, a fear of science, seeking research funding, etc.
A bioassay has the potential to sense whether hazardous substances are present at concentrations that pose risk. It is not necessary to know the identity of the substance in advance. Like a food taster in ancient times, a king did not need to know the identity of the poison; the bioassay gave a dose related response. If a taster died or became ill, the identity of the poison and the poisoner could be investigated. Soil microbial respiration quotient (SMRQ) is a sensitive bioassay for chemical challenges. The SMRQ (the respiration rate per unit mass of biomass) increases when the biomass is stressed.
Biochemical processes in soil are mediated by the soil microbial biomass (see for example Powlson et al., 2011). The size of the microbial biomass normally increases after available carbon substrates have been added, e.g. bulky organic manures, or crop residues. Soil microbial activity can also be affected by soil temperature, moisture, pH, agrochemicals and heavy metals or organic contaminants, which could accumulate in soil following the long-term application of enriched organic manures including biosolids and livestock wastes.
