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Integrated Assessment of Soil Quality, Environmental Emissions and Agronomic Benefits from Land Application of Organic Waste Products

Final Report Summary - SOILWASTEBENEFITS (Integrated Assessment of Soil Quality, Environmental Emissions and Agronomic Benefits from Land Application of Organic Waste Products)

In Europe, the recycling of organic biodegradable waste is expected to increase in future, while decreasing waste landfilling and incineration without energy recovery. Land application of the biodegradable fraction wastes after biological treatment (e.g. composting or anaerobic digestion) is likely to be one of the most environmentally-friendly waste management options. Soil application of organic amendments derived from waste materials (referred to as organic waste products: OWP) can be used to maintain or even increase soil organic carbon (SOC) levels. Soil application of OWP results in various direct and indirect agronomic and environmental effects. Some positive effects are related to the enhanced SOC contents and changed SOC quality, such as improved aggregate stability and soil porosity. Other effects are due to the potential substitution of mineral fertilisers through the inputs of nutrients such as N, P, K provided by OWP, thus avoiding high fossil energy costs and hence the global warming impact of synthetic fertilisers. Finally, OWP application on soil has a potential for climate change mitigation through the sequestration of atmospheric CO2-C in SOC.

Life cycle assessment (LCA) is a powerful standardized method to compare the environmental impacts associated with different household waste management options or treatment processes, enabling to take into account the whole process of waste creation, disposal and its effects. Although LCA has been applied to assess land application of different types of OWP, these LCA studies do not adequately take into account the environmental emissions related to changes in soil properties. This is due to the lack of reliable methods for estimating the environmental effects of OWP addition on soil, which therefore impedes the identification of the best biological treatment in terms of environmental performance.

The objectives of the SoilWasteBenefits project were to
• Develop analytical techniques to determine and characterize the effects of OWP on the stability and recalcitrance of SOC
• Measure the effect of OWP application on the reduction of the energy necessary for soil tillage in a field experiment
• Estimate the environmental emissions created by land application of OWP using an agroecosystem model
• To conduct a life cycle assessment for the OWP previously studied, accounting for the energy savings for soil tillage and the emissions estimated by agroecosystem modelling to determine the environmental performance of household waste composting followed by land application compared with incineration with energy recovery of the wastes.

The first phase of the project demonstrated the potential of Fourier-transformed mid-infrared photoacoustic spectroscopy (FTIR-PAS) as a powerful tool for detecting and characterizing organic compounds of different stability in soil and to characterize the composition of different organic waste products. This technique offers benefits over reflectance spectroscopy techniques for dark samples as soil and waste products because the level of sample reflectance has little effect on the PAS signal. The results showed that FTIR-PAS performed similar or better than near infrared spectroscopy for the prediction of total organic carbon, total N content and the fraction of labile soil organic carbon. The identification and interpretation of the spectral bands that correlated positively and negatively with the most biodegradable fraction of soil organic carbon revealed that bands corresponding to aliphatic, methyls, amide III and polysaccharides were positively correlated with the labile fraction of SOC, whereas bands corresponding to aromatics, amides II, amines and carboxylic acids were negatively correlated with this fraction. An interpretation of this kind therefore provided an advance in understanding of the chemical composition of labile C in soil, which is a fraction that is difficult to separate experimentally. Overall, this provided proof of the concept that FTIR-PAS spectroscopy can be used to predict soil properties both quickly and inexpensively, including SOM lability or stability and used to characterise soil organic matter.

In a second part of the project, the effect of changes in SOC induced by repeated soil application of OWP on the energy necessary for soil tillage and tractor fuel consumption was investigated in a field experiment in Denmark where different OWP where repeatedly applied for 11 year. The results showed that a change in specific energy use for soil tillage could be significantly explained by the soil clay content, bulk density, SOC content and cohesion. Specific draught force could be predicted from clay and SOC content, explaining 67 % of the variance. The change in energy required for soil tillage may result in fuel savings of up to 25 % on soil tillage for compost applied at an accelerated rate (reaching 3.5 % SOC, compared to 1.4 % SOC for NPK fertilised) and up to 14 % for compost applied at a normal rate (reaching 2.3 % SOC). This could represent an important environmental benefit from the soil application of organic waste.

The last phase of the project consisted in using dynamic simulation modelling and the life cycle assessment methodology to determine the environmental performance of soil application of household wastes after composting compared to incineration with energy recovery. The tractor fuel savings for soil tillage as a result of the increase in soil carbon content after applying compost to the soil were also accounted for in the LCA model. Although substantial reduction in tractor fuel for soil tillage can be achieved, our study suggested that such savings are negligible when compared to all the other emissions along the waste life cycle, as assessed with LCA. Integrating other effects of compost on soil properties, as the increased resistance of soil to erosion, or the enhanced biological activity might however further improve the environmental performance of the waste recycling through biological treatment and soil application.

This project resulted in four manuscripts for high ranking scientific journals. The fellow acquired new skills in a variety of disciplines, including characterization techniques, modeling and life cycle assessment and gained experience in project management, teaching, student supervision and dissemination of the results to scientific and general public.

Overall, this project yielded important development of methods to characterize the composition and stability of organic waste product and soil organic matter; and provided important data on the effect of long term organic waste application on environmental emission and environmental performance of waste recycling through biological treatment and soil application. The outcomes of this project will be of interest for researchers, as well as for farmers and authorities, providing a better estimation of the environmental effects of applying organic waste amendment to the soil.