Profiles of volatile organic compound emissions from soils amended with organic waste products

Highlights

• VOCs emissions were affected by the type of organic amendment to soil.

• Most emitted compounds were acetone, butanone and acetaldehyde for all OWP.

• 21 compounds had statistically different emissions between the OWPs.

• Organic matter content and pH jointly influenced total VOC emissions.

Abstract

Volatile Organic Compounds (VOCs) are reactive compounds essential to atmospheric chemistry. They are mainly emitted by living organisms, and mostly by plants. Soil microbes also contribute to emissions of VOCs. However, these emissions have not yet been characterised in terms of quality and quantity. Furthermore, long-term organic matter amendments are known to affect the microbial content of soils, and hence the quantity and quality of VOC emissions. This study investigates which and how much of these VOCs are emitted from soil amended with organic waste products (OWPs). Four OWPs were investigated: municipal solid waste compost (MSW), green waste and sludge co-compost (GWS), bio-waste compost (BIOW) and farmyard manure (FYM). These OWPs have been amended every two years since 1998 until now at a rate of ~4 tC ha⁻¹. A soil receiving no organic inputs was used as a reference (CN). VOCs emissions were measured under laboratory conditions using a Proton Transfer Reaction-Quadrupole ion guide Time of Flight-Mass Spectrometry (PTR-QiToF-MS). A laboratory system was set up made of two Pyrex chambers, one for samples and the second empty, to be used as a blank. Our results showed that total VOC emissions were higher in BIOW than in MSW. Further findings outlined that the most emitted compounds were acetone, butanone and acetaldehyde in all treatments, suggesting a common production mechanism for these compounds, meaning they were not affected by the OWP amendment. We isolated 21 VOCs that had statistically different emissions between the treatments and could therefore be considered as good markers of soil biological functioning. Our results suggest that organic matter and pH jointly influenced total VOC emissions. In conclusion, OWPs in soil affect the type of VOC emissions and the total flux also depends on the pH of the soil and the quantity of organic matter.

Introduction

The family of volatile organic compounds (VOCs) is composed of a large range of molecules with different functional groups and different chemical and physical properties. The common traits between VOCs are the high vapour pressure under ambient conditions and low boiling points which explain their presence in the atmosphere. Volatile organic compounds sources have been extensively studied because of their contribution to global warming and pollution (Kesselmeier and Staudt, 1999). It has been demonstrated that VOCs are involved in many chemical reactions in the atmosphere among which the production cycle of tropospheric ozone (O₃) in combination with the NO_x cycle (Seinfeld and Pandis, 2016). Another important chemical processes in the atmosphere that involves VOCs are the photolysis of formaldehyde and other carbonyls in presence of NO which are responsible of hydroxyl (OH) formation (Atkinson, 2000). Additionally, VOCs are precursors of secondary organic aerosol (SOA) (Singh et al., 1995) and are involved in the production of nitrates in the troposphere (Monson and Holland, 2001). For these reasons VOCs alter the air quality and thus they have indirectly consequences on human and ecosystems health.

Several studies demonstrate that biogenic VOCs contribute to 90% of the total VOCs emissions (Atkinson, 2000), while anthropogenic sources such as industries, solvents, transportations etc. contribute only for the 10% of the total VOCs emissions. Among the most important biogenic sources of VOCs are living plants, with a global emission of 1 Pg C y⁻¹, mainly monoterpenes from broadleaves forests (Harrison et al., 2013). Furthermore, it has been shown that the soil and litter can release around 12 to 136% of monoterpenes compared to canopy emissions in spring and fall (Faiola et al., 2014).

Another source of biogenic VOCs is the interaction between soil and microorganisms. Soils VOCs emissions are up to three times lower than the VOCs release from the canopy. Nevertheless, it has been demonstrated that under certain temperatures and soil water contents, and for specific ecosystems, VOCs emissions from soil could reach the same magnitude as canopy emissions (Peñuelas et al., 2014). Additionally, little is known about the emissions from agricultural fields, although they are recognised as large contributors of methanol and acetone to the atmosphere (Bachy et al., 2018). It however appears from recent measurements that soils may contribute to a large extent to the overall balance of VOC fluxes in crop rotations (Bachy et al., 2016).

From another perspective, intensive agriculture may lead to the decrease of soil organic matter contents (Chan et al., 2002) and Europe is encouraging the use of organic waste products (OWPs) in crops (European Commision, 2010) to favour carbon storage in soils (Peltre et al., 2012). The recycling of OWPs in soils also represents an alternative to waste management by landfilling or incineration. The application of OWPs also provides nutrients such as nitrogen (N) and phosphorous (P) which make possible the substitution of mineral fertilizer, improves soil biodiversity and has positive impacts on soil physical properties such as aggregate stability (Diacono and Montemurro, 2010). The negative impacts are not negligible though; OWPs release contaminants that can be transferred to plants, water or to the atmosphere (Belon et al., 2012). OWPs change the pH in soil. As Brinton (1998) reported, the pH is one of the chemical properties that impact on VOCs emissions. Positive and negative impacts in soil vary with the applied organic waste product (Obriot et al., 2016). For example, Seewald et al. (2010) studied VOCs production by adding glucose to soils regularly amended by different composts and mineral fertilizers. They also reduced the O₂ availability in order to impair the degradation of the produced VOCs by microorganisms. Results reported by this study shown that organic waste compost did not alter the VOC emissions compared to the untreated control, while sewage sludge composts and mineral fertilization showed distinct effects. Moreover, Potard et al. (2017) showed that different soil organic fertilizers such as pig slurry and methanized pig slurry impact in a different way soil VOCs emissions: pig slurry releases double quantities of VOCs while methanized pig slurry emits even less than the unamended samples. Contrasting type of VOCs emissions from soil have been reported from several studies. For instance, in Mediterranean soils the most emitted compound was methanol (Asensio et al., 2007a) while in soil amended with straw it was acetone (Zhao et al., 2016). In fact, emissions of VOCs are strongly related to the substrate quality and it is important to note that even small variations in nutrient composition may change the type and the amount of individual VOCs produced (Wheatley, 2002).

Significant emissions of the VOCs released by soil are produced by microorganisms (Isidorov and Jdanova, 2002). It has been reported that the composition and amount of VOCs emitted are linked to the microbial community living in the soil (Seewald et al., 2010). Microorganisms produce VOCs as a result of the degradation of sugar, alcoholic fermentation, amino acid and fatty acid degradation, terpenoid biosynthesis and sulphur reduction. For instance, acetaldehyde, which is one of the compounds most emitted from soil, is formed during the alcoholic fermentation that produces ethanol (Castaldelli et al., 2003). Previous studies have reported that microorganisms degrade sugar following three major pathways: (1) Embden-Meyerof pathway, (2) Heterolactic/homolactic pathway and (3) Entner-Douoroff pathway. In these processes, intermediate compounds are produced such as pyruvate, glyceraldehyde-3-phosphate, lactate and acetate which are precursors for the biosynthesis of several VOCs (Peñuelas et al., 2014). VOCs are also released as molecules resulting from the secondary metabolisms of micro-organisms (Werner et al., 2016). Secondary metabolites might be produced under certain circumstances or development stages; hence they might be used as indicator of the cellular state (Patti et al., 2012). Furthermore, the nature of the substrate can affect microorganisms' community and thus change VOCs emissions (Peñuelas et al., 2014). Moreover, the application of OWPs in soil stimulate its diversity, the abundance and the activities of microorganisms in soil compared to soils that only received mineral fertilizers. This is a consequence of the addition of microorganisms previously present in the OWP (Ros et al., 2006).

The aim of this study was to characterize VOCs emissions from soil amended with OWPs in order to quantify and qualify emissions from amended soil to the atmosphere. Furthermore, soils regularly amended with different organic waste products for a long period can have a differentiated volatile organic compound signature and different total VOCs fluxes. In order to reach this aim four soils amended for 20 years with organic wastes products were considered: bio-waste compost (BIOW), green waste and sludge compost (GWS), municipal solid waste compost (MSW) and farmyard manure (FYM). All these soils were compared with a control sample that had never received any OWP. We measured VOCs fluxes under standardised laboratory conditions with a Proton Transfer Reaction - Quadrupole Ion guide Time of Flight - Mass Spectrometer (PTR-QiTOF-MS). The VOCs released were identified and quantified and the relationships between VOCs emissions and soil chemical characteristics (pH, organic matter, I_roc, cation exchange capacity, carbon/nitrogen ratio, total nitrogen, organic carbon) were studied.