Profiles of volatile organic compound emissions from soils amended with organic waste products
Graphical abstract
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 NOx 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, Iroc, cation exchange capacity, carbon/nitrogen ratio, total nitrogen, organic carbon) were studied.
Section snippets
Site description
Samples were collected in the QualiAgro site, a field station taking part of the SOERE-PRO-network (https://www6.inra.fr/qualiagro_eng/Nos-partenaires/The-SOERE-PRO-network). The QualiAgro agronomic set up which is described in this study started in September 1998 and lasted until September 2015. The QualiAgro site is located at Feucherolles in northwestern France (35 km west of Paris; 48°52′N, 1°57′E, alt 150 m), on a silt loam textured soil. The soil of QualiAgro site of Feucherolles is
Soil characteristics of each treatment
The Fig. 2 summarizes soils characteristics as a function of OWP. The carbon content is in the range 10–18 g kg−1 DM which spans from typical to high carbon content for a silt-loam. Organic matter was mainly found in the clay fraction of the soil (60%) and in the smaller fraction of the particulate organic matter, mainly in the form of O-alkyls, Alkyls and Aryls (Paetsch et al., 2016).
The CN samples showed the lowest cation exchange capacity, organic matter content, total nitrogen content, pH
Most emitted VOCs: acetone, butanone, acetaldehyde, methanol, butene and ethanol
This study was focused on VOCs emissions from soils regularly amended with 4 different organic waste products compared to unamended soil. However, the soils were sampled two years after the last OWPs application. Thus the differences in VOCs emitted could be associated to long-lasting effects of the OWP applications and to changes in soil organic matter characteristics due to these repeated OWP applications. We first notice that in all treatments including the control, the 3 most emitted
Conclusions
VOCs emissions by soils are involved in atmospheric chemistry and climate change. This study underlines the importance of the VOCs emissions characterization from soil in order to fill the lack of information in this field. Our results showed that VOCs emissions were affected by the type of OWP amended to soils. The BIOW compost emitted the highest total flux rate, while MSW compost the lowest. Our results further suggested that organic matter content and pH jointly influenced total VOC
Acknowledgement
We gratefully acknowledge ANAEE-France that provided the PTR-QiTOF-MS, ADEME for partially funding this study through the projects COV3ER (n°1562C0032) and DICOV (n°1662C0020). The Qualiagro site conducted in partnership with Veolia, forms part of the SOERE-PRO integrated as a service of the infrastructure AnaEE-France, overseen by the French National Research Agency (ANR-11-INBS-0001). The QUALIAGRO field experiment forms part of the SOERE-PRO (network of long-term experiments dedicated to the
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