Recycling coffee wastes for healthy urban agriculture: do spent coffee ground and its biochar reduce the impact of antimony on soil quality and plant growth?
Résumé
Context. Environmental contaminants ubiquitous in megacities such as metals or polycyclic aromatic hydrocarbons are accumulated in soils leading ultimately to a poor quality of these soils for agriculture purpose. The economic and environmental crisis which have been modelling the 21th century awoke an awareness and a worldwide growing interest for urban organic agriculture, making the challenge of urban soil quality a priority for the next decades. Hence, coupling remediation and amendment in one-shot is a key concept for land and ecosystem restoration. Addition of organic wastes to degraded soil is a traditional, low-cost and effective approach to improve crop yield. Among the organic amendments, spent coffee ground (SCG) is the most frequent waste generated in coffee beverage production worldwide; so its revalorization is a matter of current concern (Campos-Vega et al., 2015). Direct application of fresh SCG or its charred by-product to contaminated soils and waters has evidenced intriguing outcomes that suggest these materials suitable in remediation actions or as ‘buffer’ amendments in soils receiving contaminants. Strong evidences also suggest a direct interaction between biochar and rhizospheric microorganisms such as arbuscular mycorrhizal fungi and bacteria. However, such an association is generally dependent on biochar physicochemical characteristics. Objectives. To determine whether addition of SCG and SCG-derived biochar reduce the impact of Sb on selected extracellular enzyme activities and plant performance. Accordingly, we selected several extracellular enzyme activities implied in the biogeochemical cycles of C, N and P as indicators of soil quality (Kim et al., 2014; Sanchez-Hernandez et al., 2016). Likewise, some indicators of plant fitness were determined to assess the impact of SCG and its biochar on Sb bioavailability and accumulation. Material and methods. A microcosm experiment was conducted in a greenhouse using peas (Pisum sativum L.) as the model plant. A loam agricultural soil was amended with 5% (w/w) of SCG or SCGc. Half of the pots were spiked with antimony tartrate (KSb) to reproduce Sb concentrations frequently detected in contaminated soils. Soil enzyme activities were measured and analyzed at sowing (T0mo) and harvest (T3mo). We used an ecotoxicological index of contaminant impact on organisms called “Integrated Biological Response” index (IBRv2) to compare the global response of enzymes in each treatment (Sanchez et al., 2013). Pea yield, plant growth and colonization of SCGc by microorganisms were also evaluated. Results & conclusions. Addition of fresh SCG in soil caused a slight, but statistically significant, phytotoxic effect on plant growth and pea yield (Fig.1). However, SCG-derived biochar enhanced plant performance. Both SCG and SCGc had a significant impact on soil enzymes, the activity of which was time- and treatment-dependent (Fig.2). Biochar activated extracellular enzymes implied in C- and P-cycling, coupled with an important colonization by microorganisms (Fig.3). These findings support the general idea that biochar provides a recalcitrant support for microbial development and protection from adverse environmental conditions. Moreover, SCGc reduced significantly Sb toxicity at the end of the experiment (Table 1). However, some enzymes were still affected by Sb contamination (carboxylesterase, urease, and acid phosphatase); suggesting complex interactions between Sb, SCG/SCGc and soil microbial communities, therefore making this approach difficult to generalize for recovering urban soils contaminated by Sb.
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