Constructed wetland biomass for compost production: Evaluation of effects on crops and soil

This study investigates the suitability of Phragmites australis (reed) biomass deriving from a surface flow constructed wetland (CW) to produce three compost types: reed (RC), reed mixed + potato cuttings (PC) and reed + liquid anaerobic digestate (DC), to promote both resource circularity and soil carbon sequestration. The composts were tested over 60 days on lettuce at two levels in combination or not with NH₄NO₃ (at the same kg N ha⁻¹ loading), along with NH₄NO₃ reference (Chem) and an unamended control (Ctrl). The plant tissue dry weight and N load was determined, and the N relative efficiency (N-RAE %) was calculated. On pot soil, total and labile carbon (TOC, C_L), along with the carbon management index (CMI) and δ¹³C were evaluated. Pot test showed that PC₁₀₀ yielded the best (g pot⁻¹) lettuce biomass (3.0) > DC₁₀₀ and RC₁₀₀ (2.5 and 1.6) ≈ chemical reference (3.8). A similar pattern was detected at 50% (g pot⁻¹): PC₅₀ (2.9) > DC₅₀ (2.7) > RC₅₀ (2.4). N-RAE (%) reflected this pattern: PC₁₀₀ (60) > DC₁₀₀ (21) > RC₁₀₀ (10) and PC₅₀ (76) > DC₅₀ (53) > RC₅₀ (52). Pot soil analyses showed composts well performed in TOC and CMI, in comparison to Ctrl (+42% and +13%), suggesting a positive impact on soil C amelioration. No significant differences were observed for δ¹³C distribution, suggesting the composts did not influence the microbic metabolism differently. These results indicated that the biomass harvested from the CWs can represent an interesting material for composting, combining carbon sequestration and nutrients recycling potential of these system, in addition to their wastewater treatment capacity.