Temporal Dynamics in Soil Carbon and Glomalin-Related Soil Proteins Under Sewage Sludge-Derived Biochar and Compost Treatments

Sewage sludge recycling through composting or pyrolysis offers potential to enhance soil biological activity and health indicators, yet field-based evidence across different soil conditions and crop rotations remains limited. This two-year field trial evaluated the effects of composted sludge and sludge char (applied at 5 tons per hectare) on soil glomalin, permanganate oxidisable carbon (POXC), total carbon (TC), pH, effective cation exchange capacity (ECEC), and Mehlich 3 extractable nutrients (phosphorus, potassium, calcium, Magnesium, sulphur, iron, Manganese, Zinc) in two contrasting agricultural soils. The studied soils differed substantially in carbon (3-fold), phosphorus (2-fold), and nitrogen (2-fold). Winter wheat (Triticum aestivum L., cv. Viriato) followed by mustard (Sinapis alba L., cv. Zlata) was used to evaluate glomalin accumulation under mycorrhizal host and non-mycorrhizal conditions. Both amendments increased easily extractable glomalin (EEG) during wheat cultivation, with greater responses at the site with lower phosphorus and a lower carbon-to-nitrogen (C:N) ratio (27% for sludge char and 13% for compost). TC and POXC increased significantly within the first 12 months at both sites. Neither amendment altered pH or nutrient extractability significantly compared to the unamended control. Under non-mycorrhizal mustard, glomalin continued to significantly increase where the C:N ratio and phosphorus levels were lower, indicating that phosphorus availability and the C:N ratio control arbuscular mycorrhizal fungi (AMF) activity. These findings demonstrate that sewage sludge recycling can enhance soil biological activity in phosphorus-limited soils, particularly when combined with mycorrhizal crop rotations, although long-term carbon persistence, AMF colonisation rates, and plant nutritional status require further investigation.