The effect of paper sludge and biochar addition on brown peat and coir based growing media properties

•Biochar can be used for peat replacement in substrates.•Biochar reduces the over-explotation of peatlands.•Biochar and paper sludge are phytonutrient for lettuce.•Biochar addition doubles crop yield.

Peatlands are crucial sinks for carbon in the terrestrial ecosystem, but they are jeopardized by their use as fuel or as growing media. Much research has been performed aiming to find high quality and low cost substrates from different organic wastes, such as coir, compost, sewage or paper sludges, and thus decrease peat consumption. The main objective of this work is to study the effect on peat and coir-based growing media of deinking sludge (R) and biochar obtained by pyrolysis of deinking sludge at 300 °C (B300). For this reason, mixtures of peat or coir with deinking sludge and corresponding biochar were prepared mixing them at 50/50 v/v ratios. The results showed that it is possible to improve the chemical and hydrophysical properties of peat and coir with addition of biochar and deinking sludge. Indeed, biochar increased air space, water holding capacity and total porosity of peat-based growing media whereas for coir, the best hydrophysical properties were obtained after deinking sludge addition. Finally, the use of biochar plus peat as growing media can increase lettuce yield by more than 100% with respect to peat growing media, which can be related with the improvement of hydrophysical growing media properties. This yield increment along with the reduction of the over-exploitation of peat can justify the use of biochar as growing media in spite of the cost associated to the pyrolysis process.