Effect of sewage sludge biochar on tomato plant (Solanum lycopersicum L.) cultivation

The present study refers to biochar production, its application to soil with or without combining it with compost, as well as its effect on tomato (Solanum Lycopersicum L.) cultivation. The feedstock selected for biochar production was a mixture of primary and secondary sewage sludge, which had previously been anaerobically digested and thermally dehydrated. Sewage sludge pyrolysis was conducted at 300 °C. The produced biochar was initially characterized and subsequently applied to soil, in order to study its effect on Solanum Lycopersicum L. cultivation. Pot trials which included four treatments, each comprising of three replicates, were carried out during a period of three months, using pots with a 10 kg capacity. More specifically, the following treatments were studied: i) Control (soil without biochar addition), ii) soil with biochar at a rate of 2% w/w (BC-SS), iii) soil with compost at a rate of 2% w/w (Compost), and iv) soil with a mixture of biochar and compost at a total rate of 2% w/w (BC-SS+Compost). The aim of the study was to examine whether sewage sludge biochar with or without compost affects: a) the dry weight and morphological characteristics of the plant, b) the concentrations of macronutrients, micronutrients and heavy metals in aboveground and belowground plant tissues, c) soil properties, including nutrients and heavy metals.

With biochar application a number of soil properties exhibited substantial improvement. Specifically, significant increases were noticed for TOC (67% - 85% increase), NO₃–N (55% increase) and ΝΗ₄–Ν (145% increase). Moreover, biochar with or without compost, substantially improved plant growth (25% – 34% increase in the first 40 days), and led to a significant increase of the dry weight of aboveground and belowground plant tissues. Heavy metal concentrations in plant tissues were quite low. Specifically, traces of Cr, Ni and Co were found only in plant roots, while Si was present in plant roots and stems. As, Mo and Pb were present in all plant tissues, albeit without exceeding the permissible levels established for vegetables. Finally, no traces of Hg, Se and Cd were found in any of the tissues.

Generally, sewage sludge biochar addition to soil, with or without compost, improved soil characteristics and increased plant yield. Moreover, heavy metal concentrations within permissible levels do not raise any concerns regarding safe consumption of tomato fruits.