Co-composted biochar enhances soil fertility more than individual additives

Biochar is considered a promising amendment to store carbon and improve degraded soil properties. However, this additive could have disadvantageous effects on plants and soil organisms due to its charring process toxic by-products and its ability to retain nutrients. To mitigate these negative effects, co-amendments with an organic additive such as compost have been proposed, but comparative studies are scarce. In this study, we investigated the influence of biochars applied (i) solely, (ii) mixed with matured compost or (iii) co-composted biochar on soil properties and plant growth. To this aim, three different types of biochar derived from various feedstocks were tested in two soil with different agronomic properties (Luvisol and Fluvisol). After three months of greenhouse experiment with grown lettuce (Lactuca sativa var. capitata L.), the shoot and root biomasses were quantified and the soil physicochemical properties were measured (pH, CEC, total N, organic C and soil available P). Solely applied biochar did not influence plant yield and maintained alkaline soil pH. Contrariwise, biochar mixed with matured compost maintained an average increase of lettuce growth by three times with risen soil nutrient content and kept alkaline pH. Whereas treatments with co-composted biochar and solely added compost promoted plant growth by almost six times, kept pH on neutral levels and nutrients on an average level with CEC equally enhanced, regardless of the biochar origin or the soil type. These results suggest that co-composted biochar addition to the soil is a convincing way to maintain soil fertility in a long term.