Pyrochars and hydrochars differently alter the sorption of the herbicide isoproturon in an agricultural soil

• Chars with high carbonization degree (high C stability) have high sorption capacity.
• Impact of raw materials on sorption capacity depends on carbonization conditions.
• High bioaccessibility in hydrochar amended soil irrespective of the raw material.
• Hydrochars react as H-bond acceptor, pyrochars through hydrophobic and H-bonding.
• The micro-porosity determines the amount of non-extractable residues.

Carbonaceous material from pyrolysis (pyrochars) and hydrothermal carbonization (hydrochars) are applied to soil to improve soil fertility and carbon sequestration. As a positive side effect, the mobility of pesticides and the risk of groundwater contamination can be minimized. However, the impact of various raw materials on the sorption capacity of different pyrochars and hydrochars is poorly understood. Thus, sorption experiments were performed with 14C-labeled isoproturon (IPU, 0.75 kg ha−1) in a loamy sand soil amended with either pyrochar or hydrochar (0.5% and 5% dry weight, respectively). Carbonaceous materials were produced from three different raw materials: corn digestate, miscanthus, woodchips of willow and poplar. After 72 h of incubation, a sequential extraction procedure was conducted to quantify in situ IPU bioavailability, total amount of extractable IPU, and non-extractable pesticide residues (NER).Added char amount, carbonization type, and raw materials had statistically significant effects on the sorption of IPU. The amount of in situ available IPU was reduced by a factor of 10–2283 in treatments with pyrochar and by a factor of 3–13 in hydrochar treatments. The surface area of the charred material was the most predictive variable of IPU sorption to char amended soil. Some physical and chemical char properties tend to correlate with pore water-, methanol- or non-extractable IPU amounts. Due to a low micro-porosity and ash content, high water extractable carbon contents and O-functional groups of hydrochars, the proportion of NER in hydrochar amended soils was considerably lower than in soil amended with pyrochars.