Biochar from pruning residues as a soil amendment: Effects of pyrolysis temperature and particle size

Abstract

Adding some pruning residues to the soil could result on phytotoxicity due to the presence of polyphenols. Pyrolysis of those residues could lead to a product with less polyphenols and a potential to be used as soil amendment. Soil biological properties, and, in particular soil enzyme activities have often been as indicators of soil quality. We tested phytotoxicity, using a germination test, and soil biological properties, namely soil microbial biomass C and soil microbial biomass N, soil basal respiration and several soil enzymes (β-glucosidase, β-glucosaminidase and phosphomonoesterase) in a soil amended with pruning residues and with biochars prepared at 300°C and 500°C from those residues. Moreover, both biochar and pruning residues were divided into finer (<2mm, but mostly dust) and coarser (>2mm) particles and we studied the effect of adding two different doses of amendments (1 and 3%). Biochar resulted in an increase in the germination index of tomato and lettuce seeds in comparison to pruning residues, while no difference was found for the other plant species tested (watercress and lettuce). Pruning residues showed phytototicity in the tomato germination test. In general, both, pruning residues and the use of amendments with finer size particle resulted in an increase in soil enzyme activity. Our results indicate that soil enzymes are not responsive to the presence of polyphenols and could not be used as suitable indicators of soil quality in soils amended with residues containing polyphenols, while germination tests are more suitable to detect the presence of these toxic compounds.