Evidence suggests that in addition to sequestering carbon (C), biochar amendment can increase crop yields, improve soil quality and nutrient cycling, reduce the leaching of nutrients from soil and stimulate soil microbial activity. However, biochar application primarily benefits soils of intrinsic poor quality, thus the advantages of adding biochar to temperate agricultural soils remains controversial. In addition, there is limited information about the longer term effects of biochar application, or of increasing the rate of biochar loading to soil. Therefore, the aim of this study was to determine the effect of biochar residency time and application rate on soil quality, crop performance, weed emergence, microbial growth and community composition in a temperate agricultural soil. We used replicated field plots with three wood biochar application rates (0, 25 and 50 t ha -1). Three years after biochar amendment, the plots were further split and fresh biochar added at two different rates (25 and 50 t ha -1) resulting in double-loaded reapplications of 25+25 and 50+50 t ha -1. After a soil residency time of three years, there were no significant differences in soil nutrients, microbial growth, mycorrhizal colonisation or weed emergence between biochar amended and unamended soil. In contrast, the reapplication of biochar had a significant effect on soil quality, (e.g. increased PO 43-, K + and Ca 2+, DOC, soil moisture, organic matter and EC), microbial growth, (e.g. decreased saprophytic fungal growth), increased mycorrhizal root colonisation and inhibition of weed emergence. Whilst biochar application is unquestionably a strategy for the sequestration of C, in this case, other benefits, e.g. improved soil nutrient levels or crop performance, seemed to be short lived. Reapplication of biochar exemplifies the transient nature of biochar-mediated benefits rather than any lasting differences in soil nutrient dynamics or microbial communities. These results emphasise the need for more long-term field studies to provide data that can meaningfully inform agronomic management decisions and climate change mitigation strategies.
