Plant growth responses to soil-applied hydrothermally-carbonised waste amendments: a meta-analysis

Abstract

Background and aims: Hydrothermal carbonisation (HTC) is an alternative thermochemical method for conversion of waste to carbonised material. HTC converts high moisture biomass into hydrochar, with substantially lower energy inputs than pyrolysis since pre-drying is not required. Hydrochar is increasingly being proposed as a soil amendment; however, hydrochar addition to soils has inconsistent effects on germination and plant growth. Here, we aggregated hydrochar-plant studies to ascertain the effect of hydrochar on plant production. Method: Using meta-analysis, data from 43 published articles with 437 pairwise comparisons was synthesised to investigate the effect of hydrochar on seed germination or plant growth, and the driving factors. Results: On average, hydrochar application significantly reduced both seed germination (-38 %) and shoot biomass (-10 %) across hydrochar properties and experimental conditions. Negative impacts of hydrochar on seed germination and shoot biomass were greatest when application rates of hydrochar were above 11 t/ha (for all feedstocks except woody biomass) and 16 t/ha, respectively. At a standardised application rate of 10 t/ha, unmodified sewage sludge, animal manure and green waste hydrochars had a significant negative effect on germination, whilst food waste and woody hydrochars had no effect. Importantly, modification of hydrochar to lower toxin content significantly mitigated the negative effect on both shoot biomass and germination. Conclusions: Findings provide a basis for further research to elucidate mechanisms leading to the different plant responses following hydrochar application. Fundamentally, interactions among hydrochar dose, properties and edaphic variables are essential to understand when and where benefits may be achieved.