The influence of hydrochar from biogas digestate on soil improvement and plant growth aspects

Abstract

Hydrochar (HC), produced by hydrothermal carbonization, offers technical advantages over biochar (BC) produced by pyrolysis, and is suitable for soil amelioration, carbon sequestration, and enhanced plant growth. BC grain size has been shown to influence nutrient retention, microbial colonization and aggregate formation; however, similar research for HC is lacking. Pot trials were conducted to investigate the influence of HC grain size [coarse (6.3-2 mm), medium (2-0.63 mm) and fine (< 0.63 mm)], produced from biogas digestate, for soil improvement in three soils: loamy Chernozem, sandy Podzol, and clayey Gleysol, at a 5% HC application rate (w/w). All soils including two controls (with and without plants) were analysed for water holding capacity (WHC), cation exchange capacity (CEC), wet aggregate stability, pH, plant available nutrients (PO 4 -P, K and N min ) and germination and biomass success using standard laboratory and statistical methods. Soil pH showed a compensatory shift toward the HC pH (7.2) in all soils over the course of the study. For example, the pH of the medium grained HC treatment for the Chernozem decreased from 7.9 to 7.2 and increased in the Podzol and Gleysol from 5.9 to 6.1 and 4.9 to 5.5, respectively. The nutrient-rich HC (2034 ± 38.3 mg kg −1 PO 4 -P and 2612.5 ± 268.7 mg kg −1 K content) provided only a short-term supply of nutrients, due to the relatively easily mineralized fraction of HC, which allowed for quick nutrient release. The pH and PO 4 -P effects were most pronounced in the fine grained HC treatments, with a ~ 87%, ~ 308% and ~ 2500% increase in PO 4 -P content in the Chernozem, Podzol and Gleysol, respectively, compared to the controls at the beginning of the study. The same trend was observed for the K and NH 4 + content in the fine and medium grained HC treatments in all soils. No seed germination inhibition of Chinese cabbage was observed, with average germination rates > 50% in all soils. An effect on NO 3 − content was indeterminable, while there was little to no effect on biomass production, WHC, CEC and aggregate stability. In conclusion, the application of 5% fine grained HC significantly influenced the nutrient content over a short-term. However, the application rate was insufficient to substantially improve plant growth, nor to sustain a longer-term nutrients supply, regardless of grain size.

Hydrothermal Carbonization as a Valuable Tool for Energy and Environmental Applications: A Review

“…Moreover, different studies demonstrated also the positive effect of hydrochar on the retention of pesticides, allowing an optimized utilization and reducing the possible risk of leakage and contamination of ground waters [156][157][158]. The fast mineralization of the carbon contained in the char leads to a short-term release of nutrients in the soil, reducing the carbon content by 30-40% within the first 12-19 months [159][160][161].…”

Towards Negative Emissions: Hydrothermal Carbonization of Biomass for Sustainable Carbon Materials

“…[322][323] Moreover, HTC carbon materials are suitable for soils with different textures, such as clay, sandy soils, and loamy soils. [322,[324][325] They also prove beneficial for highly weathered soils with unfavorable chemical properties, such as high pH levels and low cation exchange capacity (CEC). [326][327] The presence of acidic functional groups in HTC carbon materials allows them to effectively lower soil pH levels, particularly in alkaline and calcareous soils.…”

Effect of garden and park waste hydrochar and biochar in soil application: a comparative study

“…In addition, the application of hydrochar from urban biowaste on soil may raise crop productivity and nutrient availability [21]. These benefits have been described in soils with a wide range of textures, such as clay, loamy, and sandy soils [17,19,22]. However, direct application of biochar and hydrochar can also have an adverse impact on seed germination and plant growth since, depending on the source and operating conditions, hazardous chemicals may be present on their surfaces.…”