No 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.

The hydrothermal carbonization (HTC) of biogas digestate alters the raw materials inherent characteristics to produce a carbon (C)-rich hydrochar (HC), with an improved suitability for soil amelioration. Numerous studies report conflicting impacts of various HC application rates on soil properties and plant growth. In this study, the influence of HC application rate on soil improvement and plant growth aspects was investigated in three diverse soils (Chernozem, Podzol, and Gleysol). Pot trials were conducted in which all soils were amended with 5, 10, 20 and 30% (w/w) HC in quintuplicate, with two controls of pure soil (with and without plants, respectively) also included. Prior to potting, soil samples were collected from all HC-amended soils and controls and analyzed for soil pH, plant available nutrients (PO4-P and K), and microbial activity using standard laboratory and statistical methods. Immediately after potting, a 6-week seed germination experiment using Chinese cabbage was conducted to determine germination success, followed by a plant growth experiment of equal duration and plant species to determine biomass success. At the end of the study (after a total plant growth period of 12 weeks), each pot was sampled and comparatively analyzed for the same soil properties as at the beginning of the study. Soil pH shifted toward the pH of the HC (6.6) in all soils over the course of the study, but was most expressed in the 20% and 30% application rates, confirming the well-documented liming effect of HC. The addition of HC increased the PO4-P and K contents, particularly with 20% and 30% HC amendments. These results are proposedly due to the large labile C fraction of the HC, which is easily degradable by microorganisms. The rapid decomposition of this C fraction prompted the quick release of the HCs inherently high PO4-P and K content into the soil, and in turn, further stimulated microbial activity, until this fraction was essentially depleted. HC addition did not inhibit seed germination at any rate, presumably due to a lack of phytotoxic compounds in the HC from aging and microbial processes, and furthermore, showed no significant impact (positive or negative) on plant growth in any soil, despite improved soil conditions. In conclusion, although less pronounced, soil improvements were still achievable and maintainable at lower application rates (5% and 10%), whereas higher rates did not ensure greater benefits for plant growth. While the addition of high rates of HC did not detrimentally effect soil quality or plant growth, it could lead to leaching if the nutrient supply exceeds plant requirements and the soil’s nutrient retention capacity. Therefore, this study validates the previous study in the effectiveness of the biogas digestate HC for soil amelioration and suggests that smaller regularly repeated HC applications may be recommendable for soil improvement.

Hydrochar (HC) produced by the hydrothermal carbonization (HTC) of typically wet biomass is generally considered to be less effective for carbon (C) sequestration in soils compared to biochar (BC) by pyrolysis, due to a higher content of more easily decomposable C. Although the recalcitrance of HC is suggested to improve with increasing HTC production temperature, the way it interacts and becomes associated with soil organic matter (SOM) fractions of different stabilities against decomposition, may also influence its effectiveness for C sequestration in soils. In that respect, this study aimed to verify the potential of HCs from maize silage produced at different HTC temperatures (190, 210 and 230 °C) for C sequestration in a HC-amended sandy loam Podzol. To do this, we conducted a pot trial experiment and traced the fate of HC-derived C (HC-C) within different SOM fractions, namely the free- and occluded particulate organic matter (POMF and POMO, respectively) fractions and that comprising organic matter (OM) bound to clays (OMCl). Approx. 1 year after applying 5% of the different HTC temperature HCs to the soil, the SOM fractions were isolated by density fractionation for each HC treatment (HC190, HC210 and HC230) and the control (absent of HC). All fractions and the HCs were analyzed for organic C (OC) content and isotopic signatures (δ 13C). From the δ 13C signatures, the amount of HC-C and native soil organic carbon (SOC) within each fraction was calculated. Increased C contents and decreased H/C and O/C ratios were observed with increasing HTC production temperatures, which suggests a lower stability for the low temperature HC. After ca. 1 year, a loss of ~ 20–23% of the bulk soil TOC was found in the HC-amended soils. The POMF fraction of the HC-amended soils showed losses of 68–81% HC-C and 52–72% native SOC, which may be due to a positive priming effect caused by HC addition. The POMO and OMCl fractions of the HC-amended soils contained more OC than the control, indicating the integration of HC-C together with SOM within these more stable fractions, while the effect of HTC production temperature on the level of decomposition of the resultant HCs was negligible. In all HC treatments, the OMCl fraction comprised the least amount of HC-C, thus showing the weakest response to C amendment. In conclusion, long(er)-term research on the C net balance that accounts for the observed priming-induced TOC losses and the HC-C enrichment in more stable fractions is required to verify the potential of the different HCs for the purpose of C sequestration in soils. Graphical Abstract