An investigation of the effects of hydrochar application rate on soil amelioration and plant growth in three diverse soils

Abstract

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.

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

“…The S3 substrate exhibits the highest plant growth, probably because of the high OM content of peat, which, in combination with vermiculite, provides higher moisture retention and aeration. These results highlight the importance of choosing an adequate type of char material for each crop, plant, and soil [31]. Regarding the FHC concentration, we found that plants growing in S3 suffer a soft growth inhibition as the dose increases, while in the other substrates, a significant decrease in leaf area is observed in comparison with the control.…”

“…Moreover, addition of compost to the hydrochar diminished the phytotoxicity on cress seeds and Chinese cabbage germination [2,29]. Nevertheless, further research is needed to understand the relationship between the characteristics of hydrochar and the responses of different crops linked to soil type and dosage [30,31], time of stabilization [2], and the effect of post-treatments prior to soil application.…”

Comparison of Nutrient-Release Strategies in Hydrothermally Treated Digested Sewage Sludge

“…Therefore, those hydrochars from acid-free HTT would be suitable as a solid organic fertilizer because they contain N, P, and K, and those from HCl-mediated HTT would be suitable as soil amendment (nutrients have been mostly extracted) (Table S1). However, beyond complying with the limits established by current regulations, some authors have reported that the presence of certain organic compounds in hydrochars could inhibit plant growth, highlighting the importance of conducting studies to ensure proper synergy between HTC products and plant metabolism. , Eskandari et al (2019) tested the effects of paper mill sludge-based hydrochar on pine seedling growth, reported that the application of adequate rates of hydrochar can reduce fertilizer requirements, and showed positive effects on germination, shoot biomass, and stem diameter compared with control seedlings (without hydrochar) under the fertilizer levels tested.…”

Kinetic and Thermodynamic Evidence of the Paal–Knorr and Debus–Radziszewski Reactions Underlying Formation of Pyrroles and Imidazoles in Hydrothermal Liquefaction of Glucose–Glycine Mixtures

“…8 An alternative utilization of hydrochar as a soil amendment/fertilizer is also obstructed due to the phytotoxicity of N-heterocycles toward plants. 9 In addition, the existence of N-heterocycles in the nutrient-rich aqueousphase coproduct also complicates the nutrient separation using a membrane due to the potential fouling and blockage of the membrane pores. 10 These disadvantages demonstrate the importance of suppressing the formation of N-heterocycles in order to increase the techno-economic attractiveness of HTL operation.…”

“…While N-heterocycles removal from biocrude is possible via the catalytic hydrotreatment process in traditional oil refineries, the additional cost of this upgrading step leads to operational expenditure surpassing the economical price of the produced drop-in fuel . An alternative utilization of hydrochar as a soil amendment/fertilizer is also obstructed due to the phytotoxicity of N-heterocycles toward plants . In addition, the existence of N-heterocycles in the nutrient-rich aqueous-phase coproduct also complicates the nutrient separation using a membrane due to the potential fouling and blockage of the membrane pores .…”