Microcosm study on the decomposability of hydrochars in a Cambisol
Highlights
► Increasing processing temperature enhances stability of hydrochar in soil. ► Feedstock and processing time has only little effect on decomposability. ► Lower O/C ratio of hydrochar indicates decreased decomposability. ► The O/C ratio is a function of processing temperature. ► MRT of hydrochars is up to 5 times higher compared to wheat straw.
Introduction
Increasing population and changing human diets in several parts of the world will boost the amount of organic wastes and residues from food production in the near future. Today, such materials are either used as non-modified, composted or digested manures and soil conditioners, for energy supply (heat, electricity, fuel), or landfilling. By such uses most if not all of the carbon (C) contained in parent materials is released to the atmosphere as CO2 or CH4 in short time (days to months) without any additional benefit, thereby contributing to climate change.
Simultaneously, the world's increasing energy demand will raise attraction on crop residues, which are currently left in the field to maintain soil organic carbon (SOC) stocks. Moreover, production "systems will shift to crops such as energy maize with only little amounts of residues remaining in the field. Such developments risk to decrease the SOC level of arable soils in the longer term, thereby threatening SOC related soil properties such as soil aggregate formation and stability, water infiltration and storage, soil aeration, nutrient storage and buffering, and finally crop yield [1]. Full return of crop residues to European arable soils may increase soil carbon levels by up to 0.7 Mg C ha−1 yr−1 [2], [3], [4]. Depending on soil, climate, crops/crop rotation, and tillage system, the negative effect of residue removal on soil quality may occur after a few years already [1], [5], [6], [7], [8], [9].
The SOC level might be conserved or restored through the application of adequate amounts of poorly stabilized materials such as green or animal manure, or stabilized materials, mainly composts, and recently biochar [10]. Biochar is usually characterized by a very high recalcitrance against microbial decomposition [11], [12], [13], thus offering the potential to climate change mitigation [14].
The technical process of hydrothermal carbonization (HTC) converts biomass low in dry matter (DM) content into a C-rich lignite alike product [15], [16], [17], denoted here as hydrochar [18]. The reaction takes place in aqueous milieu under saturated pressure and elevated temperature (180–250 °C) within a few hours (4–12 h) [15], [16], [17], [19], [20]. During HTC, the various components of the initial biomass are modified by chemical transformations: hydrolysis, dehydration, decarboxylation, condensation, polymerization, and aromatization [20]. The physicochemical properties of hydrochars are affected by the type of feedstock [21], and the production conditions; namely temperature and time of processing [17], [22], [23], [24]. Hydrochars produced from simple chemical compounds and lignocellulosic feedstocks were shown to consist of sphere-like microparticles of 1–10 μm in diameter [25] with a surface area up to 30 m2 g−1 [26], and cation exchange capacity of 134 mmolc kg−1 [27]. Accordingly, hydrochar may be designated as a soil conditioner [18], [28], [29]. Preliminary short-term studies on the effects of hydrochar applied to soil showed diminished plant growth due to reduced plant available nitrogen [30], [31], reduced germination and growth as a result of phytotoxicity [32], and an increase of soil cation exchange capacity and field capacity up to 40% and 9%, respectively [33].
Compared to composting of plant wastes and residues, the process of HTC presumably has a higher C conversion efficiency, which amounts to ∼90% of the initial biomass C if the solid (hydrochar) and liquid phase C are summed up [16], [22], [34]. The C distribution in the solid, liquid, and gaseous HTC product phase was found to account for ∼75, 20, and 5%, respectively [16]. Derived from studies on biochar it is hypothesized that hydrochars (solid phase) might similarly offer an increased stability against microbial decomposition [17], [25], [27]. First studies showed enhanced microbial activity [35] and increased CO2 emissions from hydrochar amended soils [21], [30], [36], [37]. From a laboratory trial the mean residence times (MRT) of hydrothermally carbonized 13C-glucose and 13C-yeast were estimated as 4 and 29 yr, respectively [21]. There is, however, a lack of systematic studies on the effect of feedstock and hydrochar production conditions on its actual stability against decomposition in soils.
Thus, the objectives of the present study were: i) to quantify the stability of hydrochar against microbial decomposition compared to other organic materials used as soil conditioners, and ii) to elucidate the effect of hydrochar feedstock and production conditions (processing temperature and time) on its decomposability in an arable soil. Our study aimed to contribute to an evaluation of the potential of hydrochar for C sequestration and climate change mitigation, and to optimize hydrochar properties for this purpose.
Section snippets
Hydrochar production
The hydrochar production conditions were systematically varied regarding processing temperature (180, 200, and 250 °C) and time (4, 12 h), and model feedstock (sugarbeet pulp, draff). The hydrochars were produced in an experimental batch (25 l) reactor at the University of Applied Sciences Ostwestfalen-Lippe (Campus Höxter, Germany). The reactor was filled with de-ionized water and the feedstock was added in a respective amount to achieve an initial DM content of about 15%. Subsequently,
Chemical properties of hydrochars
Hydrothermal carbonization of biomasses resulted in 20–37% higher C, 22–60% lower O, and 2–12% lower H contents in comparison to the original feedstock (relative differences). Such shifts in elemental composition became more pronounced with increasing processing temperature, while processing time caused only small changes (Table 1). The changes were more pronounced when draff was used as feedstock. Hydrothermal carbonization increased the N content by 27–55% when processing sugarbeet pulp to
Discussion
The stability of organic matter and organic C against decomposition in soil is determined by three major factors: soil organisms, the physicochemical environmental conditions, and the properties of the organic matter [49]. This study focuses on the properties of the applied hydrochars resulting from different feedstocks and hydrochar production conditions.
Hydrothermal conversion of plant biomass is, among others, characterized by dehydration and decarboxylation [16], [26], [50] which was
Conclusions
Hydrothermal carbonization of sugarbeet pulp and draff taken as model feedstocks resulted in hydrochars having considerably higher C content and heat of combustion compared to the original feedstocks. Hydrochar application increased soil C release. Higher processing temperature decreased the O/C mol ratio of the hydrochar and enhanced the size and MRT of the stable hydrochar-C fraction. This emphasizes the importance of hydrochar production conditions for its stability in soil.
Mean residence
Acknowledgments
This publication is based on work funded by the Deutsche Bundesstiftung Umwelt (DBU, Germany, Project number: AZ 27436 – 35/0). Technical assistance by the staff of the University of Applied Sciences Ostwestfalen-Lippe, Campus Höxter, and the Institute of Sugar Beet Research, Göttingen, is gratefully acknowledged.
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