Hydrochar enhances growth of poplar for bioenergy while marginally contributing to direct soil carbon sequestration

Silvia Baronti,Irene Criscuoli,Federica Camin,Lorenzo Genesio,Giorgio Alberti,Franco Miglietta,Robert Mass,Luca Ziller,Francesco P Vaccari

doi:10.1111/gcbb.12450

Silvia Baronti, Irene Criscuoli + Show 7 more

Open Access

https://doi.org/10.1111/gcbb.12450

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Abstract

AbstractHydrothermal carbonization (HTC) has been proposed as an alternative method to pyrolysis for producing C‐rich amendments for soil C sequestration. However, the use of hydrochar (HC) as soil amendment is still controversial due to the limited information on the potential benefits and trade‐offs that may follow its application into soil. This study investigated the effects of HC starting from maize silage on plant growth in a 2‐year controlled experiment on poplar for bioenergy and evaluated HC stability in soil by periodic soil respiration and isotopic (δ13C) measurements. HC application caused a substantial and significant increase in plant biomass after one and two years after planting, and no evident signs of plant diseases were evident. Isotopic analysis on soil and CO2 efflux showed that slightly less than half of the C applied was re‐emitted as CO2 within 12 months. On the contrary, considering that the difference in the amount of N fixed in wood biomass in treated and not‐treated poplars was 16.6 ± 4.8 g N m−2 and that the soil N stocks after one year since application did not significantly change, we estimated that approximately 85% of the N applied with HC could have been potentially lost as leachate or volatilized into the atmosphere as N2O, in response to nitrification/denitrification processes in the soil. Thus, the permanence, additionality and leakage of C sequestration strategy using HC are deeply discussed.

Highlights

Minimal fertilizer input and high biomass yield are required to maximize the net benefit of bioenergy crops
During Hydrothermal carbonization (HTC) process, wet biomass undergoes a series of hydrolysis, condensations, decarboxylation and dehydration reactions and this transformation process leads to a two-phase mixture of solid and liquid which is conventionally named hydrochar (HC)
This study considered the use of HC as soil amendment in a poplar bioenergy crop

Summary

Introduction

Minimal fertilizer input and high biomass yield are required to maximize the net benefit of bioenergy crops. As bioenergy crops are expected to expand mainly on marginal and less fertile soils to avoid competition with food production, yield enhancement is not a trivial goal especially considering that intensification must be obtained without adverse environmental impact (Allwright & Taylor, 2016). Transformations are required to increase the recalcitrance of organic C-containing compounds while enhancing the availability of plant nutrients. Carbon-rich and recalcitrant solid residues (charred materials) can become available after the transformation, and those can be subsequently incorporated into soils becoming a sustainable negative emissions technology, eventually able to mitigate climate change (Smith, 2016). Several processes have been proposed so far for thermochemical conversion, leading to the production of a wide range of solid residues having different physical and chemical characteristics (Meyer et al, 2011). HC has been shown to be a promising sorbent of a wide range of pollutants (Sun et al, 2011; Eibisch et al, 2015; Han et al, 2016)

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