Short-term mineralization of hydrochar and its interaction with native soil carbon under contrasting moisture conditions

Hydrochar has emerged as a promising soil amendment within climate-smart agriculture due to its potential to improve soil properties and contribute to carbon (C) sequestration. However, while hydrochar is often regarded as a relatively recalcitrant material, substantial uncertainties remain regarding its short-term stability in soil and its interactions with native soil organic carbon (SOC), particularly under contrasting soil moisture conditions. Addressing these knowledge gaps is essential to better assess the role of hydrochar in soil C cycling and its implications for soil C stability.

In this study, we investigated the short-term mineralization dynamics of hydrochar and its effects on SOC decomposition through a 45-day laboratory incubation experiment. A Cambisol was amended with chicken-manure-derived hydrochar at four agronomically relevant application rates (3.25, 6.5, 13, and 26 t ha⁻¹) and incubated under two contrasting soil moisture regimes simulating well-irrigated (60% water holding capacity, WHC) and moderate water-deficit conditions (30% WHC). Soil respiration was periodically quantified on incubation days 1, 3, 7, 11, 16, 23, 30, 37, and 45, and the isotopic composition of emitted CO₂ (δ¹³C–CO₂) was determined using a GasBench interface coupled to an isotope ratio mass spectrometer. Combined with isotopic signatures of soil and hydrochar, these data allowed partitioning of CO₂ sources, estimation of hydrochar-derived CO₂ contributions, and assessment of priming effects on native SOC.

Across both moisture regimes, total soil respiration increased consistently with increasing hydrochar application rate throughout the incubation. Concurrently, δ¹³C–CO₂ values became progressively less negative at higher hydrochar doses, indicating an increasing contribution of hydrochar-derived C to total CO₂ emissions. Source partitioning confirmed that the proportion of CO₂ originating from hydrochar increased with application rate, with this effect being more pronounced and temporally consistent under well-irrigated conditions. These results demonstrate that hydrochar is not inert in the short term and can undergo measurable mineralization shortly after soil incorporation.

Priming effect analysis revealed a clear interaction between hydrochar dose and soil moisture. Under well-irrigated conditions, low hydrochar doses induced a tendency towards positive priming, whereas higher doses resulted in neutral or negative priming effects. In contrast, under water-deficit conditions, positive priming emerged predominantly at higher hydrochar application rates, increasing with dose. These patterns suggest that hydrochar-mediated stimulation or suppression of SOC mineralization is strongly context-dependent and governed by both amendment rate and water availability.

Overall, our findings challenge the common assumption of hydrochar recalcitrance by demonstrating its short-term degradability and its capacity to modulate SOC dynamics. The results highlight that hydrochar application can not only contribute directly to CO₂ emissions through its own mineralization but, depending on dose and moisture conditions, may also enhance native SOC decomposition. These insights are critical for refining assessments of hydrochar-based soil management strategies and their implications for soil C stability under future climatic scenarios.