Hydrochar as a Modulator of Soil Microbial Communities: Abundance, Activity, and Shifts in Bacterial and Fungal Composition

Hydrochar (HC), as an organic soil amendment, has the potential to improve soil fertility and crop yield. However, only recent studies have focused on its impact on soil microorganisms. At present, the effects of HC application on the abundance, activity, and taxonomic composition of distinct bacterial and fungal communities are still not fully understood. Therefore, we conducted a greenhouse pot experiment with five treatments under two different irrigation conditions (well-irrigated and water-deficit). We investigated the responses of sunflower (Helianthus annuus L.) yield, as well as soil chemical and biological properties, to two application rates of HC (3.25 and 6.5 t ha⁻¹) prepared from chicken manure. Mineral fertilizer treatments with equivalent total nitrogen contributions to those of the HC treatments were included for comparison. After 77 days of cultivation, the plants were harvested, and soil samples were collected from the topsoil (0-15 cm) for metagenomic analysis and to assess the abundance and activity of microorganisms.

At the onset of the experiment, HC application did not cause a significant change in the composition of most soil nutrients. However, in comparison to non-amended soils, HC application, particularly at elevated doses, improved plant productivity and induced changes of the soil nutrient concentrations under both irrigation conditions at the end of the experiment. Our primary hypothesis to explain our observation posits that the presence of HC in soils, can play a significant role in the development and activity of their microbial communities, which may indirectly enhance nutrient availability and affect other soil biogeochemical processes. Through microbial abundance analyses, including Colony Forming Unit (CFU) counts and qPCR (16S rRNA and ITS), soils treated with 6.5 t ha⁻¹ HC exhibited higher bacterial and fungal populations compared to untreated soils. Likewise, results from basal and substrate-induced (glucose, alpha-ketoglutarate, N-acetylglucosamine, and L-cysteine) micro-respiration (MicroResp) indicated greater microbial CO₂ production in HC-amended soils. Furthermore, soils treated with HC under well-irrigated conditions displayed a distinct microbial community-level physiological profile from that of untreated soils. These differences in microbial functional diversity suggest changes in the relative abundance of soil microbial communities in HC-treated soils, as it was confirmed by metagenomic analysis.

Our results underscore that the effects of HC amendments on soil systems should not be regarded as a straightforward linear process. Rather, it requires evaluation within the framework of the complex interplay of climatic conditions, application rate, plant physiology, and microbial composition and activity.

Acknowledgements: This work was supported by a PhD scholarship (PREDOC_00339) granted by the Junta de Andalucía, as well as by the “SequestCarb” project (PY20_01065, funded by the Junta de Andalucía) and the MarshSOIL project (PID2020-119220GB-I00, funded by the Agencia Estatal de Investigación).