<p>The hydrothermal carbonization (HTC) of organic feedstocks results in hydrochar, a product free of pathogens with the potential to be used as soil amendment as an efficient tool to recycle nutrients from organic waste and improve soil quality. However, presently the knowledge about the mechanisms of controlling hydrochar biogeochemical effects in planted soils is still scarce. Likewise, there is not much understanding about the effects under deficit irrigation conditions. In the present work, a hydrochar (250 &#186;C, 0.5 h) from chicken manure was used to evaluate the impact of this amendment on nutrient status and dynamics, the microbial activity and the quantity and quality of the organic matter of sunflower-planted soils during a 77 days greenhouse experiment. The main objective of this study was to obtain a better understanding of the interrelationship between soil organic matter (SOM), soil nutrients, microbial activity and irrigation management of soils.</p><p>The hydrochar was applied to a Cambisol with doses of 3.25 and 6.5 t ha<sup>-1</sup>. For comparison, mineral fertilizer treatments with the same contribution of total nitrogen (N) as the amendment were included in the study. Sunflower plants (<em>Helianthuus annus</em> L.) were grown in pot cultures under two irrigations conditions (60 and 30% of the soil water retention capacity).</p><p>Total macro- and micronutrients, along with soluble nutrients among the soils were monitored after 30 and 77 days. The SOM composition of these soils was characterized by solid-state NMR spectroscopy. For the microbiological studies, the microbial biomass carbon (MBC) and N (MBN) were analysed by the fumigation-extraction technique. Dissolved organic carbon (DOC) and N (DON) were also analysed. Determination of &#946;-glucosidase, acid phosphomonoesterase and dehydrogenase activities were assayed to study the microbial metabolism of soil. We test the hypothesis that hydrochar has an impact on composition and dynamics of nutrients, SOM quality and soil microbial activities, resulting in a beneficial effect on the soil-plant system in both irrigation conditions.</p>
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