The application of hydrochar promotes soil microbial growth and enhances sunflower yield, altering the nutritional composition of the seeds

In recent decades, the potential use of chars, such as biochar or hydrochar, as soil amendments to enhance plant growth, has garnered significant interest. However, understanding potential toxic impact of these materials on soil and plants remains an area requiring further exploration. In this study, the effect of applying hydrochar (HC) derived from chicken manure on the yield and nutritional quality of sunflower (Helianthus annuus L.) seeds was assessed under both well-irrigated and water deficit (WD) conditions. To address it, the HC was applied to a Cambisol at rates of 3.25 and 6.5 t ha^-1. Mineral fertilizer treatments with equivalent total nitrogen contributions as the amendment were included for comparison. Sunflower plants were cultivated in pots under two irrigations conditions (60 and 30% of the soil water holding capacity). After 77 days of cultivation, the plants were harvested, and the content of macro- and micronutrients, heavy metals in both seeds and leaves, as well as the seed fatty acid composition, were analyzed. Additionally, soil nutrient contents were assessed post-experiment

Initially, HC application did not cause a noticeable change in soil nutrient composition. However, both HC applications demonstrated high productivity under well-irrigated conditions. Likewise, under WD conditions, the best yields were achieved with a HC dose of 6.5 t ha^-1. Conversely, the composition of the different fatty acids in the seed oil remained unaffected by the treatments and irrigation conditions. Nonetheless, seed nutrient content was notably affected, particularly in plants treated with 6.5 t ha^-1. Under well-irrigated conditions, the seeds of these plants exhibited decreased K and P levels, along with higher levels of toxic elements and heavy metals such as Al, Ba, Cd, Pb, and Sr. Under WD conditions, the impact of the treatment on heavy metals contents was less pronounced, but there was a marked reduction of seed macronutrient content (Ca, K, Mg, P and S). Notably, plants treated with 6.5 t ha^-1 of HC under well-irrigated conditions exhibited a preferential accumulation of Al and Sr in the seeds, leading to the lowest concentrations of these toxic elements in their leaves. The accumulation of these metals in the seeds was accompanied by a decline of both elements in the soil after 77 days of experiment under this irrigation condition, suggesting plant uptake. Given that the concentration of both elements did not increase after the application of HC at the beginning of the experiment, it is presumed that these changes are due to a potential role of HC involved in heavy metals mobilization. Following qPCR analysis this may be related to microbial activity since the soils treated with 6.5 t ha^-1 of HC exhibited a higher abundance of 16S rRNA and ITS gene copies related to bacteria and fungi, respectively, along with an increased dehydrogenase activity. Our findings highlight that the impact of char amendment on soil systems should not be seen as a simple linear process but has to be evaluated considering the complex interactions between climatic conditions, application rate, plant physiology and microbial activity.