Drought increases root and rhizodeposition carbon inputs into soil
Aims. Increasing droughts affect crop yield and health. Plants can respond to drought by adapting their root biomass and morphology and quality and quantity of rhizodeposition to improve water and nutrient uptake. Rhizodeposition is an important link between plants and microbes. Besides droughts, agricultural management influences roots and rhizodeposition; however, it is not well studied how cropping systems can affect the response of roots and rhizodeposition to drought. Methods. A semi-continuous 13CO2 isotope labelling was performed long-term field experiment comparing organic, mixed conventional, and mineral conventional cropping systems. Rainout shelters were installed to induce drought. Root, rhizodeposition, and the rhizosphere microbiome were determined at wheat grain ripening.  Results. Drought enhanced the total root carbon mainly through the increase of fine wheat roots. However, fine root carbon was mainly enhanced in the mixed conventional and organic cropping systems, both receiving farmyard manure, whereas no increase was measured in the minerally fertilized conventional system. Rhizodeposition carbon was enhanced in all cropping systems under drought, particularly in the first 25 cm. While some plant growth-promoting genera such as Streptomyces and Rhizophagus showed relative increases under drought, other plant growth-promoting genera often involved in nitrogen fixation such as Rhodoferax and Mesorhizobium were decreased. Conclusion. This field trial suggests that drought increases belowground carbon via fine root and rhizodeposition carbon inputs, indicating an adaptation to water and nutrient limitations. However, the fine root C response to drought can differ between cropping systems suggesting different adaptations or susceptibilities to drought.