Soil additives ameliorate crop´s phosphorus nutrition and the rhizosphere microbiome during drought

Climate models predict that the frequency, magnitude, spatial extent and duration of extreme climate events such as drought will further increase throughout Europe in the 21st century. Drought not only affects water availability but also alters the rhizosphere microbiome and its functions, consequently hampering soil nutrient cycling and crop nutrition. One measure to circumvent drought conditions in soil, at least during short to intermediate dry periods, is the application of additives to enhance P availability by improving diffusion conditions in the rhizosphere. However, studies focusing on the effect of soil additives on crop nutrition and functional capabilities of the rhizosphere microbiome during drought are scarce. We conducted a rhizotron experiment planted with spring wheat and induced 11 days of drought to investigate the effect of novel soil additives such as pelleted biochar-lignocellulose hydrogels including activators and nutrient loadings versus pyrolyzed biochar on wheat´s phosphorus (P) nutrition. Besides analyses of macro- and micro-nutrients in root and shoot as well as wheat’s active gene transporters (PM ATPase, ALMT, MATE, PHT, PHO1, SWEET), we determined the co-localization of enzymatic properties (Vmax, Km), pH, and microbial functional gene abundance in rhizosphere hotspots.

The area of rhizosphere phosphomonoesterases hotspots reduced to 1% during drought without additives (non-drought condition 4%). Biochar-hydrogel pellets amended to soil shifted microbial community composition, increased their diversity, and enhanced functional gene abundances of the microbiome in rhizosphere hotspots under drought conditions. The P content in roots was up to 3-fold higher with pellets than without. Higher P mobilization was determined in soil amended with pellets rather than solely biochar or control which was in line with a doubling in abundance of phosphomonoesterase genes. Consequently, the addition of the pellets increased P availability in the rhizosphere, potentially based on improved diffusion processes. Wheat´s PHT1.6 transporter in the shoots, which are crucial for P uptake and remobilization, was 9-fold higher in pellet amended soil than in control. Moreover, there was a 3-fold increase in the abundance of the PHO1 transporter in roots, which facilitates P transport from roots to shoots. The root: shoot ratio was 3-fold lower when the pellets were added implying less investment in root development across the wheat growth period. Wheat´s active PM ATPase and SWEET gene expression in shoots was 2-fold higher with added pellets than in control during drought, highlighting the potential of H⁺-ATPase gene regulation in shoots as a strategy to increase the proton motive force and thus co-transport with phosphate.

The results suggest an ameliorated functional redundancy of the microbiome mitigating drought stress and improving soil health compared to single biochar application. Next the application of ecologically uncritical soil additives such as pelleted biodegradable lignocellulose hydrogels with pyrolyzed biochar to mitigate drought stress in crop production is going to be investigated in field trails.