Rojas Pinzon, P. A.; Siedl, B.; Kejik, S.; Karbon, I.; Sedlacek, C. J.; Prommer, J.; Pilz, K.; Bueschl, C.; Sanden, T.; Spiegel, H.; Giguere, A. T.; Pjevac, P.; Fuchslueger, L.

Modern agriculture is characterized by substantial fertilizer nitrogen (N) losses from soils, resulting in low crop N-use efficiency. Biological nitrification inhibitors (BNIs) are studied as a strategy to improve N retention in soils by suppressing nitrification. However, the impacts of applying exogenous BNIs to crops with unknown intrinsic BNI capacity remain poorly understood. In this study, we evaluated the impacts of adding three BNIs (methyl 3-(4-hydroxyphenyl) acrylate [MHPA], 6-methoxy-2(3H)-benzoxazolone [MBOA], and limonene), their mixture, and the synthetic nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on barley (Hordeum vulgare L.) growth, plant and soil N dynamics, and soil microbial communities. Using a rhizobox system with planted and bare-soil compartments, combined with 15 N isotope tracing and molecular microbial community analyses, we assessed the spatio-temporal dynamics of N transformations, losses, plant N uptake, and microbial community responses in an alkaline agricultural soil. Independent of inhibitor application, the applied fertilizer N was lost primarily through NO3- leaching (3-9% of the applied N). In contrast, N2O emissions represented only 0.001-0.028% of the applied N and varied with inhibitor type. MHPA increased dissolved inorganic N soil pools without affecting plant biomass or 15N uptake or strongly shifting microbial community composition. MBOA reduced NO3- concentrations in soil pore water without influencing plant growth or N uptake but shifted soil microbial community composition. In contrast, limonene reduced plant growth and 15N uptake and most significantly altered microbial community composition, without significantly changing N availability. Applying a BNI mixture, as well as limonene alone, was detrimental to plant growth and 15N uptake. DMPP showed only minor effects on N pools, plant growth, plant N uptake and microbial community composition. Overall, our results reveal both the potential and limitations of exogenous BNI application for improving N retention in crop systems.