Soil Physicochemical Properties, Microbial Communities, and Lucerne Performance under Organic Fertilization in a Long-Term Cropping System in Austria

Long-term organic fertilization can shape soil functioning and forage performance and quality, but its depth-dependent effects under semi-arid conditions are not well quantified. We assessed the impact of prolonged organic fertilization systems (FSs) on soil properties, soil microbiome, and lucerne (Medicago sativa L.) productivity in a long-term organic rotation in eastern Austria. Four fertilization systems were compared: FS1 (stockless, two-year lucerne green manure, GM), FS2 (stockless, GM plus municipal compost, GM+MC), FS3 (with livestock, lucerne forage-use plus farmyard manure, FU+FYM), and FS4 (with livestock, lucerne forage-use plus biogas digestate, FU+BD). Soil was sampled at 0–15 and 15–30 cm, except for aggregate stability (0–5 cm). Soil depth was the main influencing factor for chemical, physical and microbial properties. SOC, TN, plant-available P, plant-available K, pore volume, hydraulic conductivity, and copy numbers of bacteria and fungi all decreased significantly with soil depth, whereas pH and bulk density increased significantly with depth. Depth also had a pronounced effect on shaping bacterial and fungal community composition. GM+MC and FU+FYM increased topsoil P and especially K and tended to improve structure compared to GM and FU+BD. Microbial community composition was strongly depth-structured, with fungal communities more responsive to fertilization than prokaryotic communities. Lucerne biomass yield over two cuts ranged from 5.5 to 6.9 Mg ha-1, carbon yield from 2406 to 2891 kg C ha-1, and nitrogen yield from 136 to 186 kg N ha-1, with first cuts ≈34% (BMY), 24% (CY) and 16% (NY) higher than second cuts. These yields did not differ significantly among FSs. Overall, biannual lucerne maintained similar productivity and nutritional yield across long-term organic fertilization systems, while GM+MC and FU+FYM most effectively improved topsoil fertility and physical condition. The strong depth gradients highlight the need to consider soil profile stratification when designing lucerne-based organic systems for semi-arid regions