Plant and microbial-derived carbon responses to contrasting tillage practices in a 53-year field experiment

Reduced tillage (RT) is widely promoted to enhance soil organic carbon (SOC) stock, yet its long-term, depth-resolved impacts on microbial processing and SOC stabilization remain controversial. We investigated a 53-year field experiment in Göttingen, Germany, established on a Haplic Luvisol under a cereal-based cropping system, comparing RT (5–8 cm) with conventional tillage (CT, up to 25 cm). Soils were sampled from 0 to 10 and 10–20 cm depths, and SOC stocks were quantified alongside biomarkers of plant-derived C (free lipids, lignin phenols) and microbial-derived C (amino sugars), microbial C use efficiency (CUE), and phospholipid fatty acids (PLFA). RT increased SOC stocks by 18% in the 0–10 cm soils through increased enzymatic C acquisition and higher microbial CUE, stimulating bacterial necromass formation from plant inputs. Conversely, RT decreased SOC stocks by 14% in 10–20 cm soils due to constrained substrate allocation. CT amplifies SOC stabilization via physical protection in macroaggregates and via fungal necromass accrual (+37% vs. RT) at 10–20 cm soils. These depth-dependent patterns emerge from tillage-induced modifications of key edaphic factors – soil pH, nitrogen availability, and bulk density – that regulate microbial community structure and function. While RT concentrates organic inputs and microbial processing in surface layers, CT facilitates vertical redistribution of organic matter and shifts microbial processing toward fungal-dominated pathways at depth. Therefore, we propose an integrated management approach that combines surface residue retention under RT with periodic (3–5 year) deeper incorporation to capitalize on both bacterial-mediated surface C accumulation and fungal-aggregate interactions at depth. This strategy, particularly when complemented with organic amendments, offers a pathway to optimize SOC storage across the entire soil profile while addressing the current limitations of single-system approaches, offering a balanced solution for climate-smart soil management.