Lauren C Breza ^1* A. Stuart Grandy ²

• ¹ Forage Seed and Cereal Research Unit, Agricultural Research Service (USDA), Corvallis, United States
• ² University of New Hampshire, Durham, North Carolina, United States

Understanding how nutrient inputs affect nitrogen (N) transformations and storage is vital for developing sustainable agroecosystems. Organic N inputs, such as crop residues (e.g., cover crop biomass and stover) and animal manures, can accelerate N cycling by increasing mineralization and immobilization rates to provide crops with more opportunities to intercept N as it moves through bioavailable pools. We aimed to understand how organic and synthetic soil amendments inhibit or promote N mineralization, immobilization, and nitrification rates. We synthesized results from studies using 15 N pool dilution methods to assess N transformation rates in response to different soil amendments. Our findings indicate that adding synthetic, manure, and crop derived residues as soil amendments increased mineralization by 60%, 135%, and 214%, respectively, relative to the unamended controls. While manure and residue produced similar mineralization rates, residue amendments induced significantly higher immobilization rates than synthetic and manure amendments -a sevenfold and fourfold increase, respectively. Furthermore, only residue N amendments enhanced the ammonium (NH4 + ) pool size, while synthetic and manure amendments resulted in no change in NH4 + pool size. These results suggest that residue amendments encourage tighter coupling of the carbon (C) and N cycles compared to manure or synthetic amendments by delivering C rich substrates (e.g., C/N ratio >20:1) to soil microbes. This tighter coupling with residue amendments leads to faster mineralizationimmobilization processes and larger NH4 + pools than those observed with manure or synthetic amendments. As such, residue amendments encourage soil N recycling between inorganic and organic forms, which is crucial to supporting crop N needs throughout the growing season while minimizing N losses.• Amendment Effect: the natural-log transformed ratio of the means (i.e., response ratio; RR) quantifying the differences between amended and unamended soils.• 𝐴𝐸 ∆ : Percent change in transformation rates (or NH4 + pool size) with respect to zero (i.e., back-transformed RR)