Quantifying Manure’s Fertilizer Nitrogen Equivalence to Optimize Chemical Fertilizer Substitution in Potato Production

Accurate quantification of the Fertilizer Nitrogen Equivalence (FNE) of manure is crucial for optimizing integrated nitrogen (N) management and reducing chemical fertilizer use in potato production. However, uncertainties persist regarding FNE’s response to varying application rates and estimation methodologies. A two-year field experiment in Inner Mongolia, China, evaluated multi-gradient sheep manure applications in potato systems to determine whether FNE exhibits diminishing returns with increasing manure rates and to assess the influence of different estimation approaches. Potato N uptake, tuber yield, and growth parameters were measured. FNE was estimated using four methods: total N uptake, fertilizer-derived N uptake, absolute tuber yield, and yield increment. The key findings were: (1) Potato yield and total N uptake increased with higher N inputs but followed the law of diminishing returns. Notably, FNE values remained statistically stable across a wide application range (180–1200 kg N ha−1, equivalent to 8–53 t ha−1 of sheep manure), with no significant decline observed (p > 0.05), regardless of the estimation method. (2) Yield-based FNE values were, on average, 41% lower than those based on N uptake, indicating inefficiencies in converting absorbed N into tuber biomass. Among the methods, the yield increment approach demonstrated the highest consistency and robustness across treatments. In conclusion, our study demonstrates that the FNE of sheep manure remains stable across a broad application range in potato systems, with no evidence of diminishing returns. For practical fertilizer substitution, we recommend using the yield increment-based FNE estimation, as it provides a reliable and agronomically relevant measure for guiding manure application aimed at reducing chemical N inputs while maintaining crop productivity.