Quantifying Soil Gaseous Nitrogen Losses From Nitrification and Denitrification Based on Nitrogen Isotope Model

Gaseous nitrogen (N) losses from nitrification and denitrification (NO + N₂O + N₂) pathways contribute a significant fraction of the total N losses from cropland ecosystems. The N mass balance and process-based models are commonly applied to estimate the NO + N₂O + N₂ losses but have suffered from systematic error accumulations or model over-parameterization, leading to a large uncertainty in estimation, hindering effective management of the global N budget. Here, we proposed a novel N isotope model, which considers fertilizer, ammonia volatilization and harvest after testing steady-state assumption of soil δ¹⁵N and N pool for croplands, and justified if it could be successfully applied to constrain NO + N₂O + N₂ losses from cropland ecosystems. We compiled the first bulk-soil δ¹⁵N data set of 0–30 cm soils (n = 738) from croplands and produced a global map of cropland soil δ¹⁵N, which is crucial input data for an isotope model to quantify NO + N₂O + N₂ losses. The results show that the cropland soil δ¹⁵N ranges from 3.5 to 9.0‰, with a mean value of 6.6 ± 0.8‰ (mean ± standard deviation). The estimated NO + N₂O + N₂ losses accounted for an average of 17 ± 9% of N outputs and were 35.86 ± 24.17 kg N ha⁻¹ yr⁻¹ in China's rice paddies, with an increasing trend from Central China to South or North China. The estimations were comparable with the results from observation-constrained denitrification-decomposition modeling (38.9 ± 4.8 kg N ha⁻¹ yr⁻¹) and in good agreement with experimental observations at site scale (R² = 0.58). Our results suggest that soil N isotopes, as a quantitative tracer, provide a valuable alternative approach to constrain the NO + N₂O + N₂ losses in croplands at large geographic scales.