Can N Fertilizer Addition Affect N₂O Isotopocule Signatures for Soil N₂O Source Partitioning?

Isotopocule signatures of N₂O (δ¹⁵N^bulk, δ¹⁸O and site preference) are useful for discerning soil N₂O source, but sometimes, N fertilizer is needed to ensure that there is enough N₂O flux for accurate isotopocule measurements. However, whether fertilizer affects these measurements is unknown. This study evaluated a gradient of NH₄NO₃ addition on N₂O productions and isotopocule values in two acidic subtropical soils. The results showed that N₂O production rates obviously amplified with increasing NH₄NO₃ (p < 0.01), although a lower N₂O production rate and an increasing extent appeared in forest soil. The δ¹⁵N^bulk of N₂O produced in forest soil was progressively enriched when more NH₄NO₃ was added, while becoming more depleted of agricultural soil. Moreover, the N₂O site preference (SP) values collectively elevated with increasing NH₄NO₃ in both soils, indicating that N₂O contributions changed. The increased N₂O production in agricultural soil was predominantly due to the added NH₄NO₃ via autotrophic nitrification and fungal denitrification (beyond 50%), which significantly increased with added NH₄NO₃, whereas soil organic nitrogen contributed most to N₂O production in forest soil, probably via heterotrophic nitrification. Lacking the characteristic SP of heterotrophic nitrification, its N₂O contribution change cannot be accurately identified yet. Overall, N fertilizer should be applied strictly according to the field application rate or N deposition amount when using isotopocule signatures to estimate soil N₂O processes.