Can N Fertilizer Addition Affect N(2)O Isotopocule Signatures for Soil N(2)O Source Partitioning?

Isotopocule signatures of N(2)O (δ(15)N(bulk), δ(18)O and site preference) are useful for discerning soil N(2)O source, but sometimes, N fertilizer is needed to ensure that there is enough N(2)O flux for accurate isotopocule measurements. However, whether fertilizer affects these measurements is unknown. This study evaluated a gradient of NH(4)NO(3) addition on N(2)O productions and isotopocule values in two acidic subtropical soils. The results showed that N(2)O production rates obviously amplified with increasing NH(4)NO(3) (p < 0.01), although a lower N(2)O production rate and an increasing extent appeared in forest soil. The δ(15)N(bulk) of N(2)O produced in forest soil was progressively enriched when more NH(4)NO(3) was added, while becoming more depleted of agricultural soil. Moreover, the N(2)O site preference (SP) values collectively elevated with increasing NH(4)NO(3) in both soils, indicating that N(2)O contributions changed. The increased N(2)O production in agricultural soil was predominantly due to the added NH(4)NO(3) via autotrophic nitrification and fungal denitrification (beyond 50%), which significantly increased with added NH(4)NO(3), whereas soil organic nitrogen contributed most to N(2)O production in forest soil, probably via heterotrophic nitrification. Lacking the characteristic SP of heterotrophic nitrification, its N(2)O 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 N(2)O processes.