Can N Fertilizer Addition Affect NO Isotopocule Signatures for Soil NO Source Partitioning?

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