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

Peiyi Zhang,Jinbo Zhang,Teng Wen,Yangmei Hu,Zucong Cai

doi:10.3390/ijerph18095024

Abstract

Isotopocule signatures of N2O (δ15Nbulk, 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 δ15Nbulk 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.

Highlights

The nitrous oxide (N2 O) emitted from soil is governed by various pathways, which often occur simultaneously in different soil micro-sites [1,2]
To ensure precise and robust isotopocule measurements, this method is mainly performed in soils with a high N2 O concentration, such as agricultural and grassland soil, while it is seldom utilized in forest soil due to the relatively small N2 O flux [3]
We have shown that N fertilizer addition led to an obvious change in the N2O flux, N2O isotopocule signatures and N2O source contributions in agricultural and forest soil incubation experiments

Summary

Introduction

The nitrous oxide (N2 O) emitted from soil is governed by various pathways, which often occur simultaneously in different soil micro-sites [1,2]. A position-specific nitrogen (N) isotope method measuring the intramolecular distribution of 15 N in N2 O (site preference, SP) has served as a useful tool to source partition N2 O in various soils [3,4,5,6]. It has clear advantages, such as the minimal disturbance of soil over the 15 N tracing method, independence of the isotopic signature of the substrate over the traditional natural abundance (15 N, 18 O) method and applicability in spatial–temporal scales with a low cost [7,8,9].

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