Reactions between nitrite and soil organicmatter and their role in nitrogen trace gasemissions and nitrogen retention in soil

As a key intermediate of both nitrification and denitrification, nitrite (NO2) is highly chemically reactive to soil organic matter (SOM), and it was proved previously that considerable amounts of nitrogen (N) trace gases, including nitrous oxide (N2O) and nitrogen oxides (NOx), were produced from the reactions of NO2 with SOM in chemical assays decades ago. However, the role of NO2‒ SOM reactions in nitrogen trace gas emissions and nitrogen retention in soils has been neglected until recently. It is vital to identify and quantify major sources and sinks of nitrogen trace gases for the sake of the environment. On the other hand, better understanding of N2O sources and nitrogen retention is also essential to improve the nitrogen use efficiency and soil fertility in agriculture. Therefore, this thesis aimed to gain a better understanding of the contribution of NO2‒SOM reactions to nitrogen trace gas emissions and nitrogen retention in soil. Emissions of N2O and carbon dioxide (CO2) from the reactions of NO2 with lignin and lignin derivatives (4‐hydroxybenzoic acid, 4‐hydroxybenzaldehyde, 4‐hydroxy‐3‐methoxybenzoic acid, 4‐ hydroxy‐3‐methoxybenzaldehyde, 4‐hydroxy‐3,5‐dimethoxybenzoic acid, 4‐hydroxy‐3,5‐ dimethoxybenzaldehyde), as well as N2O isotopic signatures, were studied in chemical assays at pH 3‒6. Among the six tested lignin derivatives, the highest N2O emission was found in the 4‐hydroxy‐ 3,5‐dimethoxybenzaldehyde treatment, and the dependency of N2O and CO2 on pH varied according to the structures of the organic substances. Most interestingly, N2O N site preference (SP) varied largely from 11.9‒37.4 ‰ depending on pH and structures of lignin derivatives, which was undistinguishable from other N2O sources, such as nitrification, denitrification, and abiotic hydroxylamine oxidation. Furthermore, real‐time N2O isotopic characterization revealed that SP also shifted largely during the reaction of NO2 with lignin derivatives. Hyponitrous acid and nitramide pathways, which could be responsible for N2O formation, were proposed to explain the shift of N2O SP values. Nitrite‐driven N2O and NOx emissions in spruce forest soils and SOM fractions were investigated online and simultaneously with a quantum cascade laser and a chemoluminescence analyzer, respectively. 17‒52 % and 3.3‒7.1 % of NO2 was immediately transformed to NOx and N2O, respectively, when NO2 was applied into soils. Abiotic N2O sources contributed 19.5‒42.3 % of the total N2O emission, and among four SOM fractions (dissolved organic matter, fulvic acid, humic acid, and humin), fulvic acid accounted for most of the abiotic N2O emission from NO2‒SOM reactions. Since the SP values of N2O from NO2‒SOM reactions were not distinguishable from that of microbial sources (denitrification and fungal denitrification for this experiment), end‐member maps failed to distinguish abiotic from biotic N2O sources, and application of a two‐end‐member mixing model Reactions between nitrite and soil organic matter and their role in nitrogen trace gas emissions and nitrogen retention in soil III biased N2O source apportioning by overestimating the contributions of both bacterial and fungal denitrification. Nitrogen retention resulting from NO2‒SOM reactions was investigated in forest, grassland, and agricultural soils using N‐NO2, and about 6 % of N‐NO2 was immobilized by SOM within 4 d. N enrichment in the fulvic acid fraction was dramatically higher compared with the humus. Solid‐state CP/MAS‐N‐NMR analysis revealed that nitro‐ and amide‐N were the dominant products of abiotic N immobilization from NO2‒SOM reactions. The effect of lignin content and composition on N2O emission, N retention, and mineral N pool dynamics were studied in agricultural soil after the application of organic soil amendments and N‐ labelled ammonium in a 114‐d laboratory incubation experiment. Both N retention and N2O emission were dramatically promoted by the combined application of N fertilizer and organic substances in the order: wheat straw > spruce sawdust > lignin. Moreover, both N retention and mineral N content were significantly (P < 0.05) correlated with lignin composition. For the first time, it was found that lignin composition regulated N partitioning of fertilizer in soil. Finally, an open pump theory was suggested to explain the combined and cross‐interacting biotic‐abiotic interactions of carbon and nitrogen in soil. This theory suggests that NO2‒SOM reactions could be a significant source of N trace gas emissions and N retention in soils. Notably, neither SP of N2O generated nor N immobilized by NO2‒SOM reactions is significantly distinct from biotic pathways, which makes it difficult to distinguish biotic and abiotic N2O production and N retention pathways. Further research is needed to identify and quantify the contribution of abiotic processes to soil N cycling. Reactions between nitrite and soil organic matter and their role in nitrogen trace gas emissions and nitrogen retention in soil