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The soilless peat-based substrate partially solves the global soil problem in greenhouse vegetable production. However, it still produces serious N 2 O emissions due to the application of nutrient solutions. The pyrolysis biochar is regarded as an effective measure to reduce soil N 2 O emissions. However, the effect and mechanism of biochar on N 2 O emissions from the soilless substrate remain unknown. Therefore, this study set up six treatments by adjusting the ratio of biochar addition of peat-based substrate: 0% (0BC), 2% (2BC), 4% (4BC), 6% (6BC), 8% (8BC) and 10% (10BC) (v/v). The results showed that compared to the control treatment, N 2 O emissions reduced by 81%, 71%, 51%, 61%, and 75% in the 2BC, 4BC, 6BC, 8BC and 10BC treatments, respectively. In addition, lettuce yield increased by 10% and 7% in the 2BC and 4BC treatments and decreased by 0.5%, 4% and 6% in the 6BC, 8BC and 10BC treatments, respectively. Combining stable isotope technology, qPCR analysis and high-throughput sequencing, five microbial pathways of N 2 O production, including bacterial and archaea nitrification (BN and AN), denitrification performed by fungi, denitrifier bacteria and nitrifier bacteria (FD, DD and ND), were roughly distinguished. In addition, the extent of N 2 O reduction was obtained by δ 18 O vs.δ 15 N SP map. For all treatments, overall, the DD process (over 50%) was the main process of N 2 O production and reduction, while ND and AN processes were almost negligible (less 5%). In detail, the decrease of N 2 O emissions was caused by decreasing the contribution of FD in the 6BC, 8BC and 10BC treatments and reducing the contribution of BN in the 0BC and 2BC treatments. In addition, biochar addition increased the extent of N 2 O reduction to N 2 . In summary, the 2% biochar addition presented the greatest extent of N 2 O reduction to N 2 (83%) and the lowest N 2 O emissions as well as the highest lettuce yields and nitrogen utilization efficiency. Therefore, 2% biochar is deemed the most optimal addition to the peat-based substrate., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Soilless culture systems (SCS) play an increasing role in greenhouse vegetable production. In the SCS, soilless substrates serve as the major substitute for soil, supplying nutrients to plants but releasing greenhouse gases into the atmosphere. Remarkably, there is a serious problem of N 2 O emission due to excessive input of N fertilizer. However, the microbial processes of N 2 O production and consumption in soilless substrates have been rarely studied resulting in difficultly interpreting for its global warming potential. Therefore, these pathways from two classic soilless substrates under two irrigation patterns were investigated by stable isotope technology combined with qPCR analysis in present study. The results according to the dual isotopocule plot of δ 15 N SP vs. δ 18 O showed that the mean contribution of denitrification and the mean extent of N 2 O reduction of case i (Reduction-Mixing) were 26.2 and 81.2 % for the treatment of peat based substrate under drip irrigation (PD), 47.7 and 70.3 % for the treatment of coir substrate under drip irrigation (CD), 29.0 and 80.8 % for the treatment of peat based substrate under tidal irrigation (PT), and 50.8 and 47.4 % for the treatment of coir substrate under tidal irrigation (CT). These results were also further confirmed by the abundance of major functional genes including AOA amoA, nirK and nosZ. Altogether, N 2 O emission and its microbial processes are determined by substrate types instead of irrigation patterns. For detail, denitrification dominated in the peat based substrate and nitrification dominated in the coir substrate. Compared to the coir substrate, the peat based substrate had higher abundance of functional genes and stronger denitrification and thus generated more N 2 O. For the two soilless substrates, moreover, the microbiome replaced the mineral N content as the limiting factor for N 2 O emission. In the SCS, in summary, the two soilless substrates play an important role in tomato growth, but might suffer from inorganic nutrient surplus and microbial shortage. More importantly, the combined analysis of N 2 O isotopocule deltas and functional genes is a robust tool and provides reliable conclusions for clarifying the microbial processes of N 2 O production and consumption, thus it is also recommended for use in environments other than soilless substrates., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022. Published by Elsevier B.V.)

Nitrous oxide (N 2 O) emissions contribute to global climate change and are regulated by the intensities of N mineralization, nitrification and denitrification processes. These processes depend on soil management, e.g., tillage and straw addition. Microbial mechanisms underlying the effects of straw addition on N 2 O emissions were investigated by 56-day incubation of soil from a long-term (27-year) no-tillage (NoTill) and conventional tillage (ConvTill) field experiment. The abundance of N functional genes (amoA , amoB , nirK , nirS and nosZ) in no-tillage and conventional tillage soils was analysed based on straw addition. The application of 15N-labelled straw allowed us to trace N 2 O sources. Conventional tillage with 15N-labelled straw (ConvTill+Straw) reduced N 2 O emissions derived from soil by 42 %, whereas no-tillage with 15N-labelled straw (NoTill+Straw) reduced N 2 O emissions by only 8 % compared with soil without straw addition. The N 2 O emissions derived from straw were higher under NoTill+Straw than under ConvTill+Straw, whereas they accounted for only ~4 % of the total N 2 O emissions under both conditions. The nirK and nirS gene abundances were higher than those in soil without straw addition. Compared to that under ConvTill, the nosZ gene abundance was higher and the N 2 O emissions were 40 % lower under ConvTill+Straw, indicating its strong capacity to reduce N 2 O to N 2. Pathway analysis showed that gene (amoA and nirK) abundance combined with soil properties (particulate organic matter nitrogen (POMN) and NO 3 −) were the main factors affecting N 2 O emissions, with increased POMN and nirK abundance driving increased N 2 O production under NoTill+Straw. However, gene (amoA , amoB , nirS and nosZ) abundances affected N 2 O emissions, with the nosZ gene being the main factor decreasing N 2 O under ConvTill+Straw. These findings revealed that straw return suppressed N 2 O emissions in conventional tillage soil, which may mitigate greenhouse gas emissions. [Display omitted] • N 2 O emissions were similar between no-tillage and conventional tillage soils. • Straw input decreased N 2 O emissions derived from soil under both tillage practices. • Straw input increased nirK and nirS gene abundances under both tillage practices. • No-tillage soil resulted in greater N 2 Oemissions derived from straw than conventional tillage soil. • Reduced N 2 O emissions under ConvTill+Straw was due to the increased nosZ gene abundance. [ABSTRACT FROM AUTHOR]

Denitrification, as both origins and sinks of N 2 O, occurs extensively, and is of critical importance for regulating N 2 O emissions in acidified soils. However, whether soil acidification stimulates N 2 O emissions, and if so for what reason contributes to stimulate the emissions is uncertain and how the N 2 O fractions from fungal (f fD) and bacterial (f bD) denitrification change with soil pH is unclear. Thus, a pH gradient (6.2, 7.1, 8.7) was set via manipulating cropland soils (initial pH 8.7) in North China to illustrate the effect of soil acidification on fungal and bacterial denitrification after the addition of KNO 3 and glucose. For source partitioning, we used and compared SP/δ18O mapping approach (SP/δ18O MAP) and acetylene inhibition technique combined isotope two endmember mixing model (AIT-IEM). The results showed significantly higher N 2 O emissions in the acidified soils (pH 6.2 and pH 7.1) compared with the initial soil (pH 8.7). The cumulative N 2 O emissions during the whole incubation period (15 days) ranged from 7.1 mg N kg−1 for pH 8.7–18.9 mg N kg−1 for pH 6.2. With the addition of glucose, relative to treatments without glucose, this emission also increased with the decrement of pH values, and were significantly stimulated. Similarly, the highest N 2 O emissions and N 2 O/(N 2 O + N 2) ratios (r N 2 O) were observed in the pH 6.2 treatment. But the difference was the highest cumulative N 2 O + N 2 emissions, which were recorded in the pH 7.1 treatment based on SP/δ18O MAP. Based on both approaches, f fD values slightly increased with the acidification of soil, and bacterial denitrification was the dominant pathway in all treatments. The SP/δ18O MAP data indicated that both the r N 2 O and f fD were lower compared to AIT-IEM. It has been known for long that low pH may lead to high r N 2 O of denitrification and f fD , but our documentation of a pervasive pH-control of r N 2 O and f fD by utilizing combined SP/δ18O MAP and AIT-IEM is new. The results of the evaluated N 2 O emissions by acidified soils are finely explained by high r N 2 O and enhanced f fD. We argue that soil pH management should be high on the agenda for mitigating N 2 O emissions in the future, particularly for regions where long-term excessive nitrogen fertilizer is likely to acidify the soils. [Display omitted] • Acidification of cropland soil in North China leads to escalating N 2 O emission. • The N 2 O/(N 2 O + N 2) ratio of denitrification was negatively correlated with soil pH. • Cropland soil acidification may enhance fungal fraction producing N 2 O. • Result obtained by SP/δ18O MAP is more in conformity with the real situation than AIT-IEM. [ABSTRACT FROM AUTHOR]

Quantitative identification of the dominant N 2 O production and consumption processes following sudden shifts in soil aeration in well-aerated soils is limited, lagging the development of N 2 O mitigation strategies. Nitrous oxide emissions from agricultural soils, the largest source of anthropogenic N 2 O, are strongly influenced by low-molecular-weight organic carbons from root exudates (RootEx). In this study, we conducted a laboratory incubation experiment to investigate the mechanisms of N 2 O emissions regulated by transient anoxic conditions under the effects of different carbon substrates. Three common components in RootEx, oxalic acid, serine, and glucose were individually added as carbon sources every 24 h to the sandy loam soil. The contribution of microbial processes to N 2 O production (fungal denitrification, nitrification, bacterial denitrification) and consumption (N 2 O reduction) was evaluated using a dual isotope mapping approach (SP/ δ 18O MAP). The result indicated that transient anoxic conditions stimulated N 2 O production primarily through bacterial denitrification process facilitated by carbon substrates, and diminished the proportion of N 2 O reduction compared to oxic conditions. Among the three components of RootEx, serine and glucose induced higher N 2 O emissions and N 2 O reduction rates than oxalic acid during incubation. The differential effects of these three components on N 2 O emissions imply that the regulatory effect of carbon substrates on N 2 O reduction could not counterbalance the influence of transient anoxic conditions on N 2 O stimulation. Our findings provide a fundamental understanding of the effects of transient anoxic conditions and RootEx on N 2 O stimulation, facilitating the study of the spatial and temporal heterogeneity of N 2 O emissions in response to soil aeration changes in various upland soils. [Display omitted] • SP/ δ 18O MAP was conducted for N 2 O source partitioning under transient anoxic condition. • Transient anoxic conditions promoted N 2 O production mainly via bacterial denitrification. • Transient anoxic conditions reduced N 2 O reduction ratio compared to oxic conditions. • Fungal denitrification and nitrification are also alternative pathway to produce N 2 O in transient anoxic conditions. • Glucose and serine have more significant promoting effects on soil N 2 O emissions than oxalic acid. [ABSTRACT FROM AUTHOR]

Nitrogen (N) fertilizer is the major deriver of nitrous oxide (N 2 O) emissions in agricultural soil. In the vegetable fields in China both inorganic and organic fertilizers are largely applied as basic sources of nitrogen. Identifying the effects of fertilizer type on soil microbial activities involved in N 2 O emissions would be of great help for future development of N 2 O reduction strategies. N 2 O isotopocule deltas, including δ 15Nbulk, δ 18O and SP (the 15N site preference in N 2 O), have been used to analyze microbial pathways of N 2 O production under different treatments, including bio–organic fertilizer treatment, half bio–organic fertilizer and half urea (mixed fertilizer) treatment, urea treatment and no fertilizer treatment. We measured environmental factors, N 2 O fluxes and N 2 O isotopocule deltas to evaluate the dynamics of N 2 O emissions and constructed the dual isotopocule plots (δ 15Nbulk vs. SP and δ 18O vs. SP) of the main N 2 O emission phases to assess contribution of the involved microbial processes (bacterial nitrification, bacterial denitrification, nitrifier denitrification and fungal denitrification). According to the results of the main N 2 O emission phases, we found that bio–organic fertilizer and mix fertilizer treatments had significantly lower N 2 O emissions compared to urea treatment, with average N 2 O fluxes of 1477 ± 204, 1243 ± 187 and 1941 ± 164 μg m−3 h−1, respectively, but there were no significant effects on mineral N and cabbage yield. In addition, the urea treatment and the mixed fertilizer treatment had close and higher nitrogen use efficiency. Furthermore, the δ 18O vs. SP plot was useful for providing insight into microbial processes, showing that fungal denitrification/bacterial nitrification was the dominant microbial pathway and bio–organic fertilizer and mix fertilizer treatments had higher denitrification and N 2 O reduction compared to urea treatment. Those findings demonstrated that the partial replacement of urea with bio–organic fertilizer was a better choice, by means of enhancing denitrification to reduce N 2 O emissions and also guaranteeing the nitrogen use efficiency and the cabbage yield. • The partial replacement of urea with bio–organic fertilizer was a better choice to reduce N 2 O emissions. • Compared with the δ 15Nbulk vs. SP plot, the δ 18O vs. SP plot provided a clearer insight into N 2 O production pathways. • Fungal denitrification might be dominant microbial pathway for all treatment in our study. [ABSTRACT FROM AUTHOR]