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Application of inorganic nitrogen (N) fertilizer and manure can increase nitrous oxide (N2O) emissions. We tested the hypothesis that increased N2O flux from soils amended with manure reflects a change in bacterial community structure and, specifically, an increase in the number of denitrifiers. To test this hypothesis, a field experiment was conducted in a drip-irrigated cotton field in an arid region of northwestern China. Treatments included plots that were not amended (Control), and plots amended with urea (Urea), animal manure (Manure) and a 50/50 mix of urea and manure (U+M). Manure was broadcast-incorporated into the soil before seeding while urea was split-applied with drip irrigation (fertigation) over the growing season. The addition treatments did not, as assessed by nextgen sequencing of PCR-amplicons generated from rRNA genes in soil, affect the alpha diversity of bacterial communities but did change the beta diversity. Compared to the Control, the addition of manure (U+M and Manure) significantly increased the abundance of genes associated with nitrate reduction (narG) and denitrfication (nirK and nosZ). Manure addition (U+M and Manure) did not affect the nitrifying enzyme activity (NEA) of soil but resulted in 39–59 times greater denitrifying enzyme activity (DEA). In contrast, urea application had no impact on the abundances of nitrifier and denitrifier genes, DEA and NEA; likely due to a limitation of C availability. DEA was highly correlated (r = 0.70–0.84, P < 0.01) with the abundance of genes narG, nirK and nosZ. An increase in the abundance of these functional genes was further correlated with soil NO3−, dissolved organic carbon, total C, and total N concentrations, and soil C:N ratio. These results demonstrated a positive relationship between the abundances of denitrifying functional genes (narG, nirK and nosZ) and denitrification potential, suggesting that manure application increased N2O emission by increasing denitrification and the population of bacteria that mediated that process. [ABSTRACT FROM AUTHOR]

It remains unclear how the source and rate of nitrogen (N) fertilizers affect N 2 O concentration and effluxes along the soil profile under the drip-fertigated agricultural system. A plot-based field study was performed in 2017 and 2018 in a cotton field in arid northwestern China, with an objective to elucidate the impact of the applications of conventional urea (Urea), polymer-coated urea (ESN) and stabilized urea (SuperU) at rates of 120 and 240 kg N ha−1 on concentration and efflux of N 2 O in the soil profile and its relationship with N 2 O surface emissions. The in-situ N 2 O concentrations at soil depths of 5, 15, 30 and 60 cm were measured and used to estimate soil profile N 2 O effluxes. Estimates of surface N 2 O flux using the concentration gradient-based (GM) were compared with those measured using the chamber-based (CM) method. In both years, soil N 2 O concentrations at all depths increased in response to basal N application at planting or in-season fertigation events. However, N rate or source did not affect soil N 2 O concentrations or effluxes at each depth. Surface emissions of N 2 O were mostly associated with that presented in the top layer of 0–15 cm. Surface N 2 O efflux determined by GM was poorly or not associated with those of chamber measurements, which was attributed to the low N 2 O production restricted by soil moisture condition under the drip-fertigated condition. These results highlight the challenge of applying the enhanced efficiency N fertilizer products in the drip-fertigated agricultural system. [Display omitted] • Soil N 2 O effluxes were estimated based on profile N 2 O concentrations. • N source or rate did not affect soil profile N 2 O concentration and efflux. • Soil N 2 O production in the 0–15 cm layer dominated surface emissions. • Gradient-based N 2 O effluxes decoupled with chamber measurements. Use of enhanced efficiency N fertilizers did not affect N 2 O concentration and efflux along the soil profile under the drip-fertigated system. Estimates of N 2 O flux based on soil profile N 2 O concentration gradients were in the same order of magnitude but higher, compared with measurement of the static chamber. [ABSTRACT FROM AUTHOR]

• Soil environmental and microbial drivers of N 2 O emissions from grazed grassland were identified. • Grazing increased cumulative N 2 O emissions over 2017–2018 growing seasons. • Soil temperature and NO 3 – availability were major contributors for variation in N 2 O flux. • Grazing enhancement of N 2 O emission was not associated with abundances of nitrifiers and denitrifiers. Grazing can degrade grassland soil and increase nitrous oxide (N 2 O) emissions, but the underlying mechanism is poorly understood. We investigated the relationship of soil N 2 O emission with nitrifying enzyme activity (NEA), denitrifying enzyme activity (DEA), and the abundances of N 2 O-producing and N 2 O-reducing functional genes, in response to three grazing intensities (none, light, intensive) over two growing seasons on an alpine grassland on Kunlun Mountain in semi-arid North China. Compared to the non-grazed control, light and intensive grazing treatments increased the 2-yr total N 2 O emissions by 27.5% and 68.1%, respectively. Daily N 2 O flux rate correlated positively with soil water-filled pore space (WFPS), temperature, and dissolved organic carbon (DOC) but not with the abundances of nitrifiers (AOB, AOA, Nitrobacter -like nxrA), nitrate reducers (narG) and denitrifiers (nirS , nirK , and nosZ). These results suggest that soil environmental factors rather than nitrifier, N 2 O-producer and N 2 O-reducer abundances were more important in determining the grazing enhancement of N 2 O emissions from the alpine grassland in semi-arid areas. The abundance of AOB and Nitrobacter -like nxrA but not AOA correlated positively with NEA, indicating they are good indicators for the potential nitrification in this arid alpine grassland. Reducing grazing intensity from intensive to light resulted in more above-ground biomass and N uptake in both years and less N 2 O emissions in 2018. These results highlight the importance of establishing a proper grazing intensity to reduce N 2 O emissions from the degraded grassland while maintaining productivity for livestock. [ABSTRACT FROM AUTHOR]

It remains unclear how the source and rate of nitrogen (N) fertilizers affect N 2 O concentration and effluxes along the soil profile under the drip-fertigated agricultural system. A plot-based field study was performed in 2017 and 2018 in a cotton field in arid northwestern China, with an objective to elucidate the impact of the applications of conventional urea (Urea), polymer-coated urea (ESN) and stabilized urea (SuperU) at rates of 120 and 240 kg N ha -1 on concentration and efflux of N 2 O in the soil profile and its relationship with N 2 O surface emissions. The in-situ N 2 O concentrations at soil depths of 5, 15, 30 and 60 cm were measured and used to estimate soil profile N 2 O effluxes. Estimates of surface N 2 O flux using the concentration gradient-based (GM) were compared with those measured using the chamber-based (CM) method. In both years, soil N 2 O concentrations at all depths increased in response to basal N application at planting or in-season fertigation events. However, N rate or source did not affect soil N 2 O concentrations or effluxes at each depth. Surface emissions of N 2 O were mostly associated with that presented in the top layer of 0-15 cm. Surface N 2 O efflux determined by GM was poorly or not associated with those of chamber measurements, which was attributed to the low N 2 O production restricted by soil moisture condition under the drip-fertigated condition. These results highlight the challenge of applying the enhanced efficiency N fertilizer products in the drip-fertigated agricultural system., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Drip irrigation is a useful practice to enhance water and fertilizer nitrogen (N) use efficiency. However, the use of drip irrigation to mitigate nitrous oxide (N 2 O) emissions in agricultural systems globally is uncertain. Here, we performed a global meta-analysis of 485 field measurements of N 2 O emissions from 74 peer-reviewed publications prior to March 2021, to quantify the fertilizer-induced N 2 O emission factor (EF) of drip irrigation and examine the influencing factors of climate, crop, soil properties, and source and rate of fertilizer N application. The results showed that drip irrigation reduced (p < 0.05) N 2 O emissions by 32% and 46% compared to furrow and sprinkler irrigation systems, respectively. The overall average EF with drip irrigation was 0.35%, being two-thirds lower than the IPCC Tier I default value of 1% (kg N 2 O-N/kg added fertilizer N). The EF was not significantly affected by climate, crop, soil texture, soil organic carbon content, and pH. The EF was also not significantly (p > 0.05) affected by synthetic N fertilizer source despite a lower numerical value with enhanced efficiency than conventional fertilizers. The EF increased significantly (p < 0.001) with N addition rate in a binomial distribution. Using the IPCC default EF overestimated N 2 O emissions inventories for drip-irrigated cropping systems by 7614 and 13,091 Mg per year for China and the globe, respectively. These results indicate that drip irrigation should be recommended as an essential N 2 O mitigation strategy for irrigated crop production., (© 2021 John Wiley & Sons Ltd.)

Spring-thaw represents a significant source for nitrous oxide (N 2 O) emissions from fertilized croplands in temperate regions. In this study, we present surface N 2 O fluxes, soil-profile N 2 O concentrations at 5, 15, 30 and 60 cm depths along with the abundance of nitrifiers and denitrifiers over the winter and spring-thaw periods in an arid, drip- fertigated cotton field, which had received spring application of 240 kg N ha -1 as urea alone (Urea), polymer-coated urea (ESN), and urea plus urease and nitrification inhibitors. Nitrous oxide emissions from December to April were generally unaffected by fertilizer treatments with a cumulative average of 186 g N ha -1 , accounting for 39% of the annual N 2 O emissions. Emission peaks occurred at spring-thaw and coincided with increasing soil-profile N 2 O concentrations at all depths, suggesting the burst in N 2 O fluxes was due to new N 2 O production, rather than a physical release of N 2 O trapped in the soil profiles over winter. The abundance of nitrifier and denitrifier genes changed over the winter and spring-thaw periods but was not affected by fertilizer treatments from the previous spring, suggesting the abundance of N 2 O-producing microorganism was more controlled by environmental conditions than N sources applied in the previous spring. The daily N 2 O flux rate from December to April was positively correlated with soil temperature, water-filled pore space, and denitrifying enzyme activity, but not with the gene copy number of AOA, AOB, narG, nirS, nirK and nosZ, indicating that variation in the abundance of these genes was not contributing to the N 2 O emissions. These results suggest that N 2 O emissions in spring-thaw are substantial for drip-fertigated croplands in the arid regions and should be considered in the annual budgets. The environmental factors such as soil temperature and moisture are likely more important than the copy-numbers of N 2 O-producing functional genes in driving the variability in spring-thaw emissions., (Copyright © 2019 Elsevier B.V. All rights reserved.)