Elevated N2O Emissions Research Articles

The impacts of film mulching and ridging on N2O emissions, relevant functional genes, and microbial communities in rain-fed potato fields

Rain-fed potato (Solanum tuberosum) fields in drylands significantly contribute to nitrous oxide (N2O) emissions, making them an important focus of agricultural greenhouse gas research. Film mulching and ridging are key agricultural methods in potato cultivation. Investigating the impact of these methods on N2O emissions, nitrifying/denitrifying functional genes, and microbial communities can provide a theoretical basis for soil emission reduction and more sustainable dryland agriculture. We examine the effects of flat tillage with mulching, ridge tillage with mulching, flat tillage without mulching, and ridge tillage without mulching, on potato fields under natural rainfall conditions in Wuchuan County, China. N2O emission fluxes were monitored using a static (dark) chamber and gas chromatography. Real-time quantitative PCR (q-PCR) was used to quantify abundances of nitrifying and denitrifying bacteria related to N2O emissions at various potato-growth stages. Illumina high-throughput sequencing was used to investigate microbial community structure by targeting 16S rRNA genes; related soil elements (soil temperatures and moisture) are analyzed. Mulching and ridging indirectly influence N2O emissions, nitrifying/denitrifying functional gene copy numbers, and microbial community structure by altering soil temperature and moisture. Cumulative N2O emissions and emission intensity were both consistently higher in ridge tillage with mulching during the potato-growing period. Ammonia-oxidizing archaea are the main microorganisms that control N2O emissions, with nitrification-coupled denitrification also being an important mechanism contributing to high N2O emissions during soil dry–wet cycles. Increased soil temperature and moisture elevated N2O emissions and functional gene copy numbers. The combination of mulching and ridging effectively uses the characteristics of both practices, making Nitrospira the dominant genus, and significantly increases N2O emissions.

Assessing the global warming potential impact of organic fertilizer strategies in rice cultivation in Sri Lanka.

Rice is the staple food in Sri Lanka, and over 15% of the national land is allocated for rice cultivation. Greenhouse gas (GHG) emissions from rice fields account for 10% of national GHG emissions. The country has committed to reducing its emissions by 14.5% between 2010 and 2030 and achieving net zero emissions by 2060. In 2021, the country banned agro-fertilizer imports and opted for organic fertilizers, leading to a notable decrease in production and posing challenges to food security. However, the impact of adopting compost fertilizers alone remains unexplored. This study evaluated the global warming impact of two organic fertilizer strategies: switching to compost fertilizer instead of urea and applying rice straw compost instead of retaining crop residue. We applied the Denitrification and Decomposition model (DNDC 95) to rice field management data from Sri Lanka's Mahaweli H agricultural region. Simulations suggest that both strategies would increase the global warming potential of rice fields, mainly owing to elevated N2O emissions. This outweighs the mitigation benefits of avoiding crop residue retention and adding organic carbon through compost. Overall, our results point to the potential risk of shifting exclusively to compost-based fertilizers.

Effects of whole‐soil warming on CH4 and N2O fluxes in an alpine grassland

AbstractGlobal climate warming could affect the methane (CH4) and nitrous oxide (N2O) fluxes between soils and the atmosphere, but how CH4 and N2O fluxes respond to whole‐soil warming is unclear. Here, we for the first time investigated the effects of whole‐soil warming on CH4 and N2O fluxes in an alpine grassland ecosystem on the Tibetan Plateau, and also studied the effects of experimental warming on CH4 and N2O fluxes across terrestrial ecosystems through a global‐scale meta‐analysis. The whole‐soil warming (0–100 cm, +4°C) significantly elevated soil N2O emission by 101%, but had a minor effect on soil CH4 uptake. However, the meta‐analysis revealed that experimental warming did not significantly alter CH4 and N2O fluxes, and it may be that most field warming experiments could only heat the surface soils. Moreover, the warming‐induced higher plant litter and available N in soils may be the main reason for the higher N2O emission under whole‐soil warming in the alpine grassland. We need to pay more attention to the long‐term response of greenhouse gases (including CH4 and N2O fluxes) from different soil depths to whole‐soil warming over year‐round, which could help us more accurately assess and predict the ecosystem‐climate feedback under realistic warming scenarios in the future.

Bacterial denitrification drives elevated N2O emissions in arid southern California drylands.

Soils are the largest source of atmospheric nitrous oxide (N2O), a powerful greenhouse gas. Dry soils rarely harbor anoxic conditions to favor denitrification, the predominant N2O-producing process, yet, among the largest N2O emissions have been measured after wetting summer-dry desert soils, raising the question: Can denitrifiers endure extreme drought and produce N2O immediately after rainfall? Using isotopic and molecular approaches in a California desert, we found that denitrifiers produced N2O within 15 minutes of wetting dry soils (site preference = 12.8 ± 3.92 per mil, δ15Nbulk = 18.6 ± 11.1 per mil). Consistent with this finding, we detected nitrate-reducing transcripts in dry soils and found that inhibiting microbial activity decreased N2O emissions by 59%. Our results suggest that despite extreme environmental conditions-months without precipitation, soil temperatures of ≥40°C, and gravimetric soil water content of <1%-bacterial denitrifiers can account for most of the N2O emitted when dry soils are wetted.

Responses of Nitrous Oxide Emissions and Bacterial Communities to Experimental Freeze-Thaw Cycles in Contrasting Soil Types.

Nitrous oxide (N2O) pulse emissions are detected in soils subjected to freeze-thaw cycles in both laboratory and field experiments. However, the mechanisms underlying this phenomenon are poorly understood. In this study, a laboratory incubation experiment that included freeze-thaw cycles (FTC), freezing (F) and control (CK) treatments was performed on three typical Chinese upland soils, namely, fluvo-aquic soil (FS), black soil (BS) and loess soil (LS). A higher similarity in soil properties and bacterial community structure was discovered between FS and LS than between FS and BS or LS and BS, and the bacterial diversity of FS and LS was higher than that of BS. FTC significantly increased the denitrification potential and the proportion of N2O in the denitrification gas products in FS and LS but decreased the denitrification potential in BS. Accordingly, with the increasing number of freeze-thaw cycles, the bacterial community composition in the FTC treatments in FS and LS diverged from that in CK but changed little in BS. Taxa that responded to FTC or correlated with denitrification potential were identified. Taken together, our results demonstrated that the effects of FTC on N2O emissions are soil-type-dependent and that the shift in the microbial community structure may contribute to the elevated N2O emissions.

Fertilizer N triggers native soil N-derived N2O emissions by priming gross N mineralization

Recent 15nitrogen (N) tracing studies have shown that unlabeled N2O emissions (UNEs) following 15N fertilizer application tend to be higher than those from zero-N controls, which indicates a potential risk for elevated N2O emissions from native soil. However, researchers do not clearly understand whether these increased UNEs are derived from a priming effect or how interactions between N in fertilizer and native soil affect various sources of N2O emissions. Here, we combined 15N tracing and 15N pool dilution in an incubation study to track gross N transformations, specific sources of N2O emissions, and N2O-production pathways following N fertilization. Four treatments, including two fertilizer N levels (0 and 120 mg N kg−1) in conjunction with two native soil N levels (0.56 and 1.25 g kg−1), were established in the study. Soil gross N mineralization (GM) increased by 33.0%–98.5% (4.59–14.97 mg N kg−1) after N fertilizer addition, which indicated a positive priming effect on soil N turnover and resulted in additional native soil N-derived N2O emissions. In addition to priming GM, the pool substitution effect after 15N addition, which was not triggered by stimulated soil N turnover, also generated substrates for the UNEs. Therefore, overlooking the pool substitution effect might overestimate native soil N-derived emissions by 104%–180%. Differentiation of the specific N2O emission sources showed that both the fertilizer N- and primed native soil N-derived N2O emissions were dominated by nitrifier-mediated processes, which were governed by ammonia-oxidizing bacteria. At a specified N fertilizer addition rate, soils with higher native N levels had greater levels of fertilizer N- and native soil N-derived N2O emissions due to the higher GM and gross nitrification rates. The study reveals that fertilizer N application triggers potential N2O emissions from native soil N by stimulating the GM and nitrifier-mediated N2O-production processes.

Characteristics of Soil Structure and Greenhouse Gas Fluxes on Ten-Year Old Skid Trails with and without Black Alders (Alnus glutinosa (L.) Gaertn.)

Forest soil compaction caused by heavy machines can cause ecosystem degradation, reduced site productivity and increased greenhouse gas (GHG) emissions. Recent studies investigating the plant-mediated alleviation of soil compaction with black alder showed promising results (Alnus glutinosa). This study aimed to measure soil recovery and GHG fluxes on machine tracks with and without black alders in North-East Switzerland. In 2008, two machine tracks were created under controlled conditions in a European beech (Fagus sylvatica) stand with a sandy loam texture. Directly after compaction, soil physical parameters were measured on one track while the other track was planted with alders. Initial topsoil bulk density and porosity on the track without alders were 1.52 g cm−3 and 43%, respectively. Ten years later, a decrease in bulk density to 1.23 g cm−3 and an increase in porosity to 57% indicated partial structure recovery. Compared with the untreated machine track, alder had no beneficial impact on soil physical parameters. Elevated cumulative N2O emission (+30%) under alder compared with the untreated track could result from symbiotic nitrogen fixation by alder. Overall, CH4 fluxes were sensitive to the effects of soil trafficking. We conclude that black alder did not promote the recovery of a compacted sandy loam while it had the potential to deteriorate the GHG balance of the investigated forest stand.

In-season split nitrogen application and cover cropping effects on nitrous oxide emissions in rainfed maize

Cover crops and in-season nitrogen (N) management are promoted as key conservation practices for reducing nitrate leaching losses from agricultural fields. However, their combined effects on soil nitrous oxide (N2O) emissions per unit of crop productivity remains uncertain. These practices might contribute to high N2O emissions by providing N and C substrates for microbial denitrification activity, especially when in-season N application coincides with cover crop decomposition. The objective of this study was to evaluate the effects of N application timing and cereal rye (CR, Secale cereale L) cover crops on area- and yield-scaled soil N2O emissions in continuous maize (Zea mays L.) over 3-yr (2018–2020). Treatments were: Pre-Season-N: 224 kg N ha-1 split-applied in fall + pre-plant in 2018, or as single pre-plant applications in 2019 and 2020; In-Season-N: pre-plant + side-dress (growth stage V6-V7); In-Season-N + CR: ; and a zero N plot as the Control. We found that shifting from pre-season to in-season split N application did not significantly affect area- or yield-scaled N2O emissions in any year. Despite high N2O spikes for the In-Season-N + CR following side-dress N application in all years, this did not translate to consistently higher cumulative N2O emissions. Compared with Pre-Season-N and In-Season-N, In-Season-N + CR significantly increased area- and yield-scaled N2O emissions by approximately 50% in 1 of 3 years. These results suggest the combination of high soil N supply, warm and wet soil conditions, and cover crop decomposition contributes to elevated N2O emissions, but the cumulative effects of side-dress N application with a CR cover crop are variable across years. With increasing adoption of these practices to reduce nitrate leaching losses from croplands, future work should simultaneously assess effects on N2O emissions and crop yield to account for potential tradeoffs in N loss pathways.

Diverse rotations impact microbial processes, seasonality and overall nitrous oxide emissions from soils

AbstractMany studies haveexamined soil‐borne nitrous oxide (N2O) emissions from crops, but little effort has gone into determining the N2O emissions from each phase of a crop rotation. A 4‐yr study on a long‐term field experiment compared growing season N2O emissions from continuous corn (CC; Zea mays L.) and a 4‐yr crop rotation involving corn (RC), oat (Avena Sativa L.) underseeded to alfalfa (Medicago sativa L.) (RO), and 2 yr of alfalfa (RA1, RA2). Molecular microbial biomass (DNA yield), as well as N‐cycling functioning genes (mineralization, nitrification, and denitrification), were also evaluated. Although 4‐yr cumulative N2O emissions from RC (9.25 kg N ha–1) were significantly greater than from CC (7.94 kg N ha–1), cumulative emissions from the entire rotation were 54% lower (3.69 kg N ha–1) than CC because of low emissions from RO (3.1 kg N ha–1), RA1, and RA2 (1.11–1.27 kg N ha–1). Years that had substantial early‐season precipitation combined with high soil inorganic N from alfalfa plow‐down contributed to elevated N2O emissions from RC. Improved soil conditions and fertility under rotation increased RC grain yields by 35% (9.45 Mg ha–1) compared with CC (7.01 Mg ha–1). Microbial biomass was 73% greater in RC compared with CC. Nitrogen mineralization genes were 19% greater in RC but they were not correlated to N2O emissions, whereas bacterial nitrifiers were positively correlated. Denitrification was likely responsible for N2O emissions under CC, while nitrifier‐denitrification appeared to be the primary pathway under RC. The N2O emissions and microbial processes from all phases of a rotation should be considered for environmental modeling and policy decisions.

Grass Growth and N2O Emissions From Soil After Application of Jellyfish in Coastal Areas

The supply of nutrients for agricultural production faces enormous challenges as food security and sustainability goals have to be ensured. Processing of marine biomass has high potential to provide nutrients for agricultural purposes in coastal areas. One underexplored resource are jellyfish, which occur as blooms and by-catch of the fishing industry. In this context, a pot experiment investigated the effects of jellyfish as a fertilizer on biomass accumulation of annual ryegrass (Lolium multiflorum), and its effect on the important greenhouse gas N2O as a sustainability indicator of novel fertilizers. Dried and ground jellyfish was applied [3 species: Aurelia aurita (AA), Cyanea capillata (CC), Periphylla periphylla (PP)] and compared with an unfertilized and a mineral fertilized (calcium-ammonium-nitrate, CAN) treatment. Dried jellyfish and CAN were applied at equal N rates of 5 g N per m2. The N2O-fluxes from soil were measured over 56 days after fertilizer application. Grass dry matter yields, when using CC and PP treatments, were not significantly different to the CAN treatment (p &amp;gt; 0.05). After reducing its salinity, AA also showed no differences to CAN on plant growth and the lowest coefficient of variation for dry matter yield as an indicator for yield stability. Accumulated N2O-emissions were lowest in the control and were 3-times higher in AA and CC compared to CAN (p &amp;lt; 0.05). If salinity levels are moderate, jellyfish application to soil can compete with artificial mineral fertilizers in terms of N-supply for above- and belowground yield response, regardless of jellyfish species used. However, elevated N2O-emissions are likely to affect its suitability for large-scale application. Nevertheless, if energy-efficient methods of drying and desalination of jellyfish can be developed, in coastal areas dried jellyfish is a valuable fertilizer in coastal areas, particularly in situations where nutrient supplies for agriculture are limited.

Greenhouse gas emissions from an irrigated cropping rotation with dairy manure utilization in a semiarid climate

AbstractThis long‐term study was established to increase knowledge of greenhouse gas (GHG) emissions from irrigated cropping systems utilizing dairy manure solids and compost in semiarid southern Idaho. The objective of this field study was to determine the effect of synthetic N fertilizer (urea or SuperU [enhanced‐efficiency synthetic fertilizer]), composted dairy manure, dairy manure (fall or spring applied), and a control (no fertilizer or manure) on nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) emissions over the growing season. The fertilizer and manure treatments were not applied to alfalfa (Medicago sativa L.) (2017) but were applied to corn (Zea mays L.) (2018; except SuperU) and barley (Hordeum vulgare L.) (2019). Cumulative N2O losses over the 3 yr ranged from 2.8 to 5.2 kg N2O‐N ha–1, with the fall and spring manure emitting the greatest amounts of N2O. Emission factors indicated that up to 0.79% of the total N applied was lost as N2O‐N during the growing seasons. Cumulative losses of CO2 and CH4 across the rotation were on average 12,170 kg CO2–C ha–1 and –0.77 kg CH4–C ha–1, respectively, with no significant differences among the treatments. Major N2O pulses were associated with early‐season irrigation events and incorporation of fertilizer and manure, but overall fluxes tended to be the greatest when soil temperatures were higher. Dairy manure and compost applications were also found to cause rapid and significant increases in soil organic carbon (SOC) in the top 30 cm of soil under corn and barley. Despite the fact that manure does cause elevated soil N2O emissions, it should be considered as an alternative to synthetic fertilizer use due to its ability to increase SOC and potentially help reduce the global warming potential.

Are annual nitrous oxide fluxes sensitive to warming and increasing precipitation in the Gurbantunggut Desert?

AbstractTemperate desert soils are an important source of atmospheric nitrous oxide (N2O). However, it is uncertain how N2O emissions respond to warming, increased rainfall and nitrogen (N) addition in such soils. A multifactorial field manipulation study was carried out in the Gurbantunggut Desert, China's second largest desert, to investigate how these factors influence desert soil N2O emissions and to assess inter‐year variation. In our 3‐year study, under current climatic conditions, the annual flux of N2O in this temperate desert soil was 0.13 ± 0.02 kg N ha−1 yr−1, with the period of no plant growth contributing 43% of the annual emission. Surprisingly, there was no significant change in annual N2O flux under warming (+1°C) or increased precipitation (30%, +60 mm yr−1). In contrast, N2O emissions were significantly affected by extreme drought followed by precipitation. Additional N input, at 30 or 60 kg N ha−1 yr−1, greatly elevated annual N2O emission by 55–133%. The combined impact of N deposition and increasing rainfall on N2O emission was greater than that of any single factor, except for N deposition. This suggests that N2O emissions in this desert are driven primarily by soil available N content and are less sensitive to variations in soil temperature and moisture.

Nitrite accumulation and nitrogen gas production increase with decreasing temperature in urea-amended soils: Experiments and modeling

Nitrite (NO2−) accumulation and associated production of nitric oxide (NO) and nitrous oxide (N2O) gases in soils amended with nitrogen (N) fertilizers are well documented, but there remains a poor understanding of their regulation and variation among soil types. We examined responses to urea inputs in two soils at five temperatures from 5 to 30 °C and developed a process-driven model to describe the dynamics. A microcosm system was used to measure ammonia gas (NH3), ammonium (NH4+), NO2−, nitrate (NO3−), NO, N2O and pH over 12 weeks. Unexpectedly, NO2−, NO and N2O production tended to increase as soil temperature declined in both soils. The maximum NO2− concentration, or compensation point (CP), differed by soil type but the time required to reach CP decreased exponentially with increasing temperature in both soils. A two-step nitrification model (’2SN’) accounted for interactions of ammonia-oxidation (AmO), nitrite oxidation (NiO), urea hydrolysis, NH4+ sorption, N gas production and pH dynamics. Both steps of nitrification (AmO and NiO) were modeled using NH3 inhibition kinetics. The model adequately simulated the observed dynamics and temperature responses and showed that increased uncoupling of AmO and NiO at colder temperatures resulted from their differential temperature responses. The dynamics observed here may be important following high-rate and banded N fertilizer applications and in ruminant urine patches. The results may help explain elevated N2O emissions observed under cold temperatures. The 2SN model can account for interactions among multiple processes and may be useful for studying the effects of management practices and climate factors, including climate change scenarios, on soil N cycling.

Warming reduces the increase in N2O emission under nitrogen fertilization in a boreal peatland

Peatlands are known as N2O sinks or low N2O sources due to nitrogen (N) limitation. However, climate warming and N deposition can modulate this limitation, and little is known about the combinative effects of them on N2O emission from boreal peatlands. In this study, experimental warming and N fertilization treatments were conducted at a boreal peatland in western Newfoundland, Canada. Contrary to previous studies on permafrost peatland and alpine meadows, the effect of warming treatment on N2O flux was not detectable during the growing seasons of 2015 and 2016. The N fertilization treatment significantly increased the N2O flux by 1.61 nmol m−2 s−1 due to increased N availability. Noticeably, warming reduced the effect of N fertilization treatment on N2O flux with high significance in the middle growing season of 2015. This can be attributed to low N availability caused by stimulated vegetation growth in the warming treatment. In addition, the results showed that total nitrogen was the main control on N2O emission under N fertilization, while dissolved organic carbon was the main driver under the combined treatment of warming and N fertilization. Due to elevated N2O emissions under N deposition/fertilization, the contribution of N2O to global warming and ozone depletion should not be ignored.

Closing the nitrogen use efficiency gap and reducing the environmental impact of wheat-maize cropping on smallholder farms in the Guanzhong Plain, Northwest China

A high crop yield with the minimum possible cost to the environment is generally desirable. However, the complicated relationships among crop production, nitrogen (N) use efficiency and environmental impacts must be clearly assessed. We conducted a series of on-farm N application rate experiments to establish the linkage between crop yield and N2O emissions in the Guanzhong Plain in Northwest China. We also examined crop yield, partial factor productivity of applied N (PFPN) and reactive N (Nr) losses through a survey of 1 529 and 1 497 smallholder farms that grow wheat and maize, respectively, in the region. The optimum N rates were 175 and 214 kg ha–1 for winter wheat and summer maize, respectively, thereby achieving the yields of 6 799 and 7 518 kg ha–1, correspondingly, with low N2O emissions based on on-farm N rate experiments. Among the smallholder farms, the average N application rates were 215 and 294 kg ha–1 season–1, thus producing 6 490 and 6 220 kg ha–1 of wheat and maize, respectively. The corresponding PFPN values for the two crops were 36.8 and 21.2 kg N kg–1, and the total N2O emissions were 1.50 and 3.88 kg ha–1, respectively. High N balance, large Nr losses and elevated N2O emissions could be explained by the overdoses of N application and low grain yields under the current farming practice. The crop yields, N application rates, PFPN and total N2O for wheat and maize were 18 and 24% higher, 42 and 37% less, 75 and 116% higher, and 42 and 47% less, correspondingly, in the high-yield and high-PFPN group than in the average smallholder farms. In conclusion, closing the PFPN gap between the current average and the value for the high-yield and high-PFPN group would increase crop production and reduce Nr losses or the total N2O emissions for the investigated cropping system in Northwest China.

Wet Spots as Hotspots: Moisture Responses of Nitric and Nitrous Oxide Emissions From Poorly Drained Agricultural Soils

AbstractA classic framework for soil nitrogen (N) cycling, the hole in the pipe (HIP) model, posits a trade‐off in emissions of nitric oxide (NO) and nitrous oxide (N2O) as a function of soil moisture. This has been incorporated into ecosystem models but not tested experimentally and remains an important uncertainty for understanding potential hotspots of reactive N emissions: poorly drained agricultural soils that experience episodically high moisture following intensive fertilization. We incubated soils at moisture ranging from 44% to 100% water‐filled pore space (WFPS). Counter to HIP, we did not observe a consistent trade‐off in NO and N2O emissions at intermediate moisture levels following fertilization, and prefertilization emissions were low. Emissions of N as N2O exceeded NO by 2–200‐fold at all moisture levels and peaked at 73–82% WFPS. Emissions of NO declined with moisture but remained significant even under saturated conditions. Increases in nitrite and reduced iron at high moisture indicated possible NO production from chemodenitrification. Potential nitrification rates were 100–1,000‐fold greater than potential denitrification. Emission factors for fertilizer N ranged from 0.05% to 0.58% (mean = 0.2%) for NO and from 0.4% to 16.9% (mean = 5.3%) for N2O. Our results caution the use of WFPS to predict NO:N2O emission ratios as often employed in ecosystem models. Subsurface N cycling may suppress emissions of NO relative to N2O, and N2O emissions can persist under saturated conditions. Elevated N2O emissions from in‐fieldwet spotscomprising a small landscape extent could potentially address disparities between top‐down and bottom‐up N2O budgets.

Short-term responses of denitrification to chlorothalonil in riparian sediments: Process, mechanism and implication

Pesticide residues in riparian zones have attracted much attention in recent decades. Their accumulations potentially deteriorate microbial activity and disturb nitrogen cycle in riparian sediments. In this study, the short-term effects of chlorothalonil (CTN, a common pesticide) on microbial denitrification were explored in riparian sediments at three levels (5, 10 and 25 mg kg−1). No significant differences were observed between control and 5 mg kg−1 treatment; however, CTN in 10 and 25 mg kg−1 treatments deteriorated sediment health condition and microbial activity. High concentrations of CTN also significantly decreased denitrification rates (15N pairing method) by 37–62%, but increased N2O emission in riparian sediments by 100–285%. Our data further revealed that CTN inhibited key enzyme activities responsible for microbial metabolism, and declined electron donor (NADH) and energy source (ATP) levels during denitrification. Key denitrifying enzyme activities were also suppressed by CTN, which explained the declined denitrification process and the elevated N2O emission. Additionally, high-throughput sequencing and quantitative-PCR analysis showed that CTN didn’t remarkably change microbial community structures and denitrifying gene abundances after short-term exposure. Overall, this study highlights that riparian pesticides could impact nitrogen cycle of the interface between terrestrial and aquatic ecosystems, potentially accelerating water pollution and global climate change.

Processes contributing to nitrite accumulation and concomitant N2O emissions in frozen soils

Nitrous oxide (N2O) emissions from frozen soils have been observed during winter. Concomitantly, nitrite (NO2−) accumulations were reported in frozen soils and may have contributed to elevated N2O emissions. The objectives of this study were to determine the processes leading to NO2− accumulation and its contribution to N2O emissions in frozen soils. Soil microcosms were incubated for 21 days under frozen (−2 °C) or unfrozen (+0.5 °C) conditions. Soils were amended with either 15N-NH4+ or 15N-NO3 to quantify the contribution of nitrification and denitrification processes to NO2− accumulation, respectively. Production of 15N-N2O was measured to determine the contribution of NO2− accumulation to N2O emissions. Nitrite progressively accumulated over 21 days in soils incubated at −2 °C to concentrations up to 8.87 μg N g−1 dry soil. However, in soils incubated at +0.5 °C, NO2− concentration increased from day 0 to day 7 (up to 3.61 μg N g−1 dry soil) and declined thereafter. At both temperatures, NO2− accumulation was associated with a progressive decrease in NO3− concentration while NH4+ concentration remained relatively stable over time. Greater N2O emissions were observed at +0.5 °C (average of 0.0135 μg N g−1 dry soil h−1) compared with −2 °C (average of 0.0045 μg N g−1 dry soil h−1) from day 2 to day 14. However, N2O emissions were 3.5 fold greater at −2 °C compared with +0.5 °C on day 21. In soils at −2 °C, 97.40–99.75% of the NO2− and over 99.56% of N2O were derived from NO3−. In soils incubated at +0.5 °C, the proportions of NO2− and N2O derived from NO3− slightly decreased while the proportions of NO2− and N2O derived from NH4+ increased up to 4.3% and 4.7%, respectively, from day 2 to day 21. At both temperatures, 15N enrichments of N2O and NO2− were similar. The results indicated that (i) denitrification was the major process leading to soil NO2− accumulation and concomitant production of N2O under both temperatures; and that (ii) N2O emissions were more sustained in frozen soils where NO2− concentrations were greater compared to unfrozen soils.

Evaluation of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) for mitigating soil N2O emissions after grassland cultivation

Grasslands within a crop rotation are temporary and thus subject to cultivation. This is typically accompanied by elevated soil N2O emissions due to residue decomposition and accumulation of mineral N in the soil for a period while N uptake by the succeeding crop is still low. This study evaluated a novel strategy for N2O mitigation, i.e., spraying of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) at 1 kg ha−1 on a grass-clover ley shortly before cultivation (+D). Tillage was performed by rotovation followed, two weeks later, by ploughing (RP), or by ploughing directly (P). We evaluated effects of DMPP and tillage strategy on net N mineralisation and dynamics of NH4+ and NO3−, and on soil N2O emissions, during the growing season of spring barley following spring cultivation of grass-clover. Rainfall between 14 April and 7 August 2015 was 298 mm, which was much higher than the 2000–2014 average for this period of 214 mm. Spraying grassland with DMPP before cultivation did not affect net N mineralization from residues, but significantly (p < 0.05) inhibited the oxidation of NH4+ for several weeks. Grassland cultivation significantly (p < 0.05) stimulated soil N2O emissions, and there was no difference in cumulative emissions between treatments RP–D (2.49 kg N ha−1) and P–D (1.76 kg N ha−1). Across both tillage treatments, spraying with DMPP significantly (p < 0.05) reduced cumulative N2O emissions. The reductions achieved for RP and P were on average 33 and 23%, respectively, but the effect for tillage treatment P was not statistically significant (0.05 < p < 0.1). The biomass and N uptake of spring barley were not significantly influenced by tillage or DMPP treatment. Results from this field validation were compared with results from previous experiments in controlled environments; the differences observed with respect to effects of residue distribution and DMPP treatments on soil N2O emissions were apparently related to soil NO3− availability. Where nitrification of residue-N is important for NO3− availability, our results indicate that spraying with DMPP prior to grassland cultivation has the potential to mitigate soil N2O emissions without affecting plant performance.

Assessment of various practices of the mitigation of N2O emissions from the arable soils of Poland

Abstract This review assesses the adaptability and effectiveness of the basic practices to mitigate the N2O emissions from the arable land in the climate, soil and agricultural conditions of Poland. We have analyzed the decrease in the nitrogen-based fertilization, selection of the fertilizer nitrogen forms, use of biological inhibitors of nitrogen transformation in the soil, control of the acidic soil reaction, reduction in the natural fertilizers use and afforestation of the low productive soils. The challenge evaluating the effectiveness of mitigation practices lies in the inadequacy of the national data on N2O soil emissions in particular agrotechnical conditions. In Poland, circumstances that favor intensive N2O emissions from the arable soils occur uncommonly, as shows the analysis of the literature reporting on the country climate, soil and agricultural conditions alongside the N2O emissions from soils under various cultivation conditions. Consequently, the effectiveness of mitigation practices that relies on an extensification of plant production may be insufficient. It can be assumed that, at the doses of nitrogen fitting the nutritional needs of crops, the soil N2O emissions are low and do not meaningfully differ from the emissions from untreated soils (literature data point to limited N2O emission from arable soils treated with N doses of ≤150-200 kg N·ha-1). The effectiveness of the nitrogen fertilization reduction as an N2O emissions mitigation practice is restricted to intensive farming. A universal registry of the mineral and natural fertilization use could help identify the agricultural holdings with a potential for high N2O emission and foster a targeted application of mitigation practices. It is suggested that normalization and maintenance of the optimum (i.e. close to neutral) soil pH should become a more common practice of N2O emissions mitigation in Poland in view of the extent of arable soils acidification and the literature data that indicate elevated N2O emissions from acid soils. Application of urease and nitrification inhibitors alongside nitrogen fertilization can be considered an effective practice of N2O emissions mitigation. Owing to economic reasons the use of nitrogen fertilizers with such additives is currently limited to non-agricultural segments of plant production. Afforestation of the low productive soils offers an attractive opportunity for mitigation of N2O emissions. Whereas N2O emissions from forest soils are considerably lower compared with those from the arable ones, the literature indicates that no N2O emissions mitigation is attained through a conversion of arable land to agroforestry. Considering the current forest area of Poland (24.9% of the total area) and the plans to increase the afforestation rate (to 33% in 2050) the measurable effects of this mitigation practice will only be seen in a long-term perspective. Besides identifying and excelling the mitigation practices the authors postulate a review of the algorithms employed by the National Centre for Emissions Management (KOBiZE) for the calculation of the GHG emissions. Solutions applied by KOBiZE appear to address mainly the area - or population-related aspects and, to a much lesser degree, the actual N2O production. In this context, the effects of certain N2O emissions mitigation practices might be difficult to be taken into consideration. The application of national statistics of the use of mineral and natural fertilizers to the calculation of the N2O emissions from the arable soils might be questioned given that the N2O emissions are driven by the actual local N dose.