A comparison of Tier 1, 2, and 3 methods for quantifying nitrous oxide emissions from soils amended with biosolids

Municipal biosolids are a nitrogen (N)-rich agricultural fertilizer which may emit nitrous oxide (N2O) after rainfall events. Due to sparse empirical data, there is a lack of biosolids-specific N2O emission factors to determine how land-applied biosolids contribute to the national greenhouse gas inventory. This study estimated N2O emissions from biosolids-amended land in Canada using Tier 1, Tier 2 (Canadian), and Tier 3 (Denitrification and Decomposition model [DNDC]) methodologies recommended by the Intergovernmental Panel on Climate Change (IPCC). Field data was from replicated plots at 8 site-years between 2017 and 2019 in the provinces of Quebec, Nova Scotia and Alberta, Canada, representing three distinct ecozones. Municipal biosolids were the major N source for the crop, applied as mesophilic anaerobically digested biosolids, composted biosolids, or alkaline-stabilized biosolids alone or combined with an equal amount of urea-N fertilizer to meet the crop N requirements. Fluxes of N2O were measured during the growing season with manual chambers and compared to N2O emissions estimated using the IPCC methods. In all site-years, the mean emission of N2O in the growing season was greater with digested biosolids than other biosolids sources or urea fertilizer alone. The emissions of N2O in the growing season were similar with composted or alkaline-stabilized biosolids, and no greater than the unfertilized control. The best estimates of N2O emissions, relative to measured values, were with the Tier 3 > adapted Tier 2 with biosolids-specific correction factors > standard Tier 2 = Tier 1 methods of the IPCC, according to the root mean square error statistic. The Tier 3 IPCC method was the best estimator of N2O emissions in the Canadian ecozones evaluated in this study. These results will be used to improve methods for estimating N2O emissions from agricultural soils amended with biosolids and to generate more accurate GHG inventories.

Arbuscular mycorrhizal fungi in oat-pea intercropping

Arbuscular mycorrhizal fungal diversity can be altered by intercropping plant species, as well as N fertilizer applications. This study examined the effects of oat-pea intercropping and N fertilizer addition on the richness and diversity of mycorrhizal species, as well as identified the most common arbuscular mycorrhizal fungi (AMF) genera recruited for oats and peas in two growing seasons (2019 and 2020). The AMF diversity was higher in an intercropped system compared to their respective monocropping system. Under drier conditions in 2019, arbuscular mycorrhizal richness decreased with N fertilizer addition in sole peas and increased with N fertilizer addition in sole oats, but no significant change in richness was observed in oat-pea intercropping. During the wetter growing season 2020, arbuscular mycorrhizal diversity increased when oat and pea were intercropped, compared to either sole oat or sole pea. Diversispora in sole pea was a significant indicator differentiating the root associated AMF community from sole oat. Claroideoglomus richness increased in peas in 2020, thus this genus could be moisture dependent. Paraglomus richness in oat-pea intercropping was similar to sole oat in 2019, and similar to sole pea in 2020. This can suggest that Paraglomus is an indicator of plant stress under intercropping, as based on the premise that stressed plants release more exudates, and the subsequent mycorrhizal associations favor these plants with higher exudation. Future investigations can further reveal the functions and benefits of these mycorrhizal genera in annual monocrop and intercropping systems.

How could simulated dewatering of slurry mitigate nitrous oxide emissions from fall and spring injections? - A modelling study in a Chernozem soil in Western Canada

Process-based ecosystem models, such as ecosys, can be useful tools to gain insights and accurately project nitrous oxide (N₂O) inventories in national, regional and global scales, and to explore potential emission reduction strategies. Our objectives are to investigate how the ecosys model simulate the effects of fall and spring slurry injections on N₂O production and if de-watering slurry could become a potential N₂O mitigation strategy for both fall and spring injections. The ecosys model was used to simulate hourly N₂O fluxes from 2014 to 2017 in a cropping system with and without slurry (fall and spring additions) in comparison with field measurements in Alberta, Canada. Furthermore, we performed simulations of de-watered fall and spring slurry applications in the same scenarios. Our results showed ecosys adequately simulated soil temperatures and moisture contents at 10 and 20 cm depths [correlation coefficients (r) ≥ 0.929 for temperatures; r ≥ 0.529 for moistures]. The divergences of modelled and measured soil water contents during spring thaws could be attributed to uncertainties in model inputs for soil hydrological parameters as well as uncertainties in field measurements. The model captured reasonably well the dynamics of N₂O fluxes from soils receiving fall and spring slurry (r = 0.356). However, the concurrent discrepancies of N₂O fluxes between modelled and measured values during the wetter spring thaw of 2017 might be a result of an unsatisfactory simulation of snowmelt infiltration and runoff. Compared to whole slurry, simulated de-watered slurry resulted in considerable reductions in cumulative N₂O emissions by 16-36 and 23-29% for fall and spring slurry injections, respectively. The model results indicate that de-watering slurry would potentially be an efficient emission mitigation strategy; however, there is still a paucity of studies addressing the feasibility of dewatering as a practice and further research can focus on this knowledge gap.

How does management legacy, nitrogen addition, and nitrification inhibition affect soil organic matter priming and nitrous oxide production?

Long-term management of croplands influences the fluxes and sources of nitrous oxide (N₂ O). We examined this premise in a greenhouse study by using soils collected from a 38-yr-old field experiment. The sampled treatments were continuous barley (Hordeum vulgare L.; CB), continuous fescue (Festuca rubra L., F. arundinacea Schreb; CF), and two phases of an 8-yr rotation: faba bean (Vicia faba L.; FB) and alfalfa (Medicago sativa L.)-bromegrass (Bromus inermis Leyss) hay. Barley was grown as a test crop in the greenhouse in each soil. The ranking of N₂ O emissions was hay > FB > CB > CF (P < .001). We quantified the ¹⁵ N-site preference to assess the N₂ O-producing processes. Denitrification was the predominant source, contributing 77.4% of the N₂ O production. We also evaluated nitrogen (N) additions: urea alone or urea with a nitrification inhibitor (nitrapyrin or DMPSA). Compared with urea alone, nitrapyrin and DMPSA reduced N₂ O emissions by 16 and 25%, respectively. We used urea labeled with ¹⁵ N to trace N to N₂ O emissions, aboveground plant N uptake, and N retention by soils. Total ¹⁵ N-recovery (N₂ O + plant + soil) was highest under FB (86%) and lowest under CB (29%). We further separated the N₂ O derived from urea versus N₂ O from soil organic matter (SOM). The inhibitor DMPSA reduced the N₂ O derived specifically from added urea-N by more than half (P < .001). With the addition of urea, N₂ O production from mineralization of SOM-N accelerated over the control (without urea), termed the priming effect. This priming of SOM-N contributed with 13% of the total N₂ O production when averaged across the four management legacies. The CB soil had the highest proportion of priming-derived N₂ O (24%). Management legacies clearly differed in soil carbon and N, which governed N₂ O production from denitrification and SOM priming.

Sources and priming of nitrous oxide production across a range of moisture contents in a soil with high organic matter

Adding nitrogen fertilizers to agricultural soils contributes to increasing concentrations of nitrous oxide (N₂ O) in the atmosphere. However, the impacts of N addition on soil organic matter (SOM) turnover, SOM availability, and the ensuing SOM-derived N₂ O emissions remain elusive. Within this context, the net change in direction and rate of SOM-derived N₂ O production triggered by added N is termed the N₂ O priming effect. This incubation study examined the sources and priming of N₂ O production as a function of urea addition and multiple moisture contents in a soil with high SOM (55 g organic C kg^-1 ). We assessed four water-filled pore space (WFPS) conditions: 28, 40, 52, and 64%. Relative to controls receiving no N, urea addition increased N₂ O production by 2.6 times (P < .001). Cumulative N₂ O production correlated well with nitrification rates (r = .75; P = .03). We used ¹⁵ N-labeled urea to trace the added urea into N₂ O. Of the N added via urea, the recovery as N₂ O-N shifted from 0.02 to 0.17% when WFPS increased from 28 to 64% (P < .05). We also partitioned the N₂ O production into urea vs. SOM sources. More N₂ O was sourced from SOM than urea, with 59 ± 2% N₂ O originating from SOM. The magnitude of SOM-derived N₂ O under urea was larger than that of the control, revealing that positive N₂ O priming was triggered by urea addition. Upon subtracting the controls, the primed N₂ O was a consistent 19 ± 2% of the total N₂ O produced by urea-amended soils. Nevertheless, the priming magnitude rose sharply with increasing moisture by more than one order of magnitude from 4 to 48 μg N₂ O-N kg^-1 soil and in exponential mode (R²   = .98). Soil moisture, SOM, and nitrification interacted to drive the sources and priming of N₂ O.

Can fertigation reduce nitrous oxide emissions from wheat and canola fields?

Increasing nitrogen fertilization and irrigation can contribute to nitrous oxide (N₂O) emissions from agriculture. Relative to the conventional practice of one-pass fertilization with all N applied at crop seeding, this study examined how splitting the total N fertilization into seeding time and in-crop fertigation impacts N₂O emission factors (EF) in irrigated wheat (Triticum aestivum) and canola (Brassica napus) in Southern Alberta, Canada during two growing seasons (May to Oct. in 2015 and 2016). With all the N applied at crop seeding, the growing-season N₂O EF of irrigated wheat and canola was in average 0.23 ± 0.03%. Conversely, implementing N fertigation lowered the magnitudes of N₂O EF in each of the four crop-years, averaging 0.16 ± 0.04%. Most of the reductions in N₂O emissions due to fertigation occurred with low and intermediate N rates (total rates of 60 and 90 kg N ha^-1) and in the second year of the study. This second year had recurrent, early-season rainfalls following seeding (and prior to fertigation) that triggered differences in the daily and cumulative N₂O fluxes. Within this year, fertigation on wheat consistently lowered the growing-season N₂O EF from a high of 0.27% to only 0.11% (P < 0.001). Also, at the intermediate rate of 90 kg N ha^-1, fertigation synergistically reduced the N₂O EF of canola by half, from 0.13% to 0.06% (P < 0.01). However, the mitigating effects of fertigation vanished with the highest N rate in the study (120 kg N ha^-1). Even with fertigation, this highest N rate resulted in high emissions in wheat, and lesser so in canola in part due to the higher N uptake of canola. Moreover, canola often manifested narrower ratios of N₂O emission-to-grain yield (EF_yield) than wheat. This interplay of crop species, rainfall and N management suggests that implementing fertigation with reduced N rates can proactively mitigates N₂O.

Nitrous oxide emissions from manured soils as a function of various nitrification inhibitor rates and soil moisture contents

Application of nitrification inhibitors (NI) coupled with nitrogen additions can reduce nitrous oxide (N₂O) emissions. The effectiveness of NIs can be impacted by environmental and soil conditions; however, more information is needed about their optimum application rates, in particular when applied with manure. This study investigated the effectiveness of a range of NIs application rates on reducing N₂O emissions from soils receiving liquid manure additions under three moisture contents. Two incubations (A and B) were conducted in Gray Luvisolic (GL) and Black Chernozemic (BC) soils using two NIs [2-chloro-6-(trichloromethyl) pyridine (nitrapyrin) and the new 3,4-dimethylpyrazole succinic acid (DMPSA)]. Soil NH₄⁺ and NO₃^- concentrations were measured. Beneficial N₂O emission reductions caused by NIs were evident at the intermediate and high soil water contents. The averaged emission reductions were 60% and 56% at the 60% and 80% water-filled pore space (WFPS) of the GL soil, respectively. Likewise, a coherent reduction of 58% was also found at the 60% WFPS of the BC soil. Conversely, this emission reduction vanished in this very carbon-rich, clayey BC soil at the highest moisture (80% WFPS). Moreover, as low N₂O fluxes occurred with the lowest moisture (40% WFPS), non-significant and minimal emission reductions by NIs were observed, with a null reduction in the BC soil and only 10% averaged reduction in the GL soil at 40% WFPS. Focusing on the N₂O emission reduction and nitrification inhibition under a broad range of NIs rates (in incubation B), as soil moisture rose from 60 to 80% WFPS, the most efficient NI rate increased from 0.25 to 1.0 kg a.i. ha^-1 for nitrapyrin and from 0.22 to 0.65 kg a.i. ha^-1 for DMPSA in both soils. In sum, results inform how soil moisture and NI application rates influence the effectiveness of NIs, aiding to improve strategies to reduce N losses from agricultural systems with NI implementation.

Odorous compounds sources and transport from a swine deep-pit finishing operation: A case study

Odor emissions from swine finishing operations are an air quality issue that affects residents at the local level. A study was conducted at a commercial swine deep-pit finishing operation in central Iowa to monitor odorous compounds emitted and transported offsite. Gaseous compounds were sampled using either sorbent tubes or canisters with GC/MS analysis, and particulates matter (PM₁₀) were sampled with high volume samplers and thermally extracted onto sorbent tubes for GC/MS analysis. Major odorous chemical classes detected at the swine facility included volatile sulfur compounds (VSC), volatile fatty acids (VFA), phenol and indole compounds. Manure storage was the main source of odorous compounds of which hydrogen sulfide (H₂S), methanethiol, 4-methylphenol, and 3-methylindole were key offenders. Only H₂S and 4-methylphenol were detected above odor threshold values (OTV) at all locations around the facility and both 4-methylphenol and 3-methylindole were detected above their OTV 1.5 km downwind from the swine facility. Odorous compounds generated during agitation and pumping of the deep pits was mainly H₂S. Odorants were mainly transported in the gas phase with less than 0.1% being associated with PM₁₀. Odor mitigation efforts should focus on gaseous compounds emitted from deep-pits and especially during manure agitation and deep-pit pumping.

Nitrate, phosphate, and ammonium loads at subsurface drains: agroecosystems and nitrogen management

Artificial subsurface drainage in cropland creates pathways for nutrient movement into surface water; quantification of the relative impacts of common and theoretically improved management systems on these nutrient losses remains incomplete. This study was conducted to assess diverse management effects on long-term patterns (1998-2006) of NO, NH, and PO loads (). We monitored water flow and nutrient concentrations at subsurface drains in lysimeter plots planted to continuous corn ( L.) (CC), both phases of corn-soybean [ (L.) Merr.] rotations (corn, CS; soybean, SC), and restored prairie grass (PG). Corn plots were fertilized with preplant or sidedress urea-NHNO (UAN) or liquid swine manure injected in the fall (FM) or spring (SM). Restored PG reduced NO eightfold compared with fields receiving UAN (2.5 vs. 19.9 kg N ha yr; < 0.001), yet varying UAN application rates and timings did not affect NO across all CCUANs and CSUANs. The NO from CCFM (33.3 kg N ha yr) were substantially higher than for all other cropped fields including CCSM (average 19.8 kg N ha yr, < 0.001). With respect to NH and PO, only manured soils recorded high but episodic losses in certain years. Compared with the average of all other treatments, CCSM increased NH in the spring of 1999 (217 vs. 680 g N ha yr), while CCFM raised PO in the winter of 2005 (23 vs. 441 g P ha yr). Our results demonstrate that fall manuring increased nutrient losses in subsurface-drained cropland, and hence this practice should be redesigned for improvement or discouraged.

Greenhouse gas fluxes in an eastern Corn Belt soil: weather, nitrogen source, and rotation

Relative contributions of diverse, managed ecosystems to greenhouse gases are not completely documented. This study was conducted to estimate soil surface fluxes of carbon dioxide (CO(2)), methane (CH(4)), and nitrous oxide (N(2)O) as affected by management practices and weather. Gas fluxes were measured by vented, static chambers in Drummer and Raub soil series during two growing seasons. Treatments evaluated were corn cropped continuously (CC) or in rotation with soybean (CS) and fertilized with in-season urea-ammonium nitrate (UAN) or liquid swine manure applied in the spring (SM) or fall (FM). Soybean (SC) rotated with CS and restored prairie grass (PG) were also included. The CO(2) fluxes correlated (P <or= 0.001) with soil temperature (rho: 0.74) and accumulated rainfall 120 h before sampling (rho: 0.53); N(2)O fluxes correlated with soil temperature (rho: 0.34). Seasonal CO(2)-C emissions were not different across treatments (4.4 Mg ha(-1) yr(-1)) but differed between years. Manured soils were net seasonal CH(4)-C emitters (0.159-0.329 kg ha(-1) yr(-1)), whereas CSUAN and CCUAN exhibited CH(4)-C uptake (-0.128 and -0.177 kg ha(-1) yr(-1), respectively). Treatments significantly influenced seasonal N(2)O-N emissions (P < 0.001) and ranged from <1.0 kg ha(-)(1) yr(-1) in PG and SC to between 3 and 5 kg ha(-1) yr(-1) in CCFM and CSUAN and >8 kg ha(-1) yr(-1) in CCSM; differences were driven by pulse emissions after N fertilization in concurrence with major rainfall events. These results suggest fall manure application, corn-soybean rotation, and restoration of prairies may diminish N(2)O emissions and hence contribute to global warming mitigation.