Xueming Yang

Ammonia loss following nitrogen fertilization can degrade air quality and impact human health, whereas nitrous oxide (N ₂ O) can contribute to global warming and climate change. Mitigation practices that target only one N‐loss pathway can lead to pollution swamping; hence, practices targeting both N‐losses are required. A 3‐year field study examined fertilizer N‐placement (broadcast urea, single‐slot injection of urea ammonium nitrate [UAN], double‐slot UAN injection) and N‐metabolization inhibitors (with/without urease and nitrification inhibitors) impacts on NH ₃ and N ₂ O losses and corn yields. Ammonia volatilization was reduced ( p < 0.05) by 26% with single‐slot UAN injection (10.6 kg N ha ⁻¹ ) and by 63% with double‐slot UAN injection (5.32 kg N ha ⁻¹ ) compared to broadcast urea (14.3 kg N ha ⁻¹ ). Dual urease and nitrification inhibitors reduced NH ₃ volatilization (0.84–3.86 kg N ha ⁻¹ ) by 57%–92% compared to no inhibitors (5.32–14.3 kg N ha ⁻¹ ). When no inhibitors were applied, N ₂ O emissions from slot injection (6.43–7.62 kg N ha ⁻¹ ) were 2.6–3.1 times greater than from broadcast urea (2.43 kg N ha ⁻¹ ). Dual inhibitors reduced N ₂ O emissions by 43% from 6.43 to 3.66 kg N ha ⁻¹ with double‐slot injection. Double‐slot UAN injection increased corn grain yields (9.73 t ha ⁻¹ ) by 12%–13% compared to single‐slot UAN injection (8.71 t ha ⁻¹ ) and broadcast urea (8.6 t ha ⁻¹ ). Double‐slot UAN injection effectively decreased NH ₃ losses and increased corn grain yields, but dual N inhibitors were required to also reduce N ₂ O. Hence, combined productivity and environmental benefits were accrued only when fertilizer containing urease and nitrification inhibitors was combined with double‐slot injection.

Many studies haveexamined soil‐borne nitrous oxide (N ₂ O) emissions from crops, but little effort has gone into determining the N ₂ O emissions from each phase of a crop rotation. A 4‐yr study on a long‐term field experiment compared growing season N ₂ O 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 N ₂ O 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 N ₂ O 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 N ₂ O emissions, whereas bacterial nitrifiers were positively correlated. Denitrification was likely responsible for N ₂ O emissions under CC, while nitrifier‐denitrification appeared to be the primary pathway under RC. The N ₂ O emissions and microbial processes from all phases of a rotation should be considered for environmental modeling and policy decisions.

Corn (Zea mays L.) stover is an increasingly important feedstock for biofuel and bioproduct production; however, excessive harvest of corn stover from agricultural fields may affect corn seedling emergence and subsequent plant population which may reduce corn growth and yield. Field experiments were conducted over four consecutive growing seasons in southwestern Ontario to determine under continuous corn cropping system the effects of no‐tillage (NT), conventional tillage (CT), and five rates of stover harvest (0, 25, 50, 75, and 100 wt. %) on corn seedling emergence in a Harrow sandy loam and in a Brookston clay loam. Time to achieve 50% seedling emergence (t_0.5), emergence rate constant (k_G), and final seedling population (M_G) were adequately simulated using the Gompertz function. Irrespective of stover removal rate and soil type, t_0.5 was 2 d longer, and k_G was 32% lower, under NT compared to CT. In Harrow sandy loam, stover removal from NT reduced t_0.5 in 1 out of 4 yr. The 100% removal reduced t_0.5 relative to 0, 25, 50, and 75% removal by 1.1–2.2 d in 2015. Averaged across both tillages and all removal rates, k_G was greater in Harrow sandy loam than in Brookston clay loam. Stover removal rates had no effect on M_G but averaged over all removal rates and soil types, M_G was 9% lower for NT compared to CT in 2016. No‐tillage practice under corn production can impede seedlings emergence early in the growing season and stover removal from the field can help mitigate against this effect.

Soybean ( Glycine max L.) is known to contribute to soil N reserves when grown in rotation with other high‐value crops such as corn ( Zea mays L.) and winter wheat ( Triticum aestivum L.). However, continuous soybean and “short” soybean rotations (e.g., corn–soybean, wheat–soybean) may cause declining soybean yields and degrading soil health over time. In this long‐term field study, we determined crop rotation effects on soybean yield and soil health indicators for a cool, humid, clay loam soil in southwestern Ontario. The study used nine soybean rotations, which included continuous soybean (S), corn–soybean (C–S), corn–soybean–soybean (C–S–S), and six rotations where winter wheat (WW) was grown with red clover ( Trifolium pratense L.) (+RC) and without underseeded red clover (i.e., winter wheat–soybean [WW–S, WW+RC–S], corn–soybean–winter wheat [C–S–WW, C–S–WW+RC], and winter wheat–soybean–soybean [WW–S–S, WW+RC–S–S]). Ten soil health indicators during the first and second soybean phase of each rotation were measured in 2018, whereas soybean yield was measured from 2002 to 2018. Soybean yields in 2‐ and 3‐yr rotations were 39–44% and 48–52% greater, respectively, relative to continuous soybean excluding the rotations with 2 of 3 yr of soybean (C–S–S, WW–S–S, WW+RC–S–S), which were only 22–35% greater than continuous soybean. Partial least squares regression indicated that inorganic N, particulate organic matter N, particulate organic matter C, potentially mineralizable N, total C, soil respiration rate, and water extractable organic soil C were the most important soil health indicators, explaining 34% of the total variation in soybean yields. It was concluded that soybean grown in 3‐yr rotations with corn and winter wheat produced the largest soybean yields and the greatest positive impacts on soil health indicators likely owing to cereal crops enhancing C inputs into soil.

Harvesting corn ( Zea mays L.) stover for production of biofuels, industrial sugars, bioproducts, and livestock bedding is increasing rapidly, but little is known of the impacts of stover removal on soil‐borne greenhouse gas (GHG) emissions. This study evaluated the impacts of removing surface corn stover (0, 25, 50, 75, 100 wt. % removal) on carbon dioxide (CO ₂ ) and nitrous oxide (N ₂ O) emissions from a sandy loam soil cropped to monoculture corn using conventional moldboard plow tillage (CT) and no‐tillage (NT). Stover removal systematically decreased CO ₂ emissions from CT, whereas stover removal had little effect on CO ₂ emissions from NT. In particular, the CT 0% stover removal treatment produced 47% greater CO ₂ emissions (5.75 Mg CO ₂ –C ha ⁻¹ ) than the CT 100% removal (3.91 Mg CO ₂ –C ha ⁻¹ ) treatment. Stover removal increased N ₂ O emissions from both tillage treatments, producing up to a 75% increase under CT (2.79 kg N ha ⁻¹ at 0% removal; 4.87 kg N ha ⁻¹ at 100% removal) and up to a 95% increase under NT (1.75 kg N ha ⁻¹ at 0% removal; 3.41 kg N ha ⁻¹ at 100% removal). Cumulative nitrate exposure increased in comparable patterns to N ₂ O emissions when stover residues were removed. There was a trade‐off in GHG emissions resulting from stover removal under CT, whereby increasing stover removal reduced CO ₂ emissions but increased N ₂ O emissions. In contrast, stover removal did not affect CO ₂ emissions under NT but it increased N ₂ O emissions especially at the 100% removal rates.

Ensuring sufficient fertilizer nitrogen (N) for crops while minimizing N losses requires best management practices optimized for climate, crop, soil, and root zone hydrology. In Ontario, pre‐plant N fertilization of corn ( Zea mays L.) is common; however, this practice extends the time between application and significant root interception of N by the plant, potentially increasing the risk of N loss through soil nitrous oxide emissions, ammonia (NH ₃ ) volatilization, and nitrate leaching. These losses contribute to greenhouse gas emissions, affect air quality (NH ₃ ), and are a substantial financial loss. This study compared three N placement methods (broadcast urea [BrUrea], broadcast incorporated urea [BrIncUrea], and injected urea ammonium nitrate [InjUAN]) and the presence or absence of N metabolite inhibitors (urease inhibitor [UI], urease plus nitrification inhibitor [UI+NI]). Fertilizer N was applied immediately before planting (150 kg N ha ⁻¹ ) to all treatments except for the control. Averaged over 3 yr (2015–2017), NH ₃ losses were reduced by 34% from BrIncUrea, by 42–55% from BrUrea+UI+NI and BrIncUrea+UI+NI, and by 99% from InjUAN relative to BrUrea (21 kg N ha ⁻¹ ). On average, N application increased corn grain yields by 83% relative to the control (6 t ha ⁻¹ ). There were no annual yield differences among N placement methods. It was concluded that incorporation or injection of N in soil and use of urease and nitrification inhibitors reduced NH ₃ emissions when N fertilizer was applied pre‐plant.

Demand for food is rising and the ability of any particular soil to support sustainable food production is dependent upon a variety of soil biochemical, chemical and physical soil parameters. However, the challenge is that the impacts of a new management practice on soil properties may take years to assess. In this field study, we investigated the effects of long‐term cropping rotation treatments (established in 2001) on soil health indicators from soils under monoculture, 2‐yr and 3‐yr crop rotations with and without a cover crop. In particular, we compared soil heath indicators under eight cropping sequences including continuous winter wheat ( Triticum aestivum L.) (WW), soybean ( Glycine max L.)–winter wheat (S–WW), corn ( Zea mays L.)–soybean–winter wheat (C–S–WW), and winter wheat–soybean–soybean (WW–S–S) all with and without red clover ( Trifolium pretense L.) (RC) in the wheat phase of the rotation. We measured ten soil health indicators collected from the wheat phase of the rotation. Crop rotation had greater effects on soil health indicators than cover crop. After 17 years, crop yields were 23–28% greater for the 2‐yr and 3‐yr rotations than monoculture WW in the presence of red clover. In the absence of red clover, yields were 32–39% greater for the 2‐yr and 3‐yr rotations than monoculture WW. However, the soil health indicators were significantly greater for monoculture WW than the 2‐yr and 3‐yr rotations. These results suggest WW enhanced soil health while crop rotations with soybean negatively impacted most biochemical soil health parameters.