Lower Nitrous Oxide Emissions from Anhydrous Ammonia Application Prior to Soil Freezing in Late Fall Than Spring Pre-Plant Application

Contribution of crop residue, soil, and fertilizer nitrogen to nitrous oxide emissions varies with long-term crop rotation and tillage

Agriculture is an important contributor to N₂O emissions - a potent greenhouse gas - with high peaks occurring when soil mineral nitrogen (N) is high (e.g., after mineralization of organic N and N fertilizer application). Nitrogen dynamics in soil and consequently N₂O emissions are affected by crop and soil management practices (e.g., crop rotation and tillage), an effect mostly assessed in the literature through comparisons of total N₂O emission. Hence, information is scarce on the effect of these management practices on specific N sources affecting N₂O emissions (i.e., N fertilizer, soil, above and belowground crop residues) - a knowledge gap explored in this study with the use of ¹⁵N tracers. The isotope approach enabled refinement on global N₂O budget by directly determining the emission factors (EF) of above and belowground crop residues that vary in chemical composition and comparison with default EF values (e.g., IPCC EFs). Our experiment was conducted over the full-cycle of long-term crop rotations to (i) compare N₂O totals and intensity, under no-tillage and conventional tillage, simple and diverse rotation; (ii) partition total N₂O emissions into soil, N fertilizer, above and belowground crop residue N sources; (iii) compare the 12-month EF of crop residue against the default values proposed by IPCC (2019). For the tillage effect, annual N₂O emissions were from 1.2- to 2.0-times higher on CT than NT soil due to 40% increased soil N derived N₂O emission in CT. The diversified crop rotation emitted 1.3-times higher N₂O than the simple rotation over the full-cycle of the rotations, but the effect was due to differences in N fertilizer rate between the rotations since emissions were equivalent when scaled by N rate. Finally, our results suggested that default IPCC EF are overestimated for crop residues under CT and NT, simple and diverse rotations as measured EFs never surpassed 0.1%.

A meta-analysis of economic and environmental benefits of conservation agriculture in South Asia

Agriculture plays a key role in ensuring food and livelihood security in South Asia. However, this region is vulnerable to climate change which is likely to impact the livelihoods of millions of marginal and small holders. Agriculture is not only impacted by climate change but also one of the major contributor to global warming in South Asia. As compared to the traditional practices, Conservation Agriculture (CA) practices help mitigate the impact of climate change through a reduction in carbon emission and conservation of natural resources. In this article, a meta-analysis of the important studies was done for the impact of CA on carbon sequestration, water use, greenhouse gas emissions and cost and net returns. Carbon sequestration potential was found significantly higher in the CA practices (+16.30%) as compared to the conventional tillage. Inclusion of legumes, clay-rich soils, irrigation and presence of soil cover are the major drivers for higher carbon sequestration potential in the region. Additionally, a significant amount of water was also saved as CA practices led to relatively less consumption of water over the conventional tillage. Further, the adoption of CA based management practices resulted in a substantial reduction of CO₂ (-4.28%) and CH₄ (-25.67%) emissions both in aerobic and anaerobic soil conditions. However, the emission of NO2 and N2O-N gases were higher under the CA, +14.45 and + 5.20% respectively. Nevertheless, the emission of N₂O-N was lesser in CA (-1.78%) under aerobic conditions whereas it is increased under anaerobic soil conditions (+12.15%). The adoption of CA practices resulted in higher returns and lower costs as compared to the conventional system. Although CA has significant environmental benefits, the study suggests judicious use of inorganic inputs under CA for managing the impact of climate change in South Asia. Therefore, CA is a sustainable agricultural practice that deserves outscaling in South Asia for mitigation and adaptation of climate change.

Predicting greenhouse gas benefits of improved nitrogen management in North American maize

Farmers, food supply companies, and policymakers need practical yet scientifically robust methods to quantify how improved nitrogen (N) fertilizer management can reduce nitrous oxide (N₂ O) emissions. To meet this need, we developed an empirical model based on published field data for predicting N₂ O emission from rainfed maize (Zea mays L.) fields managed with inorganic N fertilizer in the United States and Canada. Nitrous oxide emissions ranged widely on an area basis (0.03-32.9 kg N ha^-1 yr^-1 ) and a yield-scaled basis (0.006-4.8 kg N Mg^-1 grain yr^-1 ). We evaluated multiple modeling approaches and variables using three metrics of model fit (Akaike information criteria corrected for small sample sizes [AICc], RMSE, and R² ). Our model explains 32.8% of the total observed variation and 50% of observed site-level variation. Soil clay content was very important for predicting N₂ O emission and predicting the change in N₂ O emission due to a change in N balance, with the addition of a clay fixed effect explaining 37% of site-level variation. Sites with higher clay content showed greater reductions in N₂ O emission for a given reduction in N balance. Therefore, high-clay sites are particularly important targets for reducing N₂ O emissions. Our linear mixed model is more suitable for predicting the effect of improved N management on N₂ O emission in maize fields than other published models because it (a) requires only input data readily available on working farms, (b) is derived from field observations, (c) correctly represents differences among sites using a mixed modeling approach, and (d) includes soil texture because it strongly influences N₂ O emissions.

Extent and Variation of Nitrogen Losses from Non-legume Field Crops of Conterminous United States

Nitrogen (N) losses from field crops have raised environmental concerns. This manuscript accompanies a database of N loss studies from non-legume field crops conducted across the conterminous United States. Cumulative N losses through nitrous oxide-denitrification (CN2O), ammonia volatilization (CNH3), and nitrate leaching (CNO3−) during the growing season and associated crop, soil, and water management information were gathered to determine the extent and controls of these losses. This database consisted of 404, 26, and 358 observations of CN2O, CNH3, and CNO3− losses, respectively, from sixty-two peer-reviewed manuscripts. Corn (Zea mays) dominated the N loss studies. Losses ranged between −0.04 to 16.9, 2.50 to 50.9, and 0 to 257 kg N ha−1 for CN2O, CNH3 and CNO3−, respectively. Most CN2O and CNO3− observations were reported from Colorado (n = 100) and Iowa (n = 176), respectively. The highest values of CN2O, and CNO3− were reported from Illinois and Minnesota states, and corn and potato (Solanum tuberosum), respectively. The application of anhydrous NH3 had the highest value of CN2O loss, and ammonium nitrate had the highest CNO3− loss. Among the different placement methods, the injection of fertilizer-N had the highest CN2O loss, whereas the banding of fertilizer-N had the highest CNO3− loss. The maximum CNO3− loss was higher for chisel than no-tillage practice. Both CN2O and CNO3− were positively correlated with fertilizer N application rate and the amount of water input (irrigation and rainfall). Fertilizer-N management strategies to control N loss should consider the spatio-temporal variability of interactions among climate, crop-and soil types.

Long-term impact of conservation agriculture and diversified maize rotations on carbon pools and stocks, mineral nitrogen fractions and nitrous oxide fluxes in inceptisol of India

Given the increasing scarcity of production resources such as water, energy and labour coupled with growing climatic risks, maize-based production systems could be potential alternatives to intensive rice-wheat (RW) rotation in western Indo-Gangetic Plains (IGP). Conservation agriculture (CA) in maize systems has been widely promoted for minimizing soil degradation and ensuring sustainability under emerging climate change scenario. Such practices are also believed to provide mitigation co-benefits through reduced GHG emission and increased soil carbon sequestration. However, the combined effects of diversified crop rotations and CA-based management on GHG mitigation potential and other co-benefits are generally over looked and hence warrant greater attention. A field trial was conducted for 5-years to assess the changes in soil organic carbon fractions, mineral-N, N₂O emission and global warming potential (GWP) of maize-based production systems under different tillage & crop establishment methods. Four diversified cropping systems i.e. maize-wheat-mungbean (MWMb), maize-chickpea-Sesbania (MCS), maize-mustard-mungbean (MMuMb) and maize-maize-Sesbania (MMS) were factorially combined with three tillage & crop establishment methods i.e. zero tilled permanent beds (PB), zero-tillage flat (ZT) and conventional tillage (CT) in a split-plot design. After 5-years of continued experimentation, we recorded that across the soil depths, SOC content, its pools and mineral-N fractions were greatly affected by tillage & crop establishment methods and cropping systems. ZT and PB increased SOC stock (0-30 cm depth) by 7.22-7.23 Mg C ha^-1 whereas CT system increased it only by 0.88 Mg C ha^-1as compared to initial value. Several researchers reported that SOC & mineral-N fraction contents in the top 30 cm soil depth are correlated with N₂O-N emission. In our study, global warming potential (GWP) under CT system was higher by 18.1 and 17.4%, compared to CA-based ZT and PB, respectively. Among various maize systems, GWP of MMS were higher by 11.2, 6.7 and 6.6%, compared that of MWMb (1212 kg CO₂-eq. ha^-1), MCS (1274 kg CO₂-eq. ha^-1) and MMuMb (1275 kg CO₂-eq. ha^-1), respectively. The results of our study suggest that CA and diversified crop rotations should be promoted in north-western IGP and other similar agro-ecologies across the globe for ensuring food security, restoration of soil health and climate change mitigation, the key sustainable development goals (SDGs).

Soil pH as the chief modifier for regional nitrous oxide emissions: New evidence and implications for global estimates and mitigation

Nitrous oxide (N₂ O) is a greenhouse gas that also plays the primary role in stratospheric ozone depletion. The use of nitrogen fertilizers is known as the major reason for atmospheric N₂ O increase. Empirical bottom-up models therefore estimate agricultural N₂ O inventories using N loading as the sole predictor, disregarding the regional heterogeneities in soil inherent response to external N loading. Several environmental factors have been found to influence the response in soil N₂ O emission to N fertilization, but their interdependence and relative importance have not been addressed properly. Here, we show that soil pH is the chief factor explaining regional disparities in N₂ O emission, using a global meta-analysis of 1,104 field measurements. The emission factor (EF) of N₂ O increases significantly (p < .001) with soil pH decrease. The default EF value of 1.0%, according to IPCC (Intergovernmental Panel on Climate Change) for agricultural soils, occurs at soil pH 6.76. Moreover, changes in EF with N fertilization (i.e. ΔEF) is also negatively correlated (p < .001) with soil pH. This indicates that N₂ O emission in acidic soils is more sensitive to changing N fertilization than that in alkaline soils. Incorporating our findings into bottom-up models has significant consequences for regional and global N₂ O emission inventories and reconciling them with those from top-down models. Moreover, our results allow region-specific development of tailor-made N₂ O mitigation measures in agriculture.