Effects of nitrogen application rates on net annual global warming potential and greenhouse gas intensity in double-rice cropping systems of the Southern China

A global meta-analysis of yield-scaled N2 O emissions and its mitigation efforts for maize, wheat, and rice

Maintaining or even increasing crop yields while reducing nitrous oxide (N2 O) emissions is necessary to reconcile food security and climate change, while the metric of yield-scaled N2 O emission (i.e., N2 O emissions per unit of crop yield) is at present poorly understood. Here we conducted a global meta-analysis with more than 6000 observations to explore the variation patterns and controlling factors of yield-scaled N2 O emissions for maize, wheat and rice and associated potential mitigation options. Our results showed that the average yield-scaled N2 O emissions across all available data followed the order wheat (322 g N Mg-1 , with the 95% confidence interval [CI]: 301-346) > maize (211 g N Mg-1 , CI: 198-225) > rice (153 g N Mg-1 , CI: 144-163). Yield-scaled N2 O emissions for individual crops were generally higher in tropical or subtropical zones than in temperate zones, and also showed a trend towards lower intensities from low to high latitudes. This global variation was better explained by climatic and edaphic factors than by N fertilizer management, while their combined effect predicted more than 70% of the variance. Furthermore, our analysis showed a significant decrease in yield-scaled N2 O emissions with increasing N use efficiency or in N2 O emissions for production systems with cereal yields >10 Mg ha-1 (maize), 6.6 Mg ha-1 (wheat) or 6.8 Mg ha-1 (rice), respectively. This highlights that N use efficiency indicators can be used as valuable proxies for reconciling trade-offs between crop production and N2 O mitigation. For all three major staple crops, reducing N fertilization by up to 30%, optimizing the timing and placement of fertilizer application or using enhanced-efficiency N fertilizers significantly reduced yield-scaled N2 O emissions at similar or even higher cereal yields. Our data-driven assessment provides some key guidance for developing effective and targeted mitigation and adaptation strategies for the sustainable intensification of cereal production.

Ratoon Rice Cropping Mitigates the Greenhouse Effect by Reducing CH4 Emissions through Reduction of Biomass during the Ratoon Season

The ratoon rice cropping system (RR) is developing rapidly in China due to its comparable annual yield and lower agricultural and labor inputs than the double rice cropping system (DR). Here, to further compare the greenhouse effects of RR and DR, a two-year field experiment was carried out in Hubei Province, central China. The ratoon season showed significantly lower cumulative CH4 emissions than the main season of RR, the early season and late season of DR. RR led to significantly lower annual cumulative CH4 emissions, but no significant difference in cumulative annual N2O emissions compared with DR. In RR, the main and ratoon seasons had significantly higher and lower grain yields than the early and late seasons of DR, respectively, resulting in comparable annual grain yields between the two systems. In addition, the ratoon season had significantly lower global warming potential (GWP) and greenhouse gas intensity-based grain yield (GHGI) than the main and late seasons. The annual GWP and GHGI of RR were significantly lower than those of DR. In general, the differences in annual CH4 emissions, GWP, and GHGI could be primarily attributed to the differences between the ratoon season and the late season. Moreover, GWP and GHGI exhibited significant positive correlations with cumulative emissions of CH4 rather than N2O. The leaf area index (LAI) and biomass accumulation in the ratoon season were significantly lower than those in the main season and late season, and CH4 emissions, GWP, and GHGI showed significant positive correlations with LAI, biomass accumulation and grain yield in the ratoon and late season. Finally, RR had significantly higher net ecosystem economic benefits (NEEB) than DR. Overall, this study indicates that RR is a green cropping system with lower annual CH4 emissions, GWP, and GHGI as well as higher NEEB.

Greenhouse Gas Fluxes from Selected Soil Fertility Management Practices in Humic Nitisols of Upper Eastern Kenya

We quantified the soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes of five soil fertility management practices (inorganic fertilizer (Mf), maize residue + inorganic fertilizer (RMf), maize residue + inorganic fertilizer + goat manure (RMfM), maize residue + tithonia diversifolia + goat manure (RTiM), and a control (CtC)) in Kenya’s central highlands using a static chamber method from March 2019 to March 2020. The cumulative annual soil CH4 uptake ranged from −1.07 to −0.64 kg CH4-C ha−1 yr−1, CO2 emissions from 4.59 to 9.01 Mg CO2-C ha−1 yr−1, and N2O fluxes from 104 to 279 g N2O-N ha−1 yr−1. The RTiM produced the highest CO2 emissions (9.01 Mg CO2-C ha−1 yr−1), carbon sequestration (3.99 Mg CO2-eq ha−1), yield-scaled N2O emissions (YSE) (0.043 g N2O-N kg−1 grain yield), the lowest net global warming potential (net GWP) (−14.7 Mg CO2-eq ha−1) and greenhouse gas intensities (GHGI) (−2.81 Kg CO2-eq kg−1 grain yield). We observed average maize grain yields of 7.98 Mg ha−1 yr−1 under RMfM treatment. Integrating inorganic fertilizer and maize residue retention resulted in low emissions, increased soil organic carbon sequestration, and high maize yields.

Eco-friendly yield-scaled global warming potential assists to determine the right rate of nitrogen in rice system: A systematic literature review

Rice paddies are one of the largest greenhouse gases (GHGs) facilitators that are predominantly regulated by nitrogen (N) fertilization. Optimization of N uses based on the yield has been tried a long since, however, the improvement of the state-of-the-art technologies and the stiffness of global warming need to readjust N rate. Albeit, few individual studies started to, herein attempted as a systematic review to generalize the optimal N rate that minimizes global warming potential (GWP) concurrently provides sufficient yield in the rice system. To satisfy mounted food demand with inadequate land & less environmental impact, GHGs emissions are increasingly evaluated as yield-scaled basis. This systematic review (20 published studies consisting of 21 study sites and 190 observations) aimed to test the hypothesis that the lowest yield-scaled GWP would provide the minimum GWP of CH₄ and N₂O emissions from rice system at near optimal yields. Results revealed that there was a strong polynomial quadratic relationship between CH₄ emissions and N rate and strong positive correlation between N₂O emissions and N rate. Compared to control the low N dose emitted less (23%) CH₄ whereas high N dose emitted higher (63%) CH₄ emission. The highest N₂O emission observed at moderated N level. In total GWP, about 96% and 4%, GHG was emitted as CH₄ and N₂O, respectively. The mean GWP of CH₄ and N₂O emissions from rice was 5758 kg CO₂ eq ha^-1. The least yield-scaled GWP (0.7565 (kg CO₂ eq. ha^-1)) was recorded at 190 kg N ha^-1 that provided the near utmost yield. This dose could be a suitable dose in midseason drainage managed rice systems especially in tropical and subtropical climatic conditions. This yield-scaled GWP supports the concept of win-win for food security and environmental aspects through balancing between viable rice productivity and maintaining convincing greenhouse gases.

Ridge-furrow mulching system and supplementary irrigation can reduce the greenhouse gas emission intensity

In this study, in order to explore the greenhouse gas emissions and global warming potential (GWP) in winter wheat fields under the ridge-furrow mulching system (RF) with supplementary irrigation, three rainfall conditions (heavy rainfall = 275 mm, normal rainfall = 200 mm, and light rainfall = 125 mm) and four irrigation treatments (150, 75, 37.5, and 0 mm) were simulated during the growth period. Traditional flat planting (TF) was used as the control and we determined the emissions of N₂O, CO₂, and CH₄, as well as the GWP and greenhouse gas emission intensity (GHGI). The results obtained after three years (October 2016 to June 2019) showed that when the amount of irrigation was the same during the winter wheat growth period, the N₂O emission flux, CO₂ emission flux, and GHGI under RF decreased by 3.30-23.78%, 5.93-6.45%, and 5.01-23.72% with rainfall at 275 mm, respectively, compared with those under TF. Under the same level of supplementary irrigation, the N₂O emission flux, CO₂ emission flux, and GHGI decreased by 0.8-4.18%, 5.05-13.53%, and 7.83-13.72%, respectively, with rainfall at 200 mm, and they decreased by 17.49-32.46%, 25.57-35.35%, and 6.22-30.20% with rainfall at 125 mm. Under the three rainfall conditions, the absorption of CH4 in the winter wheat field increased as the supplementary irrigation decreased. Our results showed that the RF system can satisfy the goal of achieving high yields and saving water, as well as reducing the GHGI to contribute less to global climate warming as an environmentally friendly irrigation method.

Mitigation of greenhouse gas emissions through optimized irrigation and nitrogen fertilization in intensively managed wheat-maize production

In the wheat-maize rotation cultivation system in northern China, excessive irrigation and over-fertilization have depleted groundwater and increased nitrogen (N) losses. These problems can be addressed by optimized N fertilization and water-saving irrigation. We evaluated the effects of these practices on greenhouse gas emissions (GHG), net profit, and soil carbon (C) sequestration. We conducted a field experiment with flood irrigation (FN0, 0 kg N ha^-1 yr^-1, FN600, 600 kg N ha^-1 yr^-1) and drip fertigation treatments (DN0, 0 kg N ha^-1 yr^-1; DN420, 420 kg N ha^-1 yr^-1; DN600, 600 kg N ha^-1 yr^-1) in 2015-2017. Compared with FN600, DN600 decreased direct GHGs (N₂O + CH₄) emissions by 21%, and increased the net GHG balance, GHG intensity, irrigation water-use efficiency (IWUE), and soil organic C content (ΔSOC) by 13%, 12%, 88%, and 89.8%, respectively. Higher costs in DN600 (for electricity, labour, polyethylene) led to a 33.8% lower net profit than in FN600. Compared with FN600, DN420 reduced N and irrigation water by 30% and 46%, respectively, which increased partial factor productivity and IWUE (by 49% and 94%, respectively), but DN420 did not affect GHG mitigation or net profit. Because lower profit is the key factor limiting the technical extension of fertigation, financial subsidies should be made available for farmers to install fertigation technology.

Investigation into the Effects of Straw Retention and Nitrogen Reduction on CH4 and N2O Emissions from Paddy Fields in the Lower Yangtze River Region, China

Straw retention is a widely used method in rice planting areas throughout China. However, the combined influences of straw retention and nitrogen (N) fertilizer application on greenhouse gas (GHG) fluxes from paddy fields merits significant attention. In this work, we conducted a field experiment in the lower Yangtze River region of China to study the effects of straw retention modes and N fertilizer rates on rice yield, methane (CH4) and nitrous oxide (N2O) emission fluxes, global warming potential (GWP), and greenhouse gas intensity (GHGI) during the rice season. The experiments included six treatments: the recommended N fertilizer—240 kg N·ha−1 with (1) no straw, (2) wheat straw, (3) rice straw, and (4) both wheat and rice straw retentions; in a yearly rice–wheat cropping system (N1, WN1, RN1, and WRN1, respectively); as well as both wheat and rice straw retentions with (5) no N fertilizer and (6) 300 kg N·ha−1 conventional N fertilizer (WRN0, WRN2). The results showed that CH4 emissions were mainly concentrated in the tillering fertilizer stage and accounted for 54.2%–87.5% of the total emissions during the rice season, and N2O emissions were primarily concentrated in the panicle fertilizer stage and accounted for 46.7%–51.4% total emissions. CH4 was responsible for 87.5%–98.5% of the total CH4 and N2O GWP during the rice season, and was the main GHG contributor in the paddy field. Although straw retention reduced N2O emissions from paddy field, it significantly increased CH4 emissions, which resulted in a significant net increase in the total GWP. Compared with the N1 treatment, the total GWP of WN1, WRN1, and RN1 increased by 3.45, 3.73, and 1.62 times, respectively; and the GHGI increased by 3.00, 2.96, and 1.52 times, respectively, so the rice straw retention mode had the smallest GWP and GHGI. Under double-season’s straw retentions, N fertilizer application increased both CH4 and N2O emissions, and the WRN1 treatment not only maintained high rice yield but also significantly reduced the GWP and GHGI by 16.5% and 30.1% (p < 0.05), respectively, relative to the WRN2 treatment. Results from this study suggest that adopting the “rice straw retention + recommended N fertilizer” mode (RN1) in the rice–wheat rotation system prevalent in the lower Yangtze River region will aid in mitigating the contribution of straw retention to the greenhouse effect.

The environmental costs and benefits of high-yield farming

How we manage farming and food systems to meet rising demand is pivotal to the future of biodiversity. Extensive field data suggest impacts on wild populations would be greatly reduced through boosting yields on existing farmland so as to spare remaining natural habitats. High-yield farming raises other concerns because expressed per unit area it can generate high levels of externalities such as greenhouse gas (GHG) emissions and nutrient losses. However, such metrics underestimate the overall impacts of lower-yield systems, so here we develop a framework that instead compares externality and land costs per unit production. Applying this to diverse datasets describing the externalities of four major farm sectors reveals that, rather than involving trade-offs, the externality and land costs of alternative production systems can co-vary positively: per unit production, land-efficient systems often produce lower externalities. For GHG emissions these associations become more strongly positive once forgone sequestration is included. Our conclusions are limited: remarkably few studies report externalities alongside yields; many important externalities and farming systems are inadequately measured; and realising the environmental benefits of high-yield systems typically requires additional measures to limit farmland expansion. Yet our results nevertheless suggest that trade-offs among key cost metrics are not as ubiquitous as sometimes perceived.