Optimal nitrogen rate strategy for sustainable rice production in China.

Avoiding excessive agricultural nitrogen (N) use without compromising yields has long been a priority for both research and government policy in China1,2. Although numerous rice-related strategies have been proposed3-5, few studies have assessed their impacts on national food self-sufficiency and environmental sustainability and fewer still have considered economic risks faced by millions of smallholders. Here we established an optimal N rate strategy based on maximizing either economic (ON) or ecological (EON) performance using new subregion-specific models. Using an extensive on-farm dataset, we then assessed the risk of yield losses among smallholder farmers and the challenges of implementing the optimal N rate strategy. We find that meeting national rice production targets in 2030 is possible while concurrently reducing nationwide N consumption by 10% (6-16%) and 27% (22-32%), mitigating reactive N (Nr) losses by 7% (3-13%) and 24% (19-28%) and increasing N-use efficiency by 30% (3-57%) and 36% (8-64%) for ON and EON, respectively. This study identifies and targets subregions with disproportionate environmental impacts and proposes N rate strategies to limit national Nr pollution below proposed environmental thresholds, without compromising soil N stocks or economic benefits for smallholders. Thereafter, the preferable N strategy is allocated to each region based on the trade-off between economic risk and environmental benefit. To facilitate the adoption of the annually revised subregional N rate strategy, several recommendations were provided, including a monitoring network, fertilization quotas and smallholder subsidies.

Optimizing water and nitrogen productivity of wheat and triticale across diverse production environments to improve the sustainability of baked products.

Wheat (Triticum aestivum L.) is a major global commodity and the primary source for baked products in agri-food supply chains. Consumers are increasingly demanding more nutritious food products with less environmental degradation, particularly related to water and fertilizer nitrogen (N) inputs. While triticale (× Triticosecale) is often referenced as having superior abiotic stress tolerance compared to wheat, few studies have compared crop productivity and resource use efficiencies under a range of N-and water-limited conditions. Because previous work has shown that blending wheat with triticale in a 40:60 ratio can yield acceptable and more nutritious baked products, we tested the hypothesis that increasing the use of triticale grain in the baking supply chain would reduce the environmental footprint for water and N fertilizer use. Using a dataset comprised of 37 site-years encompassing normal and stress-induced environments in California, we assessed yield, yield stability, and the efficiency of water and fertilizer N use for 67 and 17 commercial varieties of wheat and triticale, respectively. By identifying environments that favor one crop type over the other, we then quantified the sustainability implications of producing a mixed triticale-wheat flour at the regional scale. Results indicate that triticale outyielded wheat by 11% (p < 0.05) and 19% (p < 0.05) under average and N-limited conditions, respectively. However, wheat was 3% (p < 0.05) more productive in water-limited environments. Overall, triticale had greater yield stability and produced more grain per unit of water and N fertilizer inputs, especially in high-yielding environments. We estimate these differences could translate to regional N fertilizer savings (up to 555 Mg N or 166 CO2-eq kg ha-1) in a 40:60 blending scenario when wheat is sourced from water-limited and low-yielding fields and triticale from N-limited and high-yielding areas. Results suggest that optimizing the agronomic and environmental benefits of triticale would increase the overall resource use efficiency and sustainability of the agri-food system, although such a transition would require fundamental changes to the current system spanning producers, processors, and consumers.

Impact of cropping system diversification on productivity and resource use efficiencies of smallholder farmers in south-central Bangladesh: a multi-criteria analysis.

Diversification of smallholder rice-based cropping systems has the potential to increase cropping system intensity and boost food security. However, impacts on resource use efficiencies (e.g., nutrients, energy, and labor) remain poorly understood, highlighting the need to quantify synergies and trade-offs among different sustainability indicators under on-farm conditions. In southern coastal Bangladesh, aman season rice is characterized by low inputs and low productivity. We evaluated the farm-level impacts of cropping system intensification (adding irrigated boro season rice) and diversification (adding chili, groundnut, mungbean, or lathyrus) on seven performance indicators (rice equivalent yield, energy efficiency, partial nitrogen productivity, partial potassium productivity, partial greenhouse gas footprint, benefit-cost ratio, and hired labor energy productivity) based on a comprehensive survey of 501 households. Indicators were combined into a multi-criteria performance index, and their scope for improvement was calculated by comparing an individual farmer's performance to top-performing farmers (highest 20%). Results indicate that the baseline system (single-crop aman season rice) was the least productive, while double cropped systems increased rice equivalent yield 72-217%. Despite gains in productivity, higher cropping intensity reduced resource use efficiencies due to higher inputs of fertilizer and energy, which also increased production costs, particularly for boro season rice. However, trade-offs were smaller for diversified systems including legumes, largely owing to lower N fertilizer inputs. Aman season rice had the highest multi-criteria performance index, followed by systems with mungbean and lathyrus, indicating the latter are promising options to boost food production and profitability without compromising sustainability. Large gaps between individual and top-performing farmers existed for each indicator, suggesting significant scope for improvement. By targeting indicators contributing most to the multi-criteria performance index (partial nitrogen productivity, energy efficiency, hired labor energy productivity), results suggest further sustainability gains can be achieved through future field research studies focused on optimizing management within diversified systems. Supplementary Information: The online version contains supplementary material available at 10.1007/s13593-022-00795-3.

Assessing the concept of control points for dissolved reactive phosphorus losses in subsurface drainage.

Agricultural phosphorus (P) loss, which is highly variable in space and time, has been studied using the hot spot/hot moment concept, but increasing the rigor of these assessments through a relatively newer "ecosystem control point" framework may help better target management practices that provide a disproportionate water quality benefit. Sixteen relatively large (0.85 ha) subsurface drainage plots in Illinois were used as individual observational units to assess dissolved reactive P (DRP) concentrations and losses within a given field over four study years. Three plot-months were identified as DRP control points (one export and two transport control points), where each plot-month contributed >10% of the annual DRP load from the field. These control points occurred on separate plots and in both the growing and nongrowing seasons but were likely related to agronomic P applications. Elevated soil test P, especially near a historic farmstead, and soil clay content were spatial drivers of P loss across the field. The nongrowing season was hypothesized to be the most significant period of P loss, but this was only documented in two of the four study years. A cereal rye (Secale cereale L.) cover crop did not significantly reduce DRP loss in any year, but there was also no evidence of increased drainage P losses due to freezing and thawing of the cover crop biomass. This work confirmed annual subsurface drainage DRP losses were agronomically small (<3% of P application rate), although the range of DRP concentrations relative to eutrophication criteria still demonstrated a potential for negative environmental impact. The control point concept may provide a new lens to view drainage DRP losses, but this framework should be refined through additional within-field studies because mechanisms of P export at this field were more nuanced than just the presence of tile drainage (i.e., a transport control point).

Simulated dataset of corn response to nitrogen over thousands of fields and multiple years in Illinois.

Nitrogen (N) fertilizer recommendations for corn (Zea mays L.) in the US Midwest have been a puzzle for several decades, without agreement among stakeholders for which methodology is the best to balance environmental and economic outcomes. Part of the reason is the lack of long-term data of crop responses to N over multiple fields since trial data is often limited in the number of soils and years it can explore. To overcome this limitation, we designed an analytical platform based on crop simulations run over millions of farming scenarios over extensive geographies. The database was calibrated and validated using data from more than four hundred trials in the region. This dataset can have an important role for research and education in N management, machine leaching, and environmental policy analysis. The calibration and validation procedure provides a framework for future gridded crop model studies. We describe dataset characteristics and provide thorough descriptions of the model setup.

Factors contributing to farm-level productivity and household income generation in coastal Bangladesh's rice-based farming systems.

Large changes have taken place in smallholder farming systems in South Asia's coastal areas in recent decades, particularly related to cropping intensity, input availability, climate risks, and off-farm activities. However, few studies have investigated the extent to which these changes have impacted farm-level crop productivity, which is a key driver of food security and poverty in rainfed, low-input, rice-based systems. The objective of this study was to conduct an integrated assessment of variables related to socioeconomic status, farm characteristics, and crop management practices to understand the major factors influencing crop productivity and identify promising leverage points for sustainable development in coastal Bangladesh. Using a panel survey dataset of 32 variables from 502 farm households located within polder (coastal embankment) and outside polder systems during 2005-2015, we employed statistical factor analysis to characterize five independent latent factors named here as Farming Challenges, Economic Status, Crop Management Practices, Asset Endowment, and Farm Characteristics. The factor Farming Challenges explained the most variation among households (31%), with decreases observed over time, specifically households located outside polders. Individual variables contributing to this factor included perceived cyclone severity, household distance to main roads and input-output markets, cropping intensity, and access to extension services. The most important factors for increasing crop productivity on a household and per unit area basis were Asset Endowment and Crop Management Practices, respectively. The former highlights the need for increasing gross cropped area, which can be achieved through greater cropping intensity, while the latter was associated with increased fertilizer, labor, and pesticide input use. Despite the importance of these factors, household poverty trajectory maps showed that changes in off-farm income had played the strongest role in improving livelihoods in this coastal area. This study can help inform development efforts and policies for boosting farm-level crop productivity, specifically through agricultural intensification (higher cropping intensity combined with appropriate and efficient use of inputs) and expanding opportunities for off-farm income as key pathways to bring smallholder households out of poverty.

Split-nitrogen application with cover cropping reduces subsurface nitrate losses while maintaining corn yields.

Artificial subsurface drainage is essential to sustain crop production in many areas but may also impair water quality by exacerbating nitrate (NO3 )-nitrogen (N) delivery downstream. Cover crops and split-N application have been promoted as key conservation practices for reducing NO3 -N losses, but few studies have simultaneously assessed their effect on water quality and crop productivity. A field study was conducted to evaluate the effects of N application timing and cover crops on subsurface drainage NO3 -N losses and grain yield in continuous corn (Zea mays L.). Treatments were preplant-N: 224 kg N ha-1 split-applied with 60% fall + 40% preplant in 2018, or as single preplant applications in 2019 and 2020; split-N: 40% preplant + 60% side-dress (V6-V7); split-N + cover crop (CC): Split-N + cereal rye (Secale cereale L.); and a zero N plot as the control. Across the 3-yr study period, split-N + CC significantly reduced flow-weighted NO3 -N concentration and NO3 -N loss by 35 and 37%, respectively, compared with preplant-N. However, flow-weighted NO3 -N concentration (4.3 mg L-1 ) and NO3 -N loss (22.4 kg ha-1 ) with split-N were not significantly different from either preplant-N (4.8 mg L-1 and 26.4 kg ha-1 , respectively) or split-N + CC (3.1 mg L-1 and 16.7 kg ha-1 , respectively). Corn yield was significantly lower in the control treatment but did not differ among N fertilized treatments in any year. These results indicate that combining split-N application with cover crops holds promise for meeting the statewide interim milestone NO3 -N reduction target of 15% by 2025 without negatively impacting crop productivity.

Towards actionable research frameworks for sustainable intensification in high-yielding rice systems.

New research frameworks that simultaneously address production and environmental goals are required to identify promising sustainable intensification options in high-yielding cereal systems. Here we estimated potential changes in environmental footprint associated with crop management practices aimed at breaking the yield ceiling for rice production in Uruguay. Results from a regional survey were combined with field experiments to integrate impacts on productivity and sustainability at two different intensification levels (average-yielding and high-yielding). Survey results indicate that high-yielding farmers produced 14% more grain compare to the regional average (7900 kg ha-1), with 25% to 99% lower agrochemical contamination risk and similar nitrogen use efficiency and carbon footprint. In on-farm trials, the alternative management practices increased yield beyond that of high-yielding farmers by up to 7% in small plots (8 site-years) and 15% in field-scale comparisons (6 site-years), yet an ex post assessment of environmental indicators shows significant decline of resource use efficiencies and increased carbon footprint. Thus, yield gains were not able to compensate for increased environmental footprint, highlighting the challenge of advancing the dual goals of SI in production systems nearing the yield ceiling. This study provides a simple but powerful framework for advancing SI in mainstream cereal production systems based on cost-effective modifications to existing agronomic experiments.

Agronomic, economic, and environmental performance of nitrogen rates and source in Bangladesh's coastal rice agroecosystems.

Farmers in low-elevation coastal zones in South Asia face numerous food security and environmental sustainability challenges. This study evaluated the effects of nitrogen (N) rate and source on the agronomic, economic, and environmental performance of transplanted and rainfed 'aman' (monsoon-season) rice in Bangladesh's non-saline coastal areas. Fifty-one farmers participated in trials distributed across two landscape positions described as 'highlands' (on which field water inundation depth typically remains <30 cm) and 'medium-highlands' (inundation depths 30-90 cm) planted singly with varieties appropriate to each position (BRRI dhan 39 for highlands and the traditional variety Bhushiara for medium-highlands). Researcher designed but farmer-managed dispersed plots were located across three district sub-units (Barisal Sadar, Hizla, Mehendigonj) and compared N source (broadcast prilled urea or deep-placed urea super granules (USG)) at four N rates. Rice grown on medium-highlands did not respond to increasing N rates beyond 28 kg N ha-1, indicating that little fertilization is required to maintain yields and profitability while limiting environmental externalities. In highland locations, clear trade-offs between agronomic and environmental goals were observed. To increase yields and profits for BRRI dhan 39, 50 or 75 kg N ha-1 was often needed, although these rates were associated with declining energy and increasing greenhouse gas (GHG) efficiencies. Compared to prilled urea, USG had no impact on yield, economic, energy and GHG efficiencies in medium-highland locations. USG conversely led to 4.2-5.8% yield improvements at higher N rates on highlands, while also increasing energy efficiency. Given the observed yield, agronomic and economic benefit of USG, our preliminary results that farmers can consider use of USG at 50 kg N ha-1 to produce yields equivalent to 75 kg N ha-1 of prilled urea in highland landscapes, while also reducing environmental externalities. These results suggest that when assessing sustainable intensification (SI) strategies for rice in South Asia's coastal zones, N requirements should be evaluated within specific production contexts (e.g. cultivar type within landscape position) to identify options for increasing yields without negatively influencing environmental and economic indicators. Similar studies in other parts of coastal South Asia could help policy-makers prioritize investments in agriculture with the aim of improving rice productivity while also considering income generation and environmental outcomes.

Simulating nitrogen management impacts on maize production in the U.S. Midwest.

Nutrient loss reduction strategies have recently been developed in the U.S. Midwest to decrease the environmental footprint associated with nitrogen (N) fertilizer use. Although these strategies generally suggest decreasing N rates and shifting the timing of N application from fall to spring, the spatiotemporal impacts of these practices on maize yield and fertilizer N use efficiency (NUE, kg grain yield increase per kg N applied) have not been assessed at the watershed scale using crop simulation models. We simulated the effects of N fertilizer rate (0, 168, 190, 224 kg N ha-1) and application timing [fall-applied N (FN): 100% N applied on 1 December; spring-applied N (SN): 100% N applied 10 days before planting; split N: 66% N applied on 1 December + 34% N applied 10 days before planting] on maize grain yield (GY) across 3042 points in Illinois during 2011-2015 using the DSSAT-CERES-Maize model. When simulations were scaled up to the watershed level, results suggest that increases in average maize GY for SN compared to FN occurred in years with higher than average winter rainfall (2011, 2013), whereas yields were similar (+/- 4%) in 2012, 2014, and 2015. Accordingly, differences in NUE for SN compared to FN were small (0.0-1.4 kg GY/kg N) when cumulative winter rainfall was < 300 mm, but increased to 0.1-9.2 kg GY/kg N when winter rainfall was > 500 mm at both 168 kg N ha-1 and 224 kg N ha-1. The combined practice of reducing N fertilizer amounts from 224 kg N ha-1 to 190 kg N ha-1 and shifting from FN to SN resulted in a wide range of yield responses during 2011-2015, with the probability of increasing yields varying from <10% to >70% of simulation points within a watershed. Positive impacts on both GY and NUE occurred in only 60% of simulations for this scenario, highlighting the challenge of simultaneously improving yield and NUE with a 15% N rate reduction in this region.

Tillage and Fertilizer Management Effects on Phosphorus Runoff from Minimal Slope Fields.

Phosphorus fertilization can increase P losses in surface runoff, but limited information is available for fields with <2% slopes in the US Midwest. Our objectives were to determine the effects of tillage-fertilizer placement (no-till-broadcast, strip till-broadcast; or strip till-deep placement, -15-cm subsurface band) and fertilizer rate applied in the fall (0, 52, or 90 kg PO ha yr) on runoff P concentrations and loads in fields with <2% slopes near Pesotum, IL, during fall and spring simulation runoff events, and to measure corn ( L.) and soybean [ (L.) Merr.] grain yield. Across four simulated runoff events, deep placement reduced dissolved reactive P (DRP) loads by 69 to 72% compared with the broadcast treatments. A tillage-fertilizer placement × P rate interaction showed that DRP and total P (TP) concentrations remained low when P was deep placed, regardless of P rate, whereas concentrations increased with increasing P rate for the broadcast treatments, but no differences existed for bioavailable P (BAP) (α = 0.05). At one site, rainfall simulation in the spring versus fall increased runoff volumes but sharply decreased BAP concentrations. During fall runoff simulations, deep placement reduced TP loads, and greater TP loads occurred with the 90- than the 52-kg PO ha yr rate. Similarly, when P was broadcast in the fall, DRP and TP concentrations were greater than deep-placed P, but no treatment differences occurred in the spring. Deep banding P and K did not reduce crop yield but reduced runoff losses of P from flat fields compared with broadcast P applications, particularly at high rates of P application.

Population and community structure shifts of ammonia oxidizers after four-year successive biochar application to agricultural acidic and alkaline soils.

Long-term studies that advance our mechanistic understanding of biochar (BC)­nitrogen (N) interactions in agricultural soils are lacking. In this study, soil potential nitrification rates (PNR), the abundance and composition of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) communities following 4-year of BC application were investigated using the shaken-slurry procedure and molecular sequencing techniques for an acidic Oxisol (QU) and an alkaline Cambisol (YU). Soils were obtained from an outdoor soil column experiment with straw-BC application rates of 0 (BC0), 2.25 (BC2.25) and 11.3 (BC11.3) Mgha-1 per cropping season for eight consecutive wheat/millet seasons. Quantitative polymerase chain reaction (qPCR) and 454 high-throughput pyrosequencing techniques were performed to quantify and sequence amoA gene copies and composition of AOA and AOB. Results showed that QU had lower PNR and a higher ratio of amoA gene copies of AOA to AOB than YU, PNR of QU with BC application was significantly associated with the amoA gene of AOB. Similar to previous short-term findings, BC application enhanced QU soil nitrification, which may be explained by the significant increase in AOB abundance and a shift in AOB community structure from Nitrosospira cluster 2 toward cluster 3, along with the disappearance of some obligate acidophile AOA groups, leading to the appearance of ammonia-oxidizers from neutral-alkaline soils in BC-amended acid soils. Canonical correspondence analysis (CCA) showed that soil pH was the most important factor driving shifts in ammonia-oxidizers composition. Although BC application did not have significant effects on PNR in YU, BC11.3 decreased AOA and AOB gene copies and influenced the relative abundance of community structure. Our findings represent the first investigation of long-term BC effects on AOA and AOB communities in agricultural soils using 454 high-throughput pyrosequencing, showing that BC application can alter soil characteristics and influence ammonia oxidizer community composition, abundance, especially in acid soils.

Potential Nitrogen Losses in Relation to Spatially Distinct Soil Management History and Biochar Addition.

Soil functional zone management strategies including ridge tillage have been shown to increase in-row (IR) compared with between-row (BR) soil nitrogen (N) availability due to the development of contrasting soil properties and carbon (C) and N cycling processes. However, potential N losses may also increase for IR positions, representing an environmental tradeoff. Although biochar has putative contributions to tightening N cycles, its effect within spatially distinct management zones has not been quantified. This study evaluated the potential for biochar to reduce soil nitrous oxide (NO) emissions and leachate nitrate (NO) concentrations as influenced by changes in soil properties after 5 yr of ridge tillage in a silty clay loam soil. Soil columns for IR and BR positions were treated with and without biochar at 10 Mg ha in a 100-d laboratory incubation ( = 4 replications). Results indicate that biochar increased average daily NO emissions in IR by 30% but decreased them by 39% in BR. In both positions, biochar decreased NO concentrations in soil solution and leachate by 11 to 20% and 10 to 15%, respectively. Our findings suggest that long-term changes in soil C and N cycling processes may increase the potential for NO emissions in IR compared with BR positions. Although most biochar studies have not accounted for within-field soil variability, our results indicate that biochar may differentially affect soil NO emissions depending on spatially distinct soil management history. Findings from this laboratory study stress the need for field research evaluating spatial differences in biochar NO mitigation potential.

Tile Drainage Nitrate Losses and Corn Yield Response to Fall and Spring Nitrogen Management.

Nitrogen (N) management strategies that maintain high crop productivity with reduced water quality impacts are needed for tile-drained landscapes of the US Midwest. The objectives of this study were to determine the effect of N application rate, timing, and fall nitrapyrin addition on tile drainage nitrate losses, corn ( L.) yield, N recovery efficiency, and postharvest soil nitrate content over 3 yr in a corn-soybean [ (L.) Merr.] rotation. In addition to an unfertilized control, the following eight N treatments were applied as anhydrous ammonia in a replicated, field-scale experiment with both corn and soybean phases present each year in Illinois: fall and spring applications of 78, 156, and 234 kg N ha, fall application of 156 kg N ha + nitrapyrin, and sidedress (V5-V6) application of 156 kg N ha. Across the 3-yr study period, increases in flow-weighted NO concentrations were found with increasing N rate for fall and spring N applications, whereas N load results were variable. At the same N rate, spring vs. fall N applications reduced flow-weighted NO concentrations only in the corn-soybean-corn rotation. Fall nitrapyrin and sidedress N treatments did not decrease flo8w-weighted NO concentrations in either rotation compared with fall and spring N applications, respectively, or increase corn yield, crop N uptake, or N recovery efficiency in any year. This study indicates that compared with fall N application, spring and sidedress N applications (for corn-soybean-corn) and sidedress N applications (for soybean-corn-soybean) reduced 3-yr mean flow-weighted NO concentrations while maintaining yields.

A Vision for Incorporating Environmental Effects into Nitrogen Management Decision Support Tools for U.S. Maize Production.

Meeting crop nitrogen (N) demand while minimizing N losses to the environment has proven difficult despite significant field research and modeling efforts. To improve N management, several real-time N management tools have been developed with a primary focus on enhancing crop production. However, no coordinated effort exists to simultaneously address sustainability concerns related to N losses at field- and regional-scales. In this perspective, we highlight the opportunity for incorporating environmental effects into N management decision support tools for United States maize production systems by integrating publicly available crop models with grower-entered management information and gridded soil and climate data in a geospatial framework specifically designed to quantify environmental and crop production tradeoffs. To facilitate advances in this area, we assess the capability of existing crop models to provide in-season N recommendations while estimating N leaching and nitrous oxide emissions, discuss several considerations for initial framework development, and highlight important challenges related to improving the accuracy of crop model predictions. Such a framework would benefit the development of regional sustainable intensification strategies by enabling the identification of N loss hotspots which could be used to implement spatially explicit mitigation efforts in relation to current environmental quality goals and real-time weather conditions. Nevertheless, we argue that this long-term vision can only be realized by leveraging a variety of existing research efforts to overcome challenges related to improving model structure, accessing field data to enhance model performance, and addressing the numerous social difficulties in delivery and adoption of such tool by stakeholders.

Methane and Nitrous Oxide Emissions from Flooded Rice Systems following the End-of-Season Drain.

Large CH and NO fluxes can occur from flooded rice ( L.) systems following end-of-season drainage, which contribute significantly to the total growing-season greenhouse gas (GHG) emissions. Field and laboratory studies were conducted to determine under what soil water conditions these emissions occur. In three field studies, GHG fluxes and dissolved CH in the soil pore water were measured before and after drainage. Across all fields, approximately 10% of the total seasonal CH emissions and 27% of the total seasonal NO emissions occurred following the final drain, confirming the importance of quantifying postdrainage CH and NO emissions. Preplant fertilizer N had no effect on CH emissions or dissolved CH; however, increased postdrainage NO fluxes were observed at higher N rates. To determine when postdrainage sampling needs to take place, our laboratory incubation study measured CH and NO fluxes from intact soil cores from these fields as the soil dried. Across fields, maximum CH emissions occurred at approximately 88% water-filled pore space (WFPS), but emissions were observed between 47 and 156% WFPS. In contrast, maximum NO emissions occurred between 45 and 71% WFPS and were observed between 16 and 109% WFPS. For all fields, gas samplings between 76 and 100% WFPS for CH emissions and between 43 and 78% WFPS for NO emissions was necessary to capture 95% of these postdrainage emissions. We recommend that frequent gas sampling following drainage be included in the GHG protocol of total GHG emissions.

Productivity limits and potentials of the principles of conservation agriculture.

One of the primary challenges of our time is to feed a growing and more demanding world population with reduced external inputs and minimal environmental impacts, all under more variable and extreme climate conditions in the future. Conservation agriculture represents a set of three crop management principles that has received strong international support to help address this challenge, with recent conservation agriculture efforts focusing on smallholder farming systems in sub-Saharan Africa and South Asia. However, conservation agriculture is highly debated, with respect to both its effects on crop yields and its applicability in different farming contexts. Here we conduct a global meta-analysis using 5,463 paired yield observations from 610 studies to compare no-till, the original and central concept of conservation agriculture, with conventional tillage practices across 48 crops and 63 countries. Overall, our results show that no-till reduces yields, yet this response is variable and under certain conditions no-till can produce equivalent or greater yields than conventional tillage. Importantly, when no-till is combined with the other two conservation agriculture principles of residue retention and crop rotation, its negative impacts are minimized. Moreover, no-till in combination with the other two principles significantly increases rainfed crop productivity in dry climates, suggesting that it may become an important climate-change adaptation strategy for ever-drier regions of the world. However, any expansion of conservation agriculture should be done with caution in these areas, as implementation of the other two principles is often challenging in resource-poor and vulnerable smallholder farming systems, thereby increasing the likelihood of yield losses rather than gains. Although farming systems are multifunctional, and environmental and socio-economic factors need to be considered, our analysis indicates that the potential contribution of no-till to the sustainable intensification of agriculture is more limited than often assumed.

Optimizing rice yields while minimizing yield-scaled global warming potential.

To meet growing global food demand with limited land and reduced environmental impact, agricultural greenhouse gas (GHG) emissions are increasingly evaluated with respect to crop productivity, i.e., on a yield-scaled as opposed to area basis. Here, we compiled available field data on CH4 and N2 O emissions from rice production systems to test the hypothesis that in response to fertilizer nitrogen (N) addition, yield-scaled global warming potential (GWP) will be minimized at N rates that maximize yields. Within each study, yield N surplus was calculated to estimate deficit or excess N application rates with respect to the optimal N rate (defined as the N rate at which maximum yield was achieved). Relationships between yield N surplus and GHG emissions were assessed using linear and nonlinear mixed-effects models. Results indicate that yields increased in response to increasing N surplus when moving from deficit to optimal N rates. At N rates contributing to a yield N surplus, N2 O and yield-scaled N2 O emissions increased exponentially. In contrast, CH4 emissions were not impacted by N inputs. Accordingly, yield-scaled CH4 emissions decreased with N addition. Overall, yield-scaled GWP was minimized at optimal N rates, decreasing by 21% compared to treatments without N addition. These results are unique compared to aerobic cropping systems in which N2 O emissions are the primary contributor to GWP, meaning yield-scaled GWP may not necessarily decrease for aerobic crops when yields are optimized by N fertilizer addition. Balancing gains in agricultural productivity with climate change concerns, this work supports the concept that high rice yields can be achieved with minimal yield-scaled GWP through optimal N application rates. Moreover, additional improvements in N use efficiency may further reduce yield-scaled GWP, thereby strengthening the economic and environmental sustainability of rice systems.

Optimal fertilizer nitrogen rates and yield-scaled global warming potential in drill seeded rice.

Drill seeded rice ( L.) is the dominant rice cultivation practice in the United States. Although drill seeded systems can lead to significant CH and NO emissions due to anaerobic and aerobic soil conditions, the relationship between high-yielding management practices, particularly fertilizer N management, and total global warming potential (GWP) remains unclear. We conducted three field experiments in California and Arkansas to test the hypothesis that by optimizing grain yield through N management, the lowest yield-scaled global warming potential (GWP = GWP Mg grain) is achieved. Each growing season, urea was applied at rates ranging from 0 to 224 kg N ha before the permanent flood. Emissions of CH and NO were measured daily to weekly during growing seasons and fallow periods. Annual CH emissions ranged from 9.3 to 193 kg CH-C ha yr across sites, and annual NO emissions averaged 1.3 kg NO-N ha yr. Relative to NO emissions, CH dominated growing season (82%) and annual (68%) GWP. The impacts of fertilizer N rates on GHG fluxes were confined to the growing season, with increasing N rate having little effect on CH emissions but contributing to greater NO emissions during nonflooded periods. The fallow period contributed between 7 and 39% of annual GWP across sites years. This finding illustrates the need to include fallow period measurements in annual emissions estimates. Growing season GWP ranged from 130 to 686 kg CO eq Mg season across sites and years. Fertilizer N rate had no significant effect on GWP; therefore, achieving the highest productivity is not at the cost of higher GWP.