Achieving High Crop Yields with Low Nitrogen Emissions in Global Agricultural Input Intensification

Influence mechanisms underlying the degradation of petroleum hydrocarbons in response to various nitrogen dosages supplementation through metatranscriptomics analysis

Exogenous nitrogen supplementation for the bioremediation of petroleum-contaminated soils is a widely adopted and effective environmentally friendly strategy. However, the mechanism by which varying nitrogen dosages affect hydrocarbon degradation pathways remains unclear. This study conducted bioremediation on soil with a total petroleum hydrocarbon (TPH) content of 17,090 mg/kg over 210 days. The results indicated that low-dosage nitrogen supplementation (136 mg N/kg, LN) achieved a 24.8 % TPH removal, significantly outperforming high-dosage nitrogen treatment (1360 mg N/kg, HN), which achieved only 12.8 % TPH removal. The LN treatment demonstrated nitrogen availability efficiency (NAVE) and nitrogen partial factor availability (NPFA) values of 6.03 mg/mg and 31.11 mg/mg, respectively, compared to -0.90 mg/mg and 1.60 mg/mg for the HN treatment. The metatranscriptomic data were employed to investigate differential gene expression across individual samples and for GO and KEGG functional annotation. The annotation results revealed a significant increase in the number of differentially expressed genes (DEGs) associated with each functional category as the nitrogen dose increased. Notably, the LN treatment upregulated genes such as bbsE, bbsF, golA, and badH, which are crucial for encoding aromatic hydrocarbon degradation. Additionally, pyruvate metabolism genes including aceE, acdA, and atoB were enriched due to the LN treatment. In contrast, the HN treatment promoted soil nitrogen metabolism through enhanced expression of nitrogen cycling-related genes such as narK, narG, narH, nirA, and nirK, contributing to competitive interactions with carbon metabolism and impeding hydrocarbon degradation by soil microorganisms. These results suggest that the regulation of nitrogen application is crucial for enhancing hydrocarbon biodegradation efficiency in petroleum-contaminated soils.

Mitigating nitrogen losses with almost no crop yield penalty during extremely wet years

Climate change-induced precipitation anomalies during extremely wet years (EWYs) result in substantial nitrogen losses to aquatic ecosystems (Nw). Still, the extent and drivers of these losses, and effective mitigation strategies have remained unclear. By integrating global datasets with well-established crop modeling and machine learning techniques, we reveal notable increases in Nw, ranging from 22 to 56%, during historical EWYs. These pulses are projected to amplify under the SSP126 (SSP370) scenario to 29 to 80% (61 to 120%) due to the projected increases in EWYs and higher nitrogen input. We identify the relative precipitation difference between two consecutive years (diffPr) as the primary driver of extreme Nw. This finding forms the basis of the CLimate Extreme Adaptive Nitrogen Strategy (CLEANS), which scales down nitrogen input adaptively to diffPr, leading to a substantial reduction in extreme Nw with nearly zero yield penalty. Our results have important implications for global environmental sustainability and while safeguarding food security.

Evaluation of Nitrogen Fertilizer Fates and Related Environmental Risks for Main Cereals in China's Croplands from 2004 to 2018

Assessment of the nitrogen (N) inputs and outputs in croplands would help effectively manage the distribution of N to improve crop growth and environmental sustainability. To better understand the N flow of the main cereal systems in China, soil N balance, N use efficiency (NUE), N losses and the potential environmental impacts of maize, wheat and rice cropping systems were estimated at the regional and national scales from 2004 to 2018. Nationally, the soil N balance (N inputs-N outputs) of maize, wheat, single rice and double rice decreased by 28.8%,13.3%, 30.8% and 34.1% from 2004-2008 to 2014-2018, equivalent to an average of 33.3 to 23.7 kg N ha-1, 82.4 to 71.4 kg N ha-1, 93.6 to 64.8 kg N ha-1 and 51.8 to 34.1 kg N ha-1, respectively. The highest soil N balance were observed in Southeast (SE) region for maize and double rice, North central (NC) region for wheat single rice and Northwest region for wheat, whereas Northeast (NE) region had the lowest N balance for all crops. The NUE increased from 49.8%, 41.2%, 49.7% and 53.7% in 2004-2008 to 54.8%, 45.9%, 55.5% and 56.5% in 2014-2018 for maize, wheat, single rice and double rice, respectively. The fertilizer N losses (i.e., N2O emission, NO emission, N2 emission, NH3 volatilization, N leaching and N runoff) were estimated as 43.7%, 38.3%, 40.2% and 36.6% of the total N inputs for maize, wheat, single rice and double rice, respectively in 2014-2018. Additionally, the highest global warming potential and acidification effects were found in NE and NC regions for maize, NC region for wheat, the middle and lower reaches of Yangtze River for single rice and SE region for double rice, respectively. The highest risk of water contamination by N leaching and surface runoff was observed in NC region for all crops mainly due to high N fertilizer input. Furthermore, the dynamics of N balance for all crops were closely tied with grain yields, except for single rice, the N balance of which was mainly correlated with N fertilizer input. Our results could help researchers and policy makers effectively establish optimized fertilization strategies and adjust the regional allocation of grain cropping areas in response to environmental risks and climate change caused by food crop cultivation in China.

Impact of a transformation from flood to drip irrigation on groundwater recharge and nitrogen leaching under variable climatic conditions

The sustainability of agriculture in the Mediterranean climate is challenged by high irrigation water demands and nitrogen fertilizer losses to the environment, causing significant pressure on groundwater resources and groundwater-dependent ecosystems. Advanced irrigation technologies and improved fertilizer management have been promoted as key solutions to reduce the agricultural impact on aquatic systems. However, it remains unclear how different irrigation-fertilizer practices perform on the long-term under a highly variable climate, such as the Mediterranean one. Here, we conduct hydrological simulations over a fifty-year period to quantify the magnitude and dynamics of groundwater recharge and nitrogen leaching under five real-case irrigation-fertilizer practices observed in Valencia (eastern Spain). The Valencian Region is the largest citrus-producing region of Europe and current irrigation-fertilizer practices reflect the ongoing transformation of irrigation systems from flood to drip irrigation. Our simulations highlight three major implications of the irrigation transformation for groundwater resources. First, the transformation from flood to drip irrigation reduces the recharge fraction (19% vs. 16%) and especially the nitrogen leaching fraction (33% vs. 18%) on the long term. Second, the long-term performance of the two irrigation practices is subject to substantial inter-annual differences controlled by precipitation variability. The sensitivity of recharge and nitrogen leaching to annual meteorological conditions is stronger in drip irrigation, which eventually leads to a similar performance of flood and drip irrigation in wet years if fertilizer inputs are similar. Third, we identify a pronounced year-to-year nitrogen memory in the soil, whereby an enhanced (decreased) nitrogen leaching is observed after anomalously dry (wet) years, affecting the performance of irrigation-fertilizer practices. Overall, the study demonstrates the highly variable nature of the performance of irrigation-fertilizer practices, and the major findings can guide future efforts in designing sustainable water management strategies for agricultural areas with a Mediterranean climate.

Significant Global Yield-Gap Closing Is Possible Without Increasing the Intensity of Environmentally Harmful Nitrogen Losses

Substantial increases in cereal yields are necessary if a growing global demand for food is to be met without further conversion of natural to agricultural land. However, since in many regions yields are limited by soil nutrient availability, this will increase the requirement for fertilizer inputs, specifically of nitrogen (N). Here we focus on maize cultivation, and investigate the trade-off between yield increases and environmentally harmful N-losses in the form of nitrous oxide (N2O) emissions and nitrate (NO3-) leaching. We model the evolution of N-losses as yield gaps—the difference between actual and potential yields—are closed. To do this we use the process-based, biogeochemical model LandscapeDNDC to perform global simulations on a 0.5° grid, and evaluate the response of yields and environmental N-losses to changes in N-inputs. Our simulations find current production (circa 2015) of 954 Tg (5.1 Mg/ha), direct and indirect N2O emissions of 416 Gg-N (2.2 kg-N/ha or 0.44 kg-N/Mg) and NO3- leaching of 5.9 Tg-N (31.5 kg-N/ha or 6.2 kg-N/Mg). We demonstrate that, under an “optimal” strategy for closing yield gaps, maize yields could be increased by 20–25% with concomitant stable or even slightly decreased yield-scaled N-losses. However, further yield increases would come at an ever accelerating cost in environmentally harmful N losses. This acceleration occurs when yields exceed ~70–80% of their potential.

Model comparison and quantification of nitrous oxide emission and mitigation potential from maize and wheat fields at a global scale

Maize and wheat are major cereals that contribute two-thirds of the food energy intake globally. The two crops consume about 35% of the nitrogen (N) fertilizer used in agriculture and thereby contribute to fertilizer-induced nitrous oxide (N₂O) emissions. Thus, estimation of spatially disaggregated N₂O emissions from maize and wheat fields on a global scale could be useful for identifying emission and mitigation hotspots. It could also be needed for prioritizing mitigation options consistent with location-specific production and environmental goals. N₂O emission from four models (CCAFS-MOT, IPCC Tier-I, IPCC Tier-II and Tropical N₂O) using a standard gridded dataset from global maize and wheat fields were compared and their performance evaluated using measured N₂O emission data points (777 globally distributed datapoints). The models were used to quantify spatially disaggregated N₂O emission and mitigation potential from maize and wheat fields globally and the values were compared. Although the models differed in their performance of capturing the level of measured N₂O emissions, they produced similar spatial patterns of annual N₂O emissions from maize and wheat fields. Irrespective of the models, predicted N₂O emissions per hectare were higher in some countries in East and South Asia, North America, and Western Europe, driven mainly by higher N application rates. The study indicated a substantial N₂O abatement potential if application of excess N in the maize and wheat systems is reduced without compromising the yield of the crops through technological and crop management innovations. N₂O mitigation potential is higher in those countries and regions where N application rates and current N₂O emissions are already high. The estimated mitigation potentials are useful for hotspot countries to target fertilizer and crop management as one of the mitigation options in their Nationally Determined Contributions (NDCs) to the United Nations Framework Convention on Climate Change (UNFCCC).

Global Phosphorus Losses from Croplands under Future Precipitation Scenarios

Phosphorus (P) losses from fertilized croplands to inland water bodies cause serious environmental problems. During wet years, high precipitation disproportionately contributes to P losses. We combine simulations of a gridded crop model and outputs from a number of hydrological and climate models to assess global impacts of changes in precipitation regimes on P losses during the 21st century. Under the baseline climate during 1991-2010, median P losses are 2.7 ± 0.5 kg P ha^-1 year^-1 over global croplands of four major crops, while during wet years, P losses are 3.6 ± 0.7 kg P ha^-1 year^-1. By the end of this century, P losses in wet years would reach 4.2 ± 1.0 (RCP2.6) and 4.7 ± 1.3 (RCP8.5) kg P ha^-1 year^-1 due to increases in high annual precipitation alone. The increases in P losses are the highest (up to 200%) in the arid regions of Middle East, Central Asia, and northern Africa. Consequently, in three quarters of the world's river basins, representing about 40% of total global runoff and home up to 7 billion people, P dilution capacity of freshwater could be exceeded due to P losses from croplands by the end of this century.

Rational trade-offs between yield increase and fertilizer inputs are essential for sustainable intensification: A case study in wheat-maize cropping systems in China

In order to feed a population of nearly 1.4 billion people with limited arable land resources, China's high crop production has been maintained by an intensive cropping system with excessive inputs of chemical fertilizers, resulting in high environmental costs. This study attempted to explore the reasonable balance between yield increase and nitrogen (N) inputs in the intensive wheat-maize cropping system in the North China Plain, which is one of the most important grain production regions in China. Based on yield simulations with the DSSAT-CERES-Wheat and DSSAT-CERES-Maize models and a household survey of 241 farmers' fields, we conducted a coupled analysis of the regional crop yields, N fertilizer inputs, and farmers' technical conversion efficiency with respect to winter wheat and summer maize production in four representative study areas. We also conducted a quantitative analysis of the equilibrium relationship between fertilizer application rates and expected yields, and the optimum N fertilization amounts for wheat and maize were recommended. The results indicated that farmers' average yields had reached almost 80% of the attainable yields, which meant that there was little room for farmers to increase their yields. However, we found that the yield gaps among the different farmers were still large, and most farmers applied excessive amounts of N while obtaining unsatisfactory yields due to poor fertilizer management techniques. Only 15% of winter wheat and 4% of summer maize on farmers' fields had achieved the synergy of high crop yields and efficient fertilization, and farmers' technical conversion efficiency was still relatively low. Therefore, farmers should be guided to appropriately lower their yield expectations and reduce the overuse of N fertilizer. In the future, if farmers receive necessary education and training and adopt advanced fertilizer management techniques, sustainable intensification of agricultural production with lower environmental costs will be feasible in China.