Carbon footprint of synthetic nitrogen under staple crops: A first cradle-to-grave analysis.

More than half of the world's population is nourished by crops fertilized with synthetic nitrogen (N) fertilizers. However, N fertilization is a major source of anthropogenic emissions, augmenting the carbon footprint (CF). To date, no global quantification of the CF induced by N fertilization of the main grain crops has been performed, and quantifications at the national scale have neglected the CO2 assimilated by plants. A first cradle-to-grave life cycle assessment was performed to quantify the CF of the N fertilizers' production, transportation, and application to the field and the uses of the produced biomass in livestock feed and human food, as well as biofuel production. We quantified the direct and indirect inventories emitted or sequestered by N fertilization of main grain crops: wheat, maize, and rice. Grain food produced with N fertilization had a net CF of 7.4 Gt CO2eq. in 2019 after excluding the assimilated C in plant biomass, which accounted for a quarter of the total CF. The cradle (fertilizer production and transportation), gate (fertilizer application, and soil and plant systems), and grave (feed, food, biofuel, and losses) stages contributed to the CF by 2%, 11%, and 87%, respectively. Although Asia was the top grain producer, North America contributed 38% of the CF due to the greatest CF of the grave stage (2.5 Gt CO2eq.). The CF of grain crops will increase to 21.2 Gt CO2eq. in 2100, driven by the rise in N fertilization to meet the growing food demand without actions to stop the decline in N use efficiency. To meet the targets of climate change, we introduced an ambitious mitigation strategy, including the improvement of N agronomic efficiency (6% average target for the three crops) and manufacturing technology, reducing food losses, and global conversion to healthy diets, whereby the CF can be reduced to 5.6 Gt CO2eq. in 2100.

Acid rain reduces plant-photosynthesized carbon sequestration and soil microbial network complexity.

Acid rain threatens the structure and function of terrestrial ecosystems; however, the mechanisms by which acid rain affects the photosynthesized carbon (C) fluxes and soil microbial communities are far less understood, thus impeding accurate projections of regional C flux in the plant-soil-atmosphere system. In this study, we performed an isotopic 13C labeling experiment to trace C footprints in a maize-soil system under acid rain pollution (pH 4.5 and 3.0; SO42-/NO3-= 2:1). Our results showed that acid rain exerted a negligible effect on total plant biomass as well as shoot biomass. Acid rain of pH 3.0 inhibited plant 13C assimilation and the flow of fixed 13C to the soil. Acid rain decreased soil total C and organic nitrogen (N) but increased inorganic N (i.e., nitrate-N) content. The acid rain of pH 3.0 enhanced soil bulk density, led to soil acidification, and promoted soil microbial diversity. However, acid rain reduced the connectivity and complexity of soil microbial network. Soil 13C content was mainly regulated by soil pH, ammonium-N, and root biomass. Our findings demonstrated that acid rain reduces photosynthesized C sequestration of maize-soil system and soil microbial taxa interactions.

Coupling the environmental impacts of reactive nitrogen losses and yield responses of staple crops in China.

Cropland reactive nitrogen losses (Nr) are of the greatest challenges facing sustainable agricultural intensification to meet the increases in food demand. The environmental impacts of Nr losses and their yield responses to the mitigation strategies were not completely evaluated. We assessed the environmental impacts of Nr losses in China and coupled the efficiency of mitigation actions with yield responses. Datasets about Nr losses in China were collected, converted into potentials of acidification (AP), global warming (GWP), and aquatic eutrophication (AEP), and analyzed by a meta-analysis program. Results showed that producing 1 Mg of rice grains had the highest AP (153 kg acid equiv.), while wheat had the highest GWP and AEP (74 kg CO2 equiv. and 0.37 kg PO4 equiv., respectively). Using the conventional rates (averagely, 200, 230, and 215 kg N ha-1) of urea as a surface application to produce 131.4, 257.2, and 212.1 Tg of wheat, maize, and rice resulted in 17-33 Tg, 7-10 Tg, and 6-87 Gg of AP, GWP, and AEP, respectively. For their balanced effect on reducing AP, GWP, and AEP while maximizing yields, inhibitors, and subsurface application could be set as the best mitigation strategies in wheat production. Inhibitors usage and biochar are strongly recommended strategies for sustainable production of maize. None of the investigated strategies had a balanced effect on rice yield and the environment, thus new mitigation technologies should be developed.

Unraveling consequences of the co-exposure of polyethylene microplastics and acid rain on plant-microbe-soil system.

Emerging microplastics (MPs) pollution and continuing acid rain (AR) co-exist in terrestrial ecosystems, and are considered as threats to ecosystems health. However, few data are available on MPs-AR interactions in plant-microbe-soil systems. Here, a microcosm experiment was manipulated to elucidate the co-exposure of polyethylene MPs (PE MPs; 1%, 5% and 10%) and AR (pH 4.0) on soil-lettuce system, in which the properties of soil and lettuce, and their links were explored. We found that 10% PE MPs increased soil CO2 emission and its temperature sensitivity (Q10) in combination with AR, while 1% PE MPs reduced soil CO2 emission irrespective of AR. PE MPs addition did not influence lettuce production (total biomass) though its photosynthesis was affected. PE MPs exerted negative impact on soil water availability. PE MPs treatments increased NH4+-N content of soil without AR, and dissolved organic carbon content of soil sprayed with AR. 10% PE MPs combined with AR reduced soil microbial biomass, while soil microbial community diversity was not affected by PE MPs or AR. Interestingly, 10% PE MPs addition altered soil microbial community structure, and promoted the complexity and connectivity of soil microbial networks. 5% and 10% PE MPs addition decreased soil urease activity under AR, but this was not the case without AR. These findings highlight the critical role of AR in regulating PE MPs impacts on plant-microbe-soil ecosystems, and the necessity to incorporate other environmental factors when evaluating the actual impacts or risks of MPs pollution in terrestrial ecosystems.

Effect of polyethylene microplastics and acid rain on the agricultural soil ecosystem in Southern China.

The increasing microplastics (MPs) pollution and continuous acid rain coincide in many areas of the world. However, how MPs interact with acid rain is still unclear. Herein, we conducted a microcosm experiment to decipher the combined effect of polyethylene (PE) MPs (1%, 5%, and 10%) and acid rain (pH 4.0) on the agricultural soil ecosystem of Southern China, in which edaphic property, microbial community, enzymatic activity and CO2 emission were investigated. The results showed that PE MPs significantly decreased soil water retention and nitrate nitrogen content regardless of acid rain. Soil total nitrogen significantly decreased under the co-exposure of 10% PE MPs and acid rain. However, PE MPs did not alter soil microbial biomass, i.e., the content of microbial biomass carbon, total phospholipid fatty acids, with or without acid rain. 10% PE MPs and acid rain treatment significantly increased the activity of catalase and soil CO2 emission. PE MPs addition did not affect the temperature sensitivity (Q10) of soil CO2 emission regardless of acid rain. These findings suggest that MPs may interact with acid rain to affect soil ecosystems, thus underscoring the necessity to consider the interaction between MPs and ambient environmental factors when exploring the impact of MPs on the soil biodiversity and function.