Global change caused by increasing greenhouse gas (GHG) emission has become a common concern of the international community. As the largest emitter of GHGs and the second largest irrigator in the world, a clear understanding of how much GHG is emitted from irrigation in China is of great importance. But no previous studies address this question. So based on Chinese official statistical data, this study estimates GHG emissions from agricultural irrigation in order to inform strategies for reasonable use of water resources and emission reduction. The study finds that in 2010 the total carbon dioxide (CO₂) equivalent (CO₂-e) emission from agricultural irrigation is 36.72~54.16 Mt. Emissions from energy activities in irrigation (including water pumping and conveyance) account for 50 % ~ 70 % of total emissions from energy activities in the agriculture sector. Ground water pumping is the biggest emission source, accounting for 60.97 % of total irrigation emissions. Given the extent of global ground water over exploitation, balancing conservation and exploitation of ground water resources is very important to both emission reduction and sustainable development. The GHG emission intensity of irrigation depends largely on water use efficiency, so improvement of water use efficiency (both technical and managerial) can be an effective way to reduce emissions. Enhanced overall management of water utilization, balanced exploitation of water resources to avoid excessive ground water consumption, and active promotion of water use efficiency can contribute to reducing GHG emissions and pressure on water resources and advance sustainable agricultural production.

Background: Intercropping (IC) has been widely adopted by farmers for enhancing crop productivity and economic returns; however, the underpinning mechanisms from the perspective of below-ground interspecific interactions are only partly understood especially when intercropping practices under saline soil conditions. By using permeable (100 μm) and impermeable (solid) root barriers in a multi-site field experiment, we aimed to study the impact of root-root interactions on nutrient accumulation, soil microbial communities, crop yield, and economic returns in a peanut/cotton IC system under non-saline, secondary-saline, and coastal saline soil conditions of China. Results: The results indicate that IC decreased the peanut pods yield by 14.00, 10.01, and 16.52% while increased the seed cotton yield by 61.99, 66.00, and 58.51%, respectively in three experimental positions, and consequently enhanced the economic returns by compared with monoculture of peanut (MP) and cotton (MC). The higher accumulations of nutrients such as nitrogen (N), phosphorus (P), and potassium (K) were also observed in IC not only in the soil but also in vegetative tissues and reproductive organs of peanut. Bacterial community structure analysis under normal growth conditions reveals that IC dramatically altered the soil bacterial abundance composition in both peanut and cotton strips of the top soil whereas the bacterial diversity was barely affected compared with MP and MC. At blossom-needling stage, the metabolic functional features of the bacterial communities such as fatty acid biosynthesis, lipoic acid metabolism, peptidoglycan biosynthesis, and biosynthesis of ansamycins were significantly enriched in MP compared with other treatments. Conversely, these metabolic functional features were dramatically depleted in MP while significantly enriched in IC at podding stage. Permeable root barrier treatments (NC-P and NC-C) counteracted the benefits of IC and the side effects were more pronounced in impermeable treatments (SC-P and SC-C). Conclusion: Peanut/cotton intercropping increases crop yield as well as economic returns under non-saline, secondary-saline, and coastal saline soil conditions probably by modulating the soil bacterial abundance composition and accelerating plant nutrients accumulation.

Maize-peanut intercropping (M//P) is widely used in the North China Plain, but little is known about the changes in soil microorganism and its interactions with soil nutrient availability. Based on three years of field experiment, this study systematically investigated the soil microorganisms and soil properties in the 0-10 cm, 10-20 cm and 20-40 cm soil layers. This study found that M//P changed the soil microbial community composition compared to monoculture modes in all the studied soil layers but had the most effects in the 10-20 cm soil layer. Soil organic carbon, available phosphorus and available potassium explained the most variation in the microbial composition, while Proteobacteria, Acidobacteria and Planctomycetes were the phyla that were most affected by environmental factors. Based on the correlations and microbial function analysis, we speculated that: 1) M//P increased the relative abundance of Chlorobi, Parcubacteria and Chlamydiae, which promoted carbon and nitrogen metabolism; 2) M//P increased the relative abundance of Chlamydiae, Saccharibacteria, and Parcubacteria and then enhanced phosphonate and phosphinate metabolism, providing more available phosphorus for crops; 3) some KEGG pathways related to immunity and disease were increased in M//P, which may be related to disease control.

Legume-based intercropping systems function in boosting crop productivity. However, the precise physiological mechanisms by which root exudates derived from these systems on crop growth have not been characterized. Here, the rhizosphere soil from a peanut/maize intercropping system was analyzed for metabolome profiles. Sucrose (SUC) and myo-inositol (MI) were significantly declined while oxalic acid (OA) was dramatically enriched compared with peanut monoculture. After concentration screening, the optimal concentrations of OA, SUC, and MI have been determined as 1.0 g/pot, 0.1 g/pot, and 0.1 g/pot, respectively. Armed with the optimal concentrations, OA, SUC, MI, and their combinations were applied to peanut soil, respectively. Agronomical and physiological assesses indicated that single application of SUC and the combination application of “OA + SUC” showed better performance on peanut growth, pod yield, and soil nitrogen (N) turnover processes including total N and NO3−-N contents as well as activities of N turnover enzymes. Consequently, the transcriptome and metabolome profiles of SUC were further determined. A total of 1036/24 up-regulated and 797/35 down-regulated differential expressed genes (DEGs)/differential accumulated metabolites (DAMs) were detected in SUC-treated peanut roots, respectively. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis advocated that DEGs were mostly enriched in crucial pathways like Glycolysis/Gluconeogenesis-tricarboxylic acid cycle (TCA cycle) and N uptake and assimilation. Moreover, DAMs like D-Aspartic acid, L-Glutamic acid, and L-Threonine were identified in “Sucrose vs. Control”. Application of root exudates like sucrose and oxalic acid derived from root exudates of peanut/maize intercropping system fulfil pivotal roles in enhancing peanut growth and productivity via modulating N turnover processes.