Long−term excessive nitrogen fertilizer input has resulted in several environmental problems, including an increase in N₂O emissions and the aggravation of nitrate leaching; monitoring nitrogen fertilizer is crucial for maize with high yield. This study aimed to optimize the amount of nitrogen applied to maize by Climate−Smart Agriculture (CSA) so as to continuously improve agricultural productivity and reduce or eliminate N₂O emissions as much as possible. Field experiments with a completely randomized design were conducted to examine the effects of six nitrogen treatments (N application levels of 0, 120, 180, 240, 300, 360 kg·ha⁻¹, respectively) on N₂O emissions, residual concentration of nitrate and ammonium nitrogen, maize yield, and nitrogen utilization efficiency in 2018 and 2019. The results indicated that the residual concentration of nitrate nitrogen (${\mathrm{NO}}_3^{-}$−N) in the two seasons significantly increased; N₂O emissions significantly increased, and the nitrogen fertilizer agronomic efficiency and partial productivity of maize fell dramatically as the nitrogen application rate increased. The maize grain yield rose when the N application amount was raised (N application amount 300 kg·ha⁻¹. An increase in the nitrogen application rate can decrease nitrogen use efficiency, increase soil ${\mathrm{NO}}_3^{-}$−N residual, and N₂O emissions. Reasonable nitrogen application can increase maize yield and reduce N₂O emissions and be conducive to improving nitrogen use efficiency. By considering summer maize yield, nitrogen use efficiency, and farmland ecological environment, 173.94~178.34 kg N kg·ha⁻¹ could be utilized as the nitrogen threshold for summer maize in the North China Plain.

Stable hydrogen and oxygen isotopes provide a powerful technique for quantifying the proportion of root water uptake (RWU) from different potential water sources. Although many models coupled with stable isotopes have been developed to estimate plant water source apportionment, inter-comparisons of different methods are still limited, especially their performance under different soil water content (SWC) conditions. In this study, three Bayesian tracer mixing models, which included MixSIAR, MixSIR and SIAR, were tested to evaluate their performances in determining the RWU of winter wheat under various SWC conditions (normal, dry and wet) in the North China Plain (NCP). The proportions of RWU in different soil layers showed significant differences (p < 0.05) among the three Bayesian models, for example, the proportion of 0–20 cm soil layer calculated by MixSIR, MixSIAR and SIAR was 69.7%, 50.1% and 48.3% for the third sampling under the dry condition (p < 0.05), respectively. Furthermore, the average proportion of the 0–20 cm layer under the dry condition was lower than that under normal and wet conditions, being 45.7%, 58.3% and 59.5%, respectively. No significant difference (p > 0.05) was found in the main RWU depth (i.e., 0–20 cm) among the three models, except for individual sampling periods. The performance of three models in determining plant water source allocation varied with SWC conditions: the performance indicators such as coefficient of determination (R²) and Nash-Sutcliffe efficiency coefficient (NS) in MixSIAR were higher than that in MixSIR and SIAR, showing that MixSIAR performed well under normal and wet conditions. The rank of performance under the dry condition was MixSIR, MixSIAR, and then SIAR. Overall, MixSIAR performed relatively better than other models in predicting RWU under the three different soil moisture conditions.

Low temperature and soil salinization during cotton sowing and seedling adversely affect cotton productivity. Exogenous melatonin (MT) can alleviate the damage caused to plants under non-biological stress; thus, applying MT is a means to improve the growth condition of crops under stress. However, achieving this goal requires a thorough understanding of the physiological regulatory mechanisms of MT on cotton seedlings under low temperature and salinity stress. This study could bring new knowledge on physio-biochemical mechanisms that improve the tolerance of cotton seedlings to combined effects of low temperature and salt stress using an exogenous foliar application of MT. The phytotron experiment comprised two temperature levels of cold stress and control and five MT treatments of 0, 50, 100, 150, and 200 μM and two salinity levels of 0 and 150 mM NaCl. Compared with the control treatments (non-salinity stress under cold stress and control), the combined stress of salt and low temperature reduced cotton seedlings’ biomass and net photosynthetic rate (P_n), aggravated the membrane damage, reduced the potassium (K⁺) content, and increased the sodium (Na⁺) accumulation in the leaves and roots. Under NaCl stress, exogenously sprayed 50–150 μM MT increased the biomass and gas exchange parameters of cotton seedlings under salt and low temperature combined with salt stress, reduced the degree of membrane damage, and regulated the antioxidant enzyme, ion homeostasis, transport, and absorption of cotton seedlings. The pairwise correlation analysis of each parameter using MT shows that the parameters with higher correlation with MT at cold stress are mainly malondialdehyde (MDA), peroxidase (POD), and catalase (CAT). The highest correlation coefficient at 25 °C is observed between the K⁺ and Na⁺ content in cotton seedlings. The conclusion indicates that under salt and low-temperature stress conditions, exogenous application of MT primarily regulates the levels of P_n, superoxide dismutase (SOD), andPOD in cotton seedlings, reduces Na⁺ and MDA content, alleviates damage to cotton seedlings. Moreover, the most significant effect was observed when an exogenous application of 50–150 μM of MT was administered under these conditions. The current study’s findings could serve as a scientific foundation for salinity and low-temperature stress alleviation during the seedling stage of cotton growth.

In the North China Plain, drip irrigation is gradually used in winter wheat production, and the improper management of water and fertilizer aggravates the risk of crop instability and groundwater pollution. A lysimeter experiment with three levels of fertilization timing (T1 = beginning; T2 = middle; and T3 = end of the irrigation cycle) and two irrigation rates (W1 of 30 mm and W2 of 20 mm) was carried out to investigate the effects of irrigation rate and fertilization timing on the soil NO₃⁻-N distribution, crop development, yield, and water–nitrogen usage efficiency of winter wheat. The results indicated that, under the condition of delayed fertilization timing (T2 and T3), the trend of NO₃⁻-N migration to the edge of moist soil became more apparent. The treatments of irrigation rate and fertilization timing significantly affected the plant height, water–nitrogen utilization efficiency, aboveground biomass, grain yield, and leaf area index. The maximum grain yield of 7688.67 kg ha⁻¹ was found at W1T2, which had a nitrogen partial factor productivity (NPFP) of 32.04 kg kg⁻¹. Moreover, W1T2 did not result in a significant reduction in irrigation water use efficiency (IWUE) (4.27 kg m⁻³) in comparison with other treatments (4.00–5.43 kg m⁻³). Based on crop growth, N uptake, yield, IWUE, and NPFP, the irrigation rate of 30 mm combined with fertilization in the middle of the irrigation duration could be considered as suitable irrigation and nitrogen timing for drip-irrigated wheat.