Through a paddy soil column experiment, we comprehensively evaluated the effects of three irrigation practices and three nitrogen (N) fertilizer application strategies on NH3 volatilization, N2O emissions, and rice yields during the rice growing season to identify the optimal irrigation and fertilization combination technique to reduce both NH3 and N2O losses in paddy soil while sustaining rice yield. In addition, we integrated molecular biology techniques (Quantitative PCR) to establish correlations between environmental factors and the abundance of N cycling-related soil microbial functional genes, revealing the intricate interactions between NH3 volatilization and N2O emissions under varied coupling irrigation and fertilization schemes. Our results clearly showed a trade-off relationship between N2O and NH3 emissions under water-saving irrigation practices (controlled irrigation (CI) and intermittent irrigation (II)) coupling with traditional fertilizer urea. Compared with continuous flooding (CF) practice, both CI and II treatments reduced NH3 volatilization by 36.3-73.9%, while increasing N2O emissions by 1483.2-2246.2% during the rice growing season. Notably, the combination application of CRF under CI mode (CI-CRF) significantly reduced NH3 volatilization by 65.0% during the rice growing season, compared to the conventional II-Urea approach. Although the impact on N2O emissions was modest, CI-CRF strategy still achieved a 4.6% reduction in N2O emissions, thus tackling the trade-offs between two important environmentally damaging gases under water-saving irrigation. The suppression of NH3 volatilization was primarily attributed to the CI-CRF strategy lowering NH4+-N concentrations in flooding water, while the reduction in N2O emissions was associated with an increase in soil nirS and nosZ gene abundances. Further estimates indicated that the CI-CRF strategy could potentially reduce NH3 volatilization by 259.2 Gg N yr-1 and N2O emissions by 3.1 Gg N yr-1 in single-crop paddy field in China, compared with traditional II-Urea approach. Therefore, the optimal reduction of gaseous N loss, coupled with yield enhancement, could be achieved through the synergistic strategy of CI-CRF in single-crop rice cultivation ecosystems. Future studies should focus on fieldbased experiments that explore the long-term effects of CI-CRF combinations under varying soil types, climates, and rice cultivation systems.

Soil shrinkage during the drying process (water stress) is one of the main issues in expansive soils of paddy fields. It occurs due to decrease in soil water content, resulting in changes in soil volume and the geometry of pores, leading to the formation of cracks and higher water loss. The aim of this study was to assess the shrinkage characteristic curve and pore size of paddy soils to determine the shrinkage -swelling behavior in Guilan province, Iran. 120 soil samples were collected from the study area. Pore size was determined using soil moisture retention curve (SMRC). It was established by plotting the soil water content (theta) versus the corresponding matric suction (h), and the shrinkage curve by plotting the void ratio (e) against the moisture ratio (upsilon). The suction-pore relationships were also determined. Furthermore, the geometric factors indicating the change in vertical (subsidence) and horizontal (crack) volume of the soils were determined and varied from 1.23 to 2.53, indicating that the vertical change in soil volume is predominant. The zero, residual and proportional shrinkage phases accounted for less than 2 %, 8-38 %, and 61-91 % of the total soil volume change, respectively. The shrinkage capacity of the soils ranged from 0.52 to 1.37. Cation exchange capacity and clay content were identified as the most important factors affecting soil shrinkage properties. In general, the studied paddy soils have great potential for swelling- shrinkage and cracking during the drying process due to the large percentage of expandable clays and the medium to fine pores. The resultant cracks negatively affect crop yield by damaging plant roots and increasing water losses through the soil profiles.

Lead (Pb) contamination in agricultural soils poses a significant threat to both ecosystems and human health. While nano-Fe3O4 exhibits promising potential for Pb remediation, its practical application in the soil is hindered by its' biotoxicity, easy aggregation, and the risk of secondary pollution. Thus, this study presents a novel approach wherein Fe3O4 was incorporated into hydrogel via a one-pot synthetic strategy (Fe3O4@LH). This incorporation enhanced the mechanical properties and environmental stability of the hydrogel composites. Based on the mechanical properties, environmental stability, and single-point adsorption results for Pb, we selected Fe3O4@LH-4 for further research. The removal mechanism and the feasibility of employing Fe3O4@LH-4 for Pb removal from paddy soil were investigated through batch adsorption experiments and soil culture studies. Results showed that the adsorption process was primarily governed by swelling adsorption, electrostatic adsorption, ion exchange, precipitation, nanometer effect, and complexation mechanisms. The application of Fe3O4@LH-4 significantly led to the reduction of 16.7 %-25.4 % in soil Pb content, with removal rates escalating alongside increased dosage and application periods of Fe3O4@LH-4. Fitting results of the prediction model indicated that the Pb content in mildly Pb-contaminated soil (186.55 mg/kg) would decrease to be below the risk control standard for soil contamination of agricultural land in China (140 mg/kg) after 112 days of continuous application. Concurrently, cadmium and arsenic contents in the soil decreased by 5.2 %-10.8 % and 7.1 %-16.7 %, respectively. Moreover, the application of Fe3O4@LH-4 positively influenced soil nutrient levels, with total nitrogen and soil organic matter content significant increments of 13.6 %-41.0 % and 4.6 %-16.1 %, respectively. Furthermore, Fe3O4@LH-4 recovery exceeded 88.3 % after a 90-day application period. These findings underscore the potential of Fe3O4 incorporated hydrogel as a promising agent for the sustained removal of heavy metal Pb from paddy soil.