The frequent occurrence of drought–flood abrupt alternation (DFAA) in Huaibei Plain has shown a great impact on local rice production. Pot experiments were performed in 2016–2018 to investigate the effects of co-occurring drought and flooding stresses on dry weight (DW), grain yield, nitrogen (N), phosphorus (P) and potassium (K) uptake and use efficiencies (NUE, PUE and KUE) in rice. The results showed that DFAA changed the accumulation of biomass and nutrients among different organs in rice. Compared with control, DFAA significantly reduced the grain yield (−29.8%) and root DW (−30.0%), but increased the DW in stem and leaf (10.2% and 9.7%). The root/shoot ratio and morphological size of the root system in DFAA-treated plants was smaller than those of drought alone and flooding alone. Under DFAA stresses, the specific absorption rate of N, P and K increased significantly (47.9%, 31.8% and 32.8%, respectively), while NUE, PUE and KUE decreased significantly (−27.9%, −10.8% and −19.7%, respectively). The decrease of nutrient use efficiencies was mainly due to the redundant growth of branches and leaves, and the key factor limiting grain yield under DFAA conditions was the effective utilization of N. Compared with the earlier drought, the subsequent flooding might have more influence on rice growth, nutrient utilization and yield formation, but the interaction of the two weakened the cumulative effect of drought and flooding. These findings provide a scientific basis for establishing a nutrient and water management system of rice cultivation under transient soil moisture conditions.

Biochar’s benign effects on agricultural production have been demonstrated. Still, no consistent conclusions have been drawn on the impact of biochar-amended paddy fields on carbon sequestration, gas emission reduction, and efficiency enhancement in typical cropping areas in the middle Yangtze River. A field experiment using five dosages of biochar (CK, BC1.5, BC3, BC4.5, and BC6) at 0, 1.5, 3, 4.5, and 6 kg·m⁻² was conducted at the Hubei Irrigation Experiment Center Station, Jingmen City, Hubei Province, China, to investigate the effects of biochar on carbon sequestration, greenhouse gas emissions, and agricultural efficiency in paddy in the middle Yangtze River Region. This study showed that the optimal biochar dosage was 4.5 kg·m⁻² (BC4.5). Biochar significantly improved soil properties, increased rice yield by 26.4–61.4%, and enhanced water use efficiency (WUE) and economic profit (EP) by 32.0–83.7% and −8.0–48.6%, respectively. Biochar increased soil carbon sequestration (SCS) and carbon pool management index (CPMI) by 23.0–198.3% and 22.9–71.5%, respectively. Biochar also reduced greenhouse gas emission intensity (GHGI), global warming potential (GWP), and emissions of CO₂, CH₄, and N₂O. Furthermore, structural equation modeling (SEM) indicated that soil organic carbon (SOC), in addition to the “biochar” influence factor, was a key positive influence factor for SCS, CPMI, and EP. Another major positive factor for GWP was silt, and for WUE it was saturated hydraulic conductivity, while TN and SOC were the major negative variables for GHGI. In summary, biochar demonstrated outstanding carbon sequestration and emission reduction impacts while ensuring crop production growth and efficiency improvement. The results provide a research basis for safeguarding food security and mitigating climate warming in the middle Yangtze River region.

Understanding the law of surface water–groundwater conversion in the face of high-intensity human activities is still a challenge. In this study, we employed statistical and system dynamics methods to investigate the surface water–groundwater conversion law in the Jianghan Plain following the impoundment of the Three Gorges Project (TGP) and the Yangtze-to-Hanjiang Water Transfer Project (YHWTP). The groundwater level’s long data set was used for the first time to study the water level change and water exchange in the research region after the impoundment of the TGP and the delivery of water from the YHWTP. The findings suggest a significant decrease in the interannual trend of the surface water level and groundwater level in the research region. It was observed that a 1m rise in the surface water level can lead to a 0.11–0.38 m rise in the groundwater level. The water level fluctuation coefficients of the surface water level and groundwater level are influenced by the impoundment of the TGP and the water delivery from the YHWTP, causing them to increase and decrease, respectively. In general, the surface water recharges the groundwater in the studied region. The water exchanges between the surface water and groundwater in the Yangtze River’s main stream, the middle region of the Hanjiang Plain, and the lower reaches of the Hanjiang River are, on average, 10⁻² m³/(d·m), 10⁻⁵ m³/(d·m), and 10⁻³ m³/(d·m) orders of magnitude, respectively. The water exchange in the Yangtze River’s main stream was reduced after TGP impoundment, and it was enhanced following YHWTP water delivery.

The development of an accurate root water-uptake model is pivotal for evaluating crop evapotranspiration; understanding the combined effect of drought and waterlogging stresses; and optimizing water use efficiency, namely, crop yield [kg/ha] per unit of ET [mm]. Existing models often lack quantitative approaches to depicting crop root water uptake in scenarios of concurrent drought and waterlogging moisture stresses. Addressing this as our objective; we modified the Feddes root water-uptake model by revising the soil water potential response threshold and by introducing a novel method to calculate root water-uptake rates under simultaneous drought and waterlogging stresses. Then, we incorporated a water stress lag effect coefficient, $\phi \left(W_s\right)$, that investigated the combined effect of historical drought and waterlogging stress events based on the assumption that the normalized influence weight of each past stress event decreases with an increase in the time interval before simulation as an exponential function of the decay rate. Further, we tested the model parameters and validated the results obtained with the modified model using data from three years (2016–2018) of rice (Oryza sativa, L) trails with pots in Bengbu, China. The modified Feddes model significantly improved precision by 9.6% on average when calculating relative transpiration rates, particularly post-stress recovery, and by 5.8% on average when simulating soil moisture fluctuations during drought periods. The root mean square error of relative transpiration was reduced by 60.8%, and soil water was reduced by 55.1%. By accounting for both the accumulated impact of past moisture stress and current moisture conditions in rice fields, the modified model will be useful in quantifying rice transpiration and rice water use efficiency in drought–waterlogging-prone areas in southern China.

《地球科学》(ISSN 1000-2383;CN42-1874/P) (原刊名《地球科学——中国地质大学学报》ISSN 1000-2383;CN42-1233/P)，创刊于1957年，是由教育部主管、中国地质大学主办的综合性地学学术理论期刊，月刊，2015年改名为《地球科学》,她以反映中国地质大学教学与科研最新的、高水平的基础地质、应用地质、资源与环境地质及地学工程技术研究成果为主要任务。

Flooded rice paddies emit large quantities of greenhouse gases, and biochar amendment serves as a promising strategy to conserve water, sequester carbon, mitigate emissions and stabilize crop yields. Nevertheless, the DNDC (DeNitrification-DeComposition) model has no specific biochar module and fails to precisely quantify coupled water–carbon–nitrogen cycles in biochar-amended paddies. Using two-season field observations from the Jianghan Plain, this study constructs the Biochar-DNDC model by introducing biochar pH and substrate adsorption parameters into the original organic fertilizer module and coupling a double-tank exponential decay model. Five biochar application rates (0, 1.5, 3, 4.5, and 6 kg∙m⁻²; CK, BC1.5, BC3, BC4.5, BC6) were set. Combined with the 40-year historical precipitation series of the study area, five hydrological year types were classified to carry out multi-scenario simulations. The results indicated that, compared with DNDC, Biochar-DNDC improved the simulation accuracy by an average of 28.4%. In terms of RRMSE, Biochar-DNDC improved the simulation accuracy of yield, SOC, CH₄, and N₂O by 37.0%, 28.7%, 28.6%, and 42.1%, respectively. Environmental temperature, precipitation, soil bulk density, optimal rice yield, and the proportion of straw returned to the field were key sensitive factors regulating water–carbon–nitrogen fluxes. Biochar reduced irrigation water use by 6.3~28.5% while promoting the accumulation of soil carbon stocks, optimizing nitrogen-use efficiency, and suppressing greenhouse gas emissions. There was a significant interaction between rainfall regime and biochar: DOC and NH₄⁺-N were higher in wet years, while NO₃⁻-N tended to accumulate in arid conditions. Biochar weakened the positive driving effect of precipitation on net primary productivity (NPP). Rice yield, NPP, and net ecosystem exchange (NEE) first increased and then decreased with increasing biochar dosage. Overall, the optimal dosage of biochar for water conservation, carbon sequestration, yield increases, and emission reductions in paddy fields on the Jianghan Plain was 4.5 kg∙m⁻². This study develops a modeling tool for the carbon and nitrogen cycles in biochar-amended paddy fields, which can provide quantitative support for low-carbon water and fertilizer management in the rice-growing regions of the middle Yangtze River Basin.