The co-incorporation of green manure and rice straw is commonly used to increase rice yield and improve soil fertility in paddy fields. However, the effects on nutrient uptake and utilization of rice under the synergistic interaction mechanism in the Taihu Plain of the Yangtze River Delta remain unclear. Based on field experiments, this study investigated the effects of green manure with rice straw return (GMS) under different nitrogen (N) fertilization rates on rice yield, nutrient use efficiency, and soil fertility. The results revealed that green manuring significantly increased rice yield while improving the uptakes and use efficiencies of N, phosphorus (P) and potassium (K). Green manure (GM) with 40% N fertilizer reduction (GM_N60) maintained the grain and straw yields and nutrient uptakes compared to winter fallow with 100% conventional N application (WF_N100). The N recovery efficiency in GM_N60 reached 45.52%, increasing by 41.26% compared to WF_N100. Rice yield and K uptake in the GMS with 40% N fertilizer reduction treatment (GMS_N60) was 10,058 and 15.41 kg/hm², increasing by 14.43% and 9.43% compared to winter fallow with rice straw return and 100% conventional N (WFS_N100). The N, P and K agronomic efficiencies in GMS_N60 increased by 77.04%, 50.22%, and 50.22% compared to WFS_N100, respectively. These findings indicate that rice straw return enhances the fertilizer-saving and yield-increasing effects of GM, promotes rice K uptake and improves P and K use efficiencies. The GM treatment increased the soil organic matter (SOM), total potassium (TK), ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃⁻-N) contents. Among the soil fertility indicators, TK and SOM were the most important factors influencing rice yield and N uptake. In conclusion, GMS can maintain or increase rice yield with 40% N fertilizer reduction, improve nutrient use efficiencies, and increase the reuse of rice straw, thereby supporting green and efficient rice production in the southern Jiangsu paddy area.

Nitrate is a prominent pollutant in water bodies around the world. The isotopes in nitrate provide an effective approach to trace the sources and transformations of nitrate in water bodies. However, determination of isotopic composition by conventional analytical techniques is time-consuming, laborious, and expensive, and alternative methods are urgently needed. In this study, the rapid determination of ¹⁵NO₃⁻ in water bodies using Fourier transform infrared attenuated total reflectance spectroscopy (FTIR-ATR) coupled with a deconvolution algorithm and a partial least squares regression (PLSR) model was explored. The results indicated that the characteristic peaks of ¹⁴NO₃⁻/¹⁵NO₃⁻ mixtures with varied ¹⁴N/¹⁵N ratios were observed, and the proportion of ¹⁵NO₃⁻ was negatively correlated with the wavenumber of absorption peaks. The PLSR models for nitrate prediction of ¹⁴NO₃⁻/¹⁵NO₃⁻ mixtures with different proportions were established based on deconvoluted spectra, which exhibited good performance with the ratio of prediction to deviation (RPD) values of more than 2.0 and the correlation coefficients (R²) of more than 0.84. Overall, the spectra pretreatment by the deconvolution algorithm dramatically improved the prediction models. Therefore, FTIR-ATR combined with deconvolution and PLSR provided a rapid, simple, and affordable method for determination of ¹⁵NO₃⁻ content in water bodies, which would facilitate and enhance the study of nitrate sources and water environment quality management.

Cold atomic absorption spectrophotometry and atomic fluorescence methods were employed to ascertain the total mercury concentration in soil samples. A systematic comparison and analysis were conducted on the operational procedures, precision and detection limits of these methodologies. The standard curve for total mercury determination in soil using the atomic fluorescence method is y = 851.98 x +16.771 ( R ² = 0.9996, 0–2.5 µg l ⁻¹ ), with a detection limit of 0.0006 mg kg ⁻¹ . This method demonstrates superior precision and accuracy not only for samples with high and medium concentrations but also for those with low concentrations. For the cold atomic absorption spectrophotometry method, the standard curve for total mercury determination is y = 0.0547 x + 0.0047 ( R ² = 0.9998, 0–15 µg l ⁻¹ ). For high-concentration samples, the curve is y = 0.0007 x − 0.0003 ( R ² = 0.9998, 0–600 µg l ⁻¹ ). The detection limit for this method is 0.06 μg kg ⁻¹ , indicating better sensitivity and accuracy compared with the atomic fluorescence method. While the precision for low-concentration (0.017 ± 0.003 mg kg ⁻¹ ) samples is moderate, it is higher for medium (0.030 ± 0.003 mg kg ⁻¹ ) and high-concentration (0.590 ± 0.05 mg kg ⁻¹ ) samples. Overall, there is no significant difference between the two detection methods. However, the cold atomic absorption spectrophotometry method has the advantage of not requiring pre-treatment, making it more convenient and environment friendly.

The winter planting of green manure (GM) is widely used in South China to reduce chemical nitrogen (N) fertilizer use, improve soil fertility, and maintain rice yields, but its effect on N runoff loss in paddy fields remains unclear. This study combines multi-site field experiments with a process model (WHCNS-Rice) to assess how GM with reduced N fertilizer impacts N runoff loss and its forms in the Yangtze River’s middle and lower reaches, considering different rainfall years. The network field experiments included four treatments: conventional fertilization (FR), conventional fertilization plus straw return (FRS), GM with a 40% N reduction (MR), and GM-straw combined return with a 40% N reduction (MRS). Monitoring the results showed that compared to the winter fallow treatment, the GM treatments reduced the peak and average total N (TN) concentrations by 11.1–57.9% (average 26.9%) and 17.1–27.3% (average 22.3%), respectively. The TN runoff loss under the GM treatment decreased by 3.50–10.61 kg N ha⁻¹ (22.5–42.1%). GM primarily reduced the runoff loss of dissolved inorganic N (DIN), with reductions at different sites ranging from 0.22 to 9.66 kg N ha⁻¹ (8.4–43.4%), indicating GM effectively decreases N runoff by reducing DIN. Model simulations of ponding water depth, runoff, TN concentration in surface water, and TN loss in paddy fields produced the consistency indices and simulation efficiencies of 0.738–0.985, 0.737–0.986, 0.912–0.986, and 0.674–0.972, respectively, indicating that the model can be used to evaluate water consumption and N runoff loss in the GM-paddy system. The simulations showed that GM with a 40% N fertilizer significantly reduced N runoff loss under all rainfall conditions, with the greatest reductions in wet years. Under wet, normal, and dry conditions, the GM treatments significantly reduced average TN loss by 0.37–5.53 kg N ha⁻¹ (12.77–29.17%), 0.21–5.32 kg N ha⁻¹ (9.95–24.51%), and 0.02–3.2 kg N ha⁻¹ (1.78–23.19%), respectively, compared to the winter fallow treatment. These results indicate that the combination of GM and a 40% reduction in N fertilizer can significantly reduce N runoff loss from paddy fields, demonstrating good effectiveness under various rainfall conditions, making it a green production model worth promoting.

Co-utilization of milk vetch as green manure (GM) and rice straw is an effective practice for reducing nitrogen (N) input while maintaining crop productivity in rice-based agroecosystems in southern China. The effects of soil carbon (C) and N pools and their fractions under green manuring and rice straw return, combined with reduced N fertilization remain to be clarified. A four-year field experiment was carried out to explore the effects of synergistic utilization of GM and rice straw (GMS) on rice yield, soil C and N fractions, and their contributions to rice productivity. The study demonstrated that compared with winter fallow (WF), GMS increased rice yield by 20.3% under 40% reduction in N fertilization (N60). GM application increased soil total N content by 16.5% and 18.0% significantly relative to WF under N0 and N60, respectively. GMS treatment demonstrated improvements in the soil organic C pool and enhanced soil N activity. Compared with WF, soil organic C, mineral-associated organic C and particulate organic C under GMS increased by 11.1% and 24.9%, 31.3% and 13.8%, 13.1% and 47.3% at N0 and N60 levels, respectively. Under N60, GMS increased heavy-fraction organic C content by 42.6% while reducing light-fraction organic C content by 28.0% compared to WF, thereby enhancing soil C pool stability. Regarding soil N fractions, GMS increased particulate organic N content by 60.8% and 79.3%, and mineral-associated organic N content by 89.7% and 43.4% at N0 and N60 levels, respectively. Under N60, GMS reduced heavy-fraction organic N content while increasing light-fraction organic N content, thereby enhancing soil N availability. Based on the results of Mantel tests and random forest prediction, our analysis found that N and particulate organic C served as the key factors affecting rice yield. In conclusion, GMS combined with 60% of the conventional N rate enhanced rice yield by mediating soil C sequestration and N availability, proving to be an effective strategy for improving soil fertility and ensuring food security in the rice-growing region of southern Jiangsu, China.