Application of nitrogen (N) fertilizer or a combination of nitrogen and zinc (Zn) can significantly increase the availability of soil Zn, improve the Zn nutrition and grain yield of winter wheat. Root exudates have been widely recognized for their critical role in mobilizing and activating nutrients within the rhizosphere, thereby facilitating their uptake by plants. In this study, we investigated the combined effects of nitrogen-zinc (N-Zn) fertilizer application and exogenous winter wheat root exudates on the transformation of different Zn forms in calcareous soils.

A hydroponic culture trial was carried out, using winter wheat cultivar Bainong 207 as the test material. Two Zn application levels (0, 10 µmol/L) and two N application levels (0.5, 7.5 mmol/L) were set up to compose 4 treatments. The root exudates were collected, and the composition and concentration of organic acids in the root exudates were analyzed. Subsequently, a rhizobox experiment was carried out using the same wheat cultivar. Each treatment was then divided into two parts: one was added with 50 mL distilled water, and the other with 50 mL root exudates collected in the hydroponic trial. The biomass, N and Zn concentration of various parts of the wheat plant, and the pH, available Zn content, and the contents of Zn fractions in rhizosphere and non-rhizosphere soil were determined.

Compared with the low-nitrogen, zinc-free treatment (N_0.5Zn₀), co-application of N-Zn fertilizers (N_7.5Zn₁₀) markedly elevated the concentrations of aconitic acid, fumaric acid, and malic acid in root exudates, whereas it significantly depressed the levels of pyruvic acid, α-ketoglutaric acid, and succinic acid. Irrespective of root exudate addition, N-Zn application notably boosted grain yield, Zn concentrations in roots, stems, leaves, and grains, as well as N concentrations in glumes and grains. In both rhizosphere and non-rhizosphere soils, this combined application significantly increased available Zn concentrations, along with the contents and proportions of exchangeable Zn, carbonate-bound Zn, and Fe-Mn oxide-bound Zn, while dramatically reducing the concentration and proportion of residual Zn. Under the regime of combined N-Zn application, the supplementation of root exudates further increased grain yield and Zn content, as well as N content in roots, stems, leaves, and glumes. It significantly elevated available Zn and exchangeable Zn concentrations in non-rhizosphere soil, raised the content and proportion of carbonate-bound Zn in rhizosphere soil, but significantly lowered the pH and decreased the content and proportion of residual Zn in rhizosphere soil.

Under conditions of combined N and Zn application, exogenous root exudates from winter wheat facilitate the conversion of residual Zn to carbonate-bound Zn in rhizosphere soils, increase the contents of carbonate-bound Zn and Fe-Mn oxide-bound Zn in both rhizosphere and non-rhizosphere soils, thereby effectively improving soil Zn bioavailability and enhancing Zn uptake by winter wheat.

This study aimed to evaluate the soil acidification characteristics of sugarcane fields in Guangxi under different fertilization treatments and to provide a theoretical basis for mitigating soil acidification in this region.

Based on a long-term field experiment initiated in 2008, four treatments were established: no-fertilizer control (CK), recommended fertilization (OPT), increased N application (OPTN, 50% higher nitrogen (N) application rate than OPT) and recommended fertilization combined with sugarcane leaf mulching and returning (OPTS). Soil pH, exchangeable acidity, acidification rate, exchangeable base cations, cation exchange capacity (CEC), soil acid-base buffering capacity (pHBC) and soil nutrients indicators were determined after 16 years under different fertilization treatments.

After 16 years of long-term experiments, compared with the initial soil, soil pH under the CK, OPT, OPTN, and OPTS treatments decreased by 0.04, 1.74, 2.00, and 1.72 units, respectively. The corresponding soil acidification rates were 0.16, 9.62, 10.41, and 9.81 kmol/(hm²·a), respectively. The soil pHBC of the CK treatment was 24.00 mmol/(kg·pH). Compared with CK, the pH buffering capacity under the other fertilization treatments increased by 20.32%−26.77%, with the greatest increase observed in the OPTS treatment. Compared with the OPT treatment, the OPTN treatment increased exchangeable acidity, exchangeable aluminum (Al³⁺), and exchangeable hydrogen (H⁺) by 19.31%, 15.25%, and 54.75% respectively. The total exchangeable base cations of the OPTN decreased by 14.03%, and base saturation decreased by 15.10%. Soil pH decreased by 0.26 units, and the acidification rate increased by 8.22% under OPTN relative to OPT. Compared with the OPT treatment, the OPTS treatment increased exchangeable acidity and exchangeable H⁺ content by 17.82% and 69.27%, respectively. The total exchangeable base cations increased by 13.45%, mainly due to a 26.22% increase in exchangeable calcium content. CEC increased by 12.67%. However, there were no significant difference in soil pH and acidification rate. Under increased N fertilization, soil total phosphorus, available phosphorus, and readily available potassium contents decreased by 6.99%, 11.06%, and 27.11%, respectively. Under sugarcane leaf mulching and returning, soil organic matter, total N, and available N contents increased by 9.96%, 12.32%, and 26.40%, respectively, while available phosphorus and readily available potassium contents decreased by 17.15% and 34.65%, respectively. In terms of the 5-year average sugarcane yield, compared with the OPT treatment, the OPTN treatment resulted in a 6.23% reduction, while the OPTS treatment resulted in a 2.68% increase.

Long-term excessive N fertilization increased exchangeable H⁺ and Al³⁺ contents while reducing total exchangeable base cations. The increased fertilizer N was not efficiently utilized by sugarcane, further accelerating soil acidification. In contrast, long-term sugarcane leaf mulching and returning effectively increased total exchangeable base cations, CEC, and organic matter content, and improved soil nutrient supply and acid-base regulation capacity. Therefore, rational chemical fertilizer application combined with sugarcane residue mulching and returning can serve as an important strategy for managing soil acidification in lateritic red soil sugarcane fields in Guangxi.

Critical nitrogen dilution curves (CNDCs) have been constructed for nitrogen nutrition diagnosis in many crops. However, the acquisition of the main variable factor, biomass, requires destructive sampling, which significantly reduces the practical feasibility of these curves. We attempted to construct a critical nitrogen dilution curve for flue-cured tobacco using accumulated growing degree days (AGDD, referred to as the “AGDD curve”) and compared the parameter values with those of the curve using shoot dry matter accumulation (referred to as the “dry matter curve”).

The field experiment was conducted in Linqu County, Shandong Province, in 2023, using the flue-cured tobacco cultivar Zhongchuan 208. The experiment employed a two-factor (planting date and nitrogen application rate) complete block design. The two transplanting dates were April 30 and May 10. The nitrogen application rates included five levels: N1 (0 kg/hm²), N2 (45 kg/hm²), N3 (90 kg/hm²), N4 (135 kg/hm²), and N5 (180 kg/hm²). Aboveground dry matter accumulation in tobacco plants was measured at key growth stages, including the rosette, vigorous growth, bud emergence, topping, and maturity stages. Using a hierarchical Bayesian framework model and the Markov Chain Monte Carlo (MCMC) algorithm, two critical nitrogen dilution curves for the aboveground dry matter accumulation of flue-cured tobacco were constructed. One curve was driven by accumulated temperature (T-curve), and the other by aboveground dry matter accumulation (M-curve). The ability of the two curves to differentiate between nitrogen-limited and non-nitrogen-limited conditions was compared. The critical nitrogen concentration and nitrogen nutrition index were calculated using measured data and fitted using the two curves to test the accuracy of the two curves.

Both planting date and N application rate significantly affected the aboveground dry matter accumulation of flue-cured tobacco. The critical nitrogen dilution curves constructed with aboveground dry matter accumulation (PDM) and accumulated growing degree days (AGDD) as driving variables, were Nc = 3.00 PDM^−0.18 and Nc = 2.32 AGDD^−0.33, respectively. For the M-curve, the 95% posterior distribution ranges for parameters A1 and A2 were 2.71−3.40 and 0.10−0.29, with mean values of 3.00 and 0.18, respectively. The curve’s uncertainty level ranged from 0.14% to 1.76%, with relative uncertainties for A1 and A2 being 0.23% and 1.12%, respectively. The discrimination ability between the nitrogen surplus group and the nitrogen deficit group was 75%. The normalized root mean square error (n-RMSE) of the nitrogen nutrition index was 23%. For the T-curve: the 95% posterior distribution ranges for parameters A1 and A2 were 2.23−2.41 and 0.23−0.44, with mean values of 2.32 and 0.33, respectively. The curve’s uncertainty level ranged from 0.12% to 1.12%, with relative uncertainties for A1 and A2 being 0.08% and 0.62%, respectively. The discrimination ability between the nitrogen surplus group and the nitrogen deficit group was 82%. The normalized root mean square error (n-RMSE) of the nitrogen nutrition index was 13%.

Compared to the dry matter-based curve, the accumulated temperature-based curve offers clearer differentiation between nitrogen deficit and nitrogen surplus conditions. The fitted nitrogen nutrition index demonstrates a stronger linear correlation with the actual nitrogen nutrition index, evidenced by a lower normalized root mean square error (n-RMSE) of 13%, indicating higher simulation accuracy. Given the practicality and accessibility of accumulated temperature data, the critical nitrogen dilution curve developed using accumulated temperature as the driving variable is well-suited for non-destructive diagnosis of nitrogen status in flue-cured tobacco.

Soil magnesium (Mg) deficiency was one of the main limiting factors for pepper production. The effect of different Mg fertilizer application amounts on pepper yield and soil exchangeable Mg content was studied, and the mechanisms was also explored.

A field experiment was conducted in east of Guizhou Province during 2018 and 2019, using chilli pepper (Capsicum annuum var. conoides) as test material. Five Mg application levels were set up, including: 0, 22.5, 45, 67.5, and 90 kg/hm², denoted as Mg0, Mg22.5, Mg45, Mg67.5, andMg90, respectively. Chilli pepper yield, yield components, leaf net photosynthetic rate (Pn), leaves chlorophyll content, shoot Mg concentration and soil exchangeable Mg content were measured.

Soil Mg application significantly increased the yield of pepper, with the Mg67.5 and Mg90 treatments showing the best results. In 2018 and 2019, the yields of chilli pepper significantly increased by 20.7%−40.6% and 14.8%−18.0%, respectively, compared to the Mg0 treatment. The yield increase was mainly attributed to improvements in the number of fruits per plant and single fruit weight. Soil Mg fertilization enhanced the Pn, chlorophyll content, and Mg content of the plants during the flowering and fruit-setting stage (a critical growth period). In 2018 and 2019, the Pn were increased by 41.8%−72.8% and 27.3%−71.3%, respectively, total chlorophyll content increased by 23.1%−37.2% and 9.3%−14.8%, respectively, compared to the Mg0 treatment. During the flowering and fruit-setting stage, plant magnesium concentration increased by 57.3%−74.7% and 29.8%−69.8%, respectively. At harvest, plant magnesium concentration increased by 24.7%−78.0% and 17.6%−42.1%, respectively. The 0−20 cm soil layer exchangeable Mg content under Mg-treated plots in 2018 and 2019 was 58.1−79.4 mg/kg and 62.1−101.7 mg/kg, respectively, representing significant increases of 26.8%−52.9% and 34.9%−121.0%, compared to the Mg0 treatment. In 2018, there were no significant differences in exchangeable Mg content among treatments in the 20−40 cm and 40−60 cm soil layers. In 2019, the exchangeable Mg content in the 20−40 cm and 40−60 cm soil layers under Mg treatments was 64.0−92.6 mg/kg and 70.9−97.0 mg/kg, respectively, showing significant increases of 18.3%−71.3% and 11.9%−53.0%, compared to Mg0 treatment. Regression analysis revealed that the Pn during the flowering and fruit-setting stage, as well as the yield of chilli pepper, exhibited a highly significant linear positive correlation with the exchangeable Mg concentration in the 0−20 cm soil layer at harvest. The Mg content in the fruit of chilli pepper reached a plateau value of 1.95 g/kg when the soil exchangeable Mg concentration reached 74.2 mg/kg.

Mg fertilization can effectively improve the exchangeable Mg concentration in 0−20 cm soil layer, providing sufficient Mg nutrition for chilli pepper during the whole growing period. As a result, Mg fertilization can enhance the net photosynthetic rate and chlorophyll content of leaves, and maintain leaf greenness during flowering and fruit-setting stage, and therefore increase the chilli pepper yield and fruit Mg content. Applying Mg 67.5 kg/hm² is recommended for high chilli pepper yield and maximum fruit Mg content, as well as the high soil exchangeable Mg content in the tested area.

Conventional fertilizers often face difficulties in controlling nutrient release rates and exhibit relatively low utilization efficiency. Thus, environment-responsive fertilizers that modulate nutrient release in response to environmental changes are progressively emerging as pivotal technological approaches for advancing sustainable agriculture.

Lignin (L) was hydrolyzed using urea/NaOH (UR/SH) at low temperatures to obtain hydrolyzed lignin (HL). The hydrolyzed lignin was then cross-linked with polyethylene glycol diglycidyl ether (PEGDGE) to prepare a hydrolyzed lignin hydrogel (HLHG). The HLHG was subsequently employed as a carrier for diammonium phosphate (DAP) fertilizer to create a pH-responsive hydrogel fertilizer.

The optimal hydrolysis conditions were determined to be 18 h and −10℃. Under these conditions, the contents of hydroxyl (—OH) and carboxyl (—COOH) groups reached their maximum levels, increasing by approximately 68% and 99%, respectively, compared to untreated L. The obtained pH-responsive lignin hydrogel [HLHG-(−10℃)-18 h] showed an equilibrium swelling ratio of 1216%, with a water retention swelling ratio of 163% after 11 hours. These values were approximately 3 and 12 times of the unhydrolyzed lignin hydrogel (LHG), respectively. pH responsiveness analysis indicated that the hydrogel demonstrated significant pH sensitivity, with equilibrium swelling ratio of 364%, 518%, and 634% at pH 6, 7, and 8, respectively. Between the adjacent gradients of pH 6−7 and pH 7−8, the equilibrium swelling ratios increased significantly by 41% and 70%, respectively, compared to LHG. SEM analysis demonstrated pH-dependent microstructural evolution of the lignin hydrogel. As the pH increased from 5 to 8, the honeycomb network exhibited a progressive pore channel expansion with concomitant wall thinning. When the pH increased to 9, the network structure contracted, resulting in a reduction in pore channels and thickening of the pore wall. This pH-responsive behavior was mechanistically attributed to the ionization/protonation equilibria of abundant —OH and —COOH groups in lignin, as validated through integrated functional group quantification and pH responsiveness assays. Furthermore, the pH-responsive hydrogel was employed as a carrier for DAP to control the release of phosphate fertilizer nutrients. The nutrient release results showed that the cumulative release rate of P₂O₅ from the lignin hydrogel fertilizer in the period of 60 h was 61%, 76%, and 85% at pH 6, 7, and 8, respectively, exhibiting a prominent pH-responsive tendency.

UR/SH hydrolysis of lignin effectively modifies the content of active functional groups and the internal spatial network structure of lignin, it is possible to create pH-responsive lignin hydrogels with enhanced water absorption and retention capacities. Utilizing this pH-responsive lignin-based hydrogel as a carrier for fertilizers offers a valuable reference for the effective utilization of lignin resources and the design of bio-based green fertilizer for nutrient control release.