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Straw return is an effective management practice. It not only utilizes agricultural waste but also introduces dissolved organic matter (DOM) into the soil. Selenium (Se) is an essential trace element in the human diet and contributes to the popularity of Se-enriched agricultural products in the Chinese market. Moreover, there are still some Se-enriched agricultural products that have yet to be utilized. This study investigated whether Se addition in soil caused component changes in the DOM extracted from rape straw. DOM extracted from rape straw grown in soil with four Se levels (0, 0.1, 0.5 and 1.0 mg Se kg−1 soil) was characterized by UV-Visible spectroscopy, fluorescence spectroscopy and FTIR spectroscopy. The UV-visible spectra revealed that 0.1 mg Se kg−1 soil reduced the molecular weight of DOM and caused the presence of more irreplaceable aromatic structures in the substituent groups of the DOM, while 0.5 and 1.0 mg Se kg−1 soil only reduced the DOM molecular weight. Fluorescence spectroscopy indicated that Se improved the humification degree but reduced the aromaticity of DOM. FTIR spectra proved that Se altered the contents of carboxylic acids, amino acids, alcohols and aromatic heterocycles in DOM, which were maximized in the 0.5 and 1.0 mg Se kg−1 treatment groups. We concluded that Se application could change the composition of DOM extracted from rape straw, potentially impacting the nutrient bioavailability in soil. This study provides basic data on Se-enriched rape straw utilization for eco-agriculture.

Cadmium (Cd) is highly toxic, even at very low concentrations, to both animals and plants. Pollen is extremely sensitive to heavy metal pollutants; however, less attention has been paid to the protection of this vital part under heavy metal stress. A pot experiment was designed to investigate the effect of foliar application of Se (1 mg/L) and Mo (0.3 mg/L) either alone or in combination on their absorption, translocation, and their impact on Cd uptake and its further distribution in Brassica napus, as well as the impact of these fertilizers on the pollen grains morphology, viability, and germination rate in B. napus under Cd stress. Foliar application of either Se or Mo could counteract Cd toxicity and increase the plant biomass, while combined application of Se and Mo solutions on B. napus has no significant promotional effect on plant root and stem, but reduces the seeds’ weight by 10–11%. Se and Mo have decreased the accumulated Cd in seeds by 6.8% and 9.7%, respectively. Microscopic studies, SEM, and pollen viability tests demonstrated that pollen grains could be negatively affected by Cd, thus disturbing the plant fertility. Se and Mo foliar application could reduce the toxic symptoms in pollen grains when the one or the other was sprayed alone on plants. In an in vitro pollen germination test, 500 μM Cd stress could strongly inhibit the pollen germination rate to less than 2.5%, however, when Se (10 μM) or Mo (1.0 μM) was added to the germination medium, the rate increased, reaching 66.2% and 39.4%, respectively. At the molecular level, Se and Mo could greatly affect the expression levels of some genes related to Cd uptake by roots (IRT1), Cd transport (HMA2 and HMA4), Cd sequestration in plant vacuoles (HMA3), and the final Cd distribution in plant tissue at the physiological level (PCS1).

The rape (Brassica napus L. cv. Xiangnongyou 571) was chosen as the experimental material to undergo solution cultivation at seedling stage to investigate the effects of selenite addition on the selenium (Se) absorption and distribution, root morphology and physiological characteristics of rape seedlings. The results showed that the bioaccumulation ability of Se decreased significantly with increasing the Se application rate, but the Se distribution coefficient remained around 0.9 with no significant influence. The application of 10 µmol . L-1 selenite stimulated the growth of rape seedlings through improving the root physiological characteristics and root morphology significantly, including significantly increasing the production of superoxide radical (O2∙-) rate and the activities of superoxide dismutase (SOD), peroxidase (POD) and fungal catalase (CAT) in the root system, which resulted in a reduction of the lipids peroxidation (MDA) content as much as 26.0%, consequently increasing the root activity as much as 17.4%. The promoting degrees of selenite on root morphological parameters were from strong to weak in such a tendency: root volumetotal surface areanumber of root forkstotal root lengthnumber of root tipsaverage diameter. However, such positive effects had no significant difference with those in treatment with 1 µmol . L-1 selenite, indicating that small amounts (≤ 10 Lmol . L-1) of selenite were able to increase the activity of antioxidant enzymes and reduce the content of MDA in root system, which could increase root activity and improve root morphology, hence increased the biomass of rape seedlings.

The high-quality, high-yield rape (Brassica napus L. cv. Xiangnongyou 571) was chosen as the experimental material to undergo seedling stage soil cultivation and solution cultivation. This study was aimed to investigate the effects of sulfur (S) on the fraction and species of amend selenium (Se) in soil by applying S fertilizer, and to reveal the mechanism through which S controls the Se uptake of rape. The results showed that applications of both Se at 5 mg.kg-1 and S at 150 mg.kg-1 could provide some boost to rape growth, however such boost was unremarkable; S application could significantly reduce Se content in shoots and roots by as much as 64. 0% and 39. 1% , respectively; S application could significantly decrease the pH value and increase the organic matter content by as much as 0. 65 units and 1.76 g.kg-1 , respectively; S application could significantly decrease soluble Se and Fe/Mn oxide-bound Se, and significantly increase organic matter-bound Se, however, there was no remarkable effect on exchangeable and residual Se. S could significantly decrease the content of Se(IV) in the soluble Se and significantly increase the content of Se(IV) in the exchangeable Se, with remarkable decrease in Se(VI) content in both fractions, and no remarkable effect on Se(II). S had a profound effect on the uptake of Se by rape seedlings when Se( IV) and Se( VI) were applied. The Se contents in shoot and root of the Se( VI)-treated rape were 7.3 and 3.2 times respectively as high as that of the Se( IV)-treated rape, while with a S rate of 2 mmolL-1, their Se contents were lower than 32.6% and 8.7% of that of the Se(IV)-treated rape. In conclusion, in addition to improving crop growth, appropriate amount of S fertilizer could also reduce soil pH and increase organic content, causing available Se in soil to be Fe/Mn oxide-bound and organic matter-bound, and retarding its transformation to Se( VI), thus reducing Se uptake by rape. This has significant value for regulation of Se content in crops from Se-rich regions, ensuring food quality and safety, benefiting the long-term use of soil Se resources and regional economic development.