Impact of land use type and altitudinal gradient on topsoil organic carbon and nitrogen stocks in the semi-arid watershed of northern Ethiopia.

Understanding the role of soils in the soil organic carbon (SOC) and total nitrogen (TN) cycle is essential, assumed that these parameters are among the key soil quality indicators in a given landscape. Nothing but their status is in a state of continual flux due to land-use, soil management practices, and nature of topographic features. Thus, this study has evaluated the effect of land-use types and altitudinal gradient on SOC and TN concentrations and stocks at a watershed scale in northern Ethiopia. A total of 450 topsoil samples (0-30 cm depth) were collected from four different land-use types (Fig.3) across three elevational categories (Fig.1(b)), and their SOC and TN distributions were studied using descriptive statistics and geostatistical methods. Results revealed significant (p < 0.05) differences in SOC and TN concentrations and stocks by land-use type, elevation, and their interactions. The highest SOC stock was recorded at the lower elevation in GL (7.24 Mg C ha-1), followed by PF (4.65 Mg C ha-1) in the middle and GL (4.61 Mg C ha-1) in the upper elevations, respectively. On the other hand, the lowest SOC stock was observed in the BL areas of the upper (2.34 Mg C ha-1) and middle (2.75 Mg C ha-1) elevations. Spatially, the mean SOC stocks of the different land-uses were in the following order: GL > PF > CL > BL in upper elevation, PF > GL > CL > BL in middle elevation, and GL˃CL in lower elevation, respectively. The estimated total SOC and TN stocks of the study watershed were about 46,868.66 ± 7747.38 Mg C and 7,008.02 ± 441.25 Mg N, respectively. The notable difference is attributable to lack of vegetation cover, unsustainable land-use system, and land degradation via water erosion. Hence, these physical landscape disturbances result in disruption of SOC and TN's storage and stability. The SOC and TN stocks have shown a significant (p < 0.05) negative correlation with soil bulk density in the study watershed. The study concludes that variations in the land-use along topographic gradients drive the soils' SOC and TN storage. Therefore, land suitability planning, soil and water conservation measures, and reforestation practices are needed and practical worth increasing SOC and TN storage in the watershed.

Rice cultivars significantly mitigate cadmium accumulation in grains and its bioaccessibility and toxicity in human HL-7702 cells.

Excessive Cd accumulation in cereals, especially in high-consumption staple crops, such as rice, is of major concern. Therefore, elucidation of cultivar-specific variation in rice grain Cd bioaccessibility and toxicity in humans would help the development of remedial strategies for Cd accumulation and toxicity. The present study combined an in vitro gastrointestinal digestion model with a human HL-7702 cell and assessed Cd bioaccessibility and toxicity to humans from the grains of 30 rice cultivars of different types harvested from Cd-contaminated paddy soil. The mean grain Cd content of cultivars within the type exceeded acceptable national standards. Cadmium bioaccessibility was high in all grains (9.08-23.6%) except the low accumulator (LA) rice cultivar (7.93%). The mean estimated daily intake of Cd via the cultivars (except LA) exceeded the FAO/WHO permissible limit based not only on the total grain Cd concentration but also on bioaccessible Cd concentration. A dose-proportional correlation between the in vitro bioaccessible and total grain Cd concentrations was observed, suggesting that Cd bioaccessibility accurately reflects the transfer of Cd from rice grain to humans. Toxicity assay results demonstrated that Cd from rice grains could commence oxidative stress and injury in HL-7702 cells, except the LA rice, which did not exhibit significant alteration in HL-7702 cells owing to its low Cd concentration. These results provide primary evidence to suggest that the cultivation of the LA rice cultivar is an effective agronomic approach to avert Cd entry into the food chain and alleviate Cd toxicity in humans.

Micro-XRF mapping and quantitative assessment of Cd in rice (Oryza sativa L.) roots.

Understanding Cd uptake and distribution in rice roots is important for breeding varieties that do not accumulate Cd in the grain to any large extent. Here, we examined the physiological and molecular factors responsible for Cd uptake and transport differences between two japonica rice cultivars prescreened as high (zhefu7) or low (Xiangzaoxian45) accumulators of Cd in the grain. No significant differences in Cd uptake between the two cultivars were observed; however, Xiangzaoxian45 retained most of the absorbed Cd in the roots, whereas zhefu7 showed higher transport of Cd from the root to the shoot, regardless of the duration of exposure to Cd. The inability to sequester Cd into root vacuoles caused high accumulation of Cd in the grain in zhefu7, whereas inefficient transport of Cd from roots to shoots in Xiangzaoxian45 caused low accumulation of Cd in the grain. Cd sequestration in the roots and transport from the root to the shoot were greatly influenced by the expression patterns of transport-related genes OsHMA3 and OsHMA2, respectively. Further, micro-X-ray fluorescence spectroscopy mapping confirmed that more Cd was sequestered in the roots of Xiangzaoxian45 than in those of zhefu7, with a significant amount of Cd localized in the root hairs, as well as in the meristematic and elongation zones, and dermal and stele tissues. Therefore, we propose that effective Cd sequestration in root vacuoles was the major determinant of divergent Cd-accumulation patterns in the two rice cultivars under study.