Monitoring soil salinity with remote sensing is difficult, but knowing the link between saline soil surface spectra, soil water, and salt transport processes might help in modeling for soil salinity monitoring. In this study, we used an indoor soil column experiment, an unmanned aerial vehicle multispectral sensor camera, and a soil moisture sensor to study the water and salt transport process in the soil column under different water addition conditions and investigate the relationship between the soil water and salt transport process and the spectral reflectance of the image on the soil surface. The observation results of the soil column show that the soil water and salt transportation process conforms to the basic transportation law of “salt moves together with water, and when water evaporates, salt is retained in the soil weight”. The salt accumulation phenomenon increases the image spectral reflectance of the surface layer of the soil column, while soil temperature has no effect on the reflectance. As the water percolates down, water and salt accumulate at the bottom of the soil column. The salinity index decreases instantly after the addition of brine and then tends to increase slowly. The experimental results indicate that this work can capture the relationship between the water and salt transport process and remote sensing spectra, which can provide theoretical basis and reference for soil water salinity monitoring.

Urban expansion has profoundly altered ecosystem carbon dynamics, yet its associations with changes in aboveground biomass (AGB) and belowground biomass (BGB) remain insufficiently understood at the national scale. This study investigated urban expansion areas in 352 Chinese cities from 2000 to 2020 using 1-km-resolution AGB and BGB datasets. By integrating temperature, precipitation, potential evapotranspiration, and atmospheric CO₂ concentration, we developed the Enhanced Aboveground Biomass Deviation Index (EAGB) and Enhanced Belowground Biomass Deviation Index (EBGB) to identify areas showing positive and negative biomass responses during urban expansion and to explore their spatial patterns and driving mechanisms. Between 2000 and 2020, China’s urban land expanded by 52,635 km², corresponding to an average annual expansion rate of 4.1%. Areas showing positive biomass responses during urban expansion accounted for 22,376 km² (42.5%) for AGB and 23,569 km² (44.8%) for BGB, mainly concentrated in East and Central China. In contrast, areas showing negative biomass responses covered 5233 km² (9.9%) for AGB and 3911 km² (7.4%) for BGB, primarily distributed in Northwest and Southwest China. During urban expansion, biomass declines were more frequently observed in rapidly expanding small- and medium-sized cities in Northwest and North China, whereas larger cities generally exhibited more widespread biomass gains. Climatic factors, particularly precipitation and atmospheric CO₂ concentration, together with human activities, were the dominant drivers of the observed spatial heterogeneity. Moreover, the dominant driving mechanisms shifted from the combined influence of climatic factors and human activities during 2000–2010 to the combined influence of climatic and topographic factors during 2010–2020. These findings provide a new framework for assessing biomass responses during urban expansion and offer scientific support for urban land-use optimization and ecological conservation.

Soil is the largest carbon reservoir in terrestrial ecosystems, and minor changes in this reservoir can dramatically affect atmospheric CO 2 concentrations. The fragile ecological environment in the arid zone of northwest China is extremely susceptible to natural and anthropogenic factors, leading to variations in ecosystem structure, function, and carbon source-sink relationships. In this study, digital soil mapping of soil organic carbon density (SOCD) was performed at a 90-m resolution for two periods (1980s and 2010s) and five depth intervals (0–5, 5–15, 15–30, 30–60, and 60–100 cm) based on historical soil profile data and the random forest model. The results showed that the prediction accuracy of SOCD in the topsoil (0–30 cm) was superior than that of the subsoil (30–100 cm). In the past 30 years, the soil organic carbon stocks (SOCS) of the northwest arid zone decreased by 585.50 Tg (1Tg = 10 12 g), with an average decrease of 19.52 Tg C yr -1 . Moreover, we quantified the amount of SOCS change caused by land-use conversion, and SOCS reduction was revealed to be mainly attributed to grassland desertification and reclamation for agriculture. Although large-scale national ecological restoration projects have increased SOCS through afforestation, sand planting, and fallowing, these projects were unable to compensate for SOCS losses in the northwest arid zone. Variance decomposition revealed that land-use conversion was the dominant factor impacting the SOCS in the northwest arid zone, followed by changes in vegetation characteristics and climate. Our results are valuable for future studies the carbon cycle processes in terrestrial ecosystems in the context of climate change and promoting the achievement of China's "carbon neutrality" goal.