Shixiang Zhao

Severe soil salinization and low fertility significantly constrain sustainable agricultural development in the Yellow River Delta, one of the three major estuarine deltas in China. Despite their ecological importance, the regional-scale spatial interactions between soil salinity and nutrients, particularly regarding their vertical variability, remain poorly understood. This study analyzed 228 soil samples from 76 sites distributed across a distinct salinity gradient, which was determined by constructing a spatial salinity distribution map after sampling. Samples were collected at three depths (0–15, 15–30, and 30–45 cm) to investigate the spatial distribution of soil organic carbon (SOC), total nitrogen (TN), and the C/N ratio, along with their underlying driving factors. SOC and TN exhibited similar spatial patterns, with higher values distributed along both banks of the Yellow River. Horizontally, SOC and TN in the 0–15 cm layer decreased gradually from west to east, whereas the 15–30 cm and 30–45 cm layers showed an opposite trend, increasing eastward. Vertically, SOC and TN contents declined significantly with soil depth (p < 0.05), although the magnitude of this decline varied regionally: the 0–15 cm layer in the western area contained markedly higher nutrient levels than deeper layers, while vertical variation was less pronounced in the eastern and estuarine regions. Both variables were positively associated with total phosphorus (TP), available potassium (AK), soil moisture content (MC), clay content, and pH, but negatively correlated with electrical conductivity (EC), particularly in the 0–15 cm layer. Our results highlight that soil texture, moisture, and salinity affect the spatial heterogeneity and vertical decline of SOC and TN in the Yellow River Delta. Future research should focus on the long-term temporal distribution of the coupling of multiple elements under changing hydrological and salinity regimes.

Rotational tillage is considered a potential option to improve soil organic carbon (SOC) stock and mitigate climate change. However, the mechanisms underlying SOC sequestration under rotational tillage remain poorly understood due to insufficient data on SOC concentration and mineralization within soil aggregates. A 12-year field experiment was conducted in Northwest China to evaluate the effects of tillage on SOC stocks, soil aggregate stability, aggregate-associated OC concentrations and mineralization. The results showed that rotational tillage had more crop residue and less soil disturbance, thus improving soil aggregate stability, aggregate-associated OC concentrations and SOC stocks. The highest MWD and SOC stocks were found in no-tillage rotated with subsoiling (NS), which were 36.0–69.7% and 16.3% higher than plowing, respectively. Macroaggregates had higher cumulative OC mineralization and lower OC mineralizability, due to physical protection. Rotational tillage treatments with higher soil aggregation contributed to decreasing OC mineralizability and increasing SOC sequestration. Meanwhile, rotational tillage decreased OC mineralization loss, mineralizability, and decomposition rate within microaggregates and silt–clay fractions. Among all treatments, NS treatment had the lowest total OC mineralization, which was lower by 5.94–27.3% than plowing at 0–40 cm depths. Considering soil structure stability, SOC mineralization and sequestration, NS treatment was a promising strategy in semiarid regions.

The spatial distribution traits of microtopography exert a profound influence on the generation of runoff and sediment. Nevertheless, the underlying mechanism through which microtopography alterations, triggered by diverse factors, impact soil erosion remains largely elusive. In light of that, this study simulated conventional farming practices on the Loess Plateau: artificial backhoe, artificial digging, and contour tillage (CT), with no tillage (CK) designated as the control group. The objective was to meticulously investigate the variations in microtopography, runoff, and sediment yield under disparate treatment conditions, rainfall intensities (60 mm/h and 90 mm/h), and slope gradients (5°, 10°, and 20°). The principal findings were as follows: With the amplification of rainfall intensity, the elevation change rate and fractal dimension of various treatments generally exhibited an upward trend, whereas the structural ratio showed a downward tendency. As the slope gradient increased, the elevation change rate and structural ratio of different treatments typically increased. However, the fractal dimension displayed no conspicuous alteration at a rainfall intensity of 60 mm/h and a decreasing trend at 90 mm/h. Under different rainfall intensity scenarios, a robust linear correlation existed between the fractal dimension and both runoff and sediment yield (R² > 0.73), rendering it an outstanding parameter for estimating these variables within the scope of this research. Path analysis revealed that the indirect effect of microtopography on sediment yield, which was mediated by runoff, constituted 77.80–96.47% of the direct effect. Moreover, under different rainfall intensities, the alterations in runoff and sediment yield ensuing from unit-scale changes in the fractal dimension varied significantly. Specifically, at a rainfall intensity of 90 mm/h, these changes were 1.70-fold and 3.75-fold those at 60 mm/h, respectively. Overall, the CT treatment engendered the lowest runoff and sediment yield, along with the highest fractal dimension, thereby emerging as the most efficacious measure for soil and water conservation in this study. The research outcomes offer valuable perspectives for further elucidating the mechanisms through which tillage practices impinge upon soil erosion.