Gully erosion threatens soil resources and agricultural production in the black soil region of Northeast China, but geological effects are difficult to separate from terrain and land use. We combined 5407 erosion gullies from the First National Water Resources Census with 1:250,000 lithological and soil-forming parent material maps, a 12.5 m digital elevation model, and land-use data for the Heihe area. We calculated area-normalized and cropland-restricted gully densities, compared five morphometric variables, and used generalized additive models to account for terrain, land cover, spatial structure, and subwatershed clustering. Gully length density differed by more than two orders of magnitude among geological units. Pleistocene basalt had the highest density among major units, whereas Late Triassic–Early Jurassic granites were nearly gully-free even within cropland. The apparent concentration of gullies in the Neogene Sunwu Formation mainly reflected its large outcrop area. Adding lithology to terrain- and cropland-adjusted grid models increased explained deviance by 1.27 percentage points at 1 km and 0.98 percentage points at 2 km. In contrast, lithology or parent material added no more than 0.24 percentage points of deviance to terrain-based models of gully morphometry. Lithology and parent material were nearly redundant, and geological units accounted for much of the elevation and relief variation. Geological substrate therefore provides a small additional signal for where gullies concentrate, whereas its association with gully form is expressed mainly through geologically inherited terrain. These results support using substrate information to refine terrain-based gully erosion risk zoning.
Black soils cover roughly 20% of cropland worldwide and constitute a critical agricultural resource for food security. Soil organic carbon (SOC) in black soil is related to arable land quality and soil fertility. However, our understanding of its compositional patterns is limited, and the relationship between SOC stability and saturation is even more difficult to grasp, thus restricting the management of carbon sequestration in black soil. Here, we integrate a recent field survey (16 soil profiles spanning 0–200 cm in depth) across a 1500-km transect in the black soil region of Northeast China with legacy data (172 topsoil records) from the region, and apply linear mixed-effects models and machine-learning to examine the multidimensional distribution and drivers of mineral-associated organic carbon (MAOC), particulate organic carbon (POC), SOC physical stability (MAOC/POC), and carbon saturation. Our results show that MAOC and POC contents decline progressively with depth, whereas SOC physical stability increases and the SOC humification degree intensifies. Linear mixed-effects models reveal that iron-bound organic carbon and calcium-bound organic carbon respectively regulate SOC physical stability in surface and deeper layers. Spatial mapping further demonstrates that topsoil SOC physical stability and carbon saturation exhibit opposing geographic patterns. Our findings underscore that policies aimed at increasing SOC content must explicitly couple horizontal and vertical carbon-pool dynamics and incorporate both SOC stability and saturation as explicit assessment metrics.
Wetlands are major global natural carbon reservoirs, yet the mechanisms controlling organic carbon persistence in tropical systems remain poorly constrained. In the Brazilian Cerrado, Veredas (groundwater-fed palm swamp wetlands) are characterized by strong hydrological gradients, dynamic redox conditions, and highly weathered soils. Despite their ecological importance, the mechanisms controlling organic carbon stabilization in these systems remain poorly understood, particularly regarding the relative roles of Fe and Al under persistent saturation. Here, we examined relationships between organic carbon and reactive Fe and Al phases along a border–intermediate–bottom toposequence using sequential selective dissolution and regression analyses. Soil moisture increased and pH decreased downslope, accompanied by a sharp decline in total Fe and substantial organic carbon accumulation in permanently saturated bottom soils. Across all positions, organic-complex (pyrophosphate-extractable) fractions dominated the extracted pools of Fe, Al, and carbon. However, contrasting behaviors emerged between metals. Iron-associated carbon decreased along the gradient and showed no consistent relationship with organic carbon, reflecting redox-driven dissolution and mobilization. In contrast, aluminum remained consistently associated with organic carbon across all positions, with significant relationships observed in the organic-complex fraction (β = 0.75, p = 0.003) and the low-crystallinity fraction (β = 0.59, p = 0.011). These results indicate a shift in dominant stabilization mechanisms along the hydrological gradient. While Fe may contribute to carbon retention under oxic conditions, its role diminishes under prolonged saturation due to redox sensitivity. In contrast, Al-mediated associations persist under reducing conditions and represent a more stable pathway for organic carbon stabilization in deeper hydromorphic environments. Overall, our findings highlight aluminum as a key, yet often overlooked, regulator of carbon persistence in highly weathered tropical wetlands.
Quantifying the erosion rates and their mechanisms is central to geomorphology, yet studies on weathering-limited landscapes remain scarce. Here we focus on the Tibetan Plateau, a largely cryospheric region with extensive weathering-limited, where sediment supply is constrained by bedrock weathering and physical breakdown. We use frost cracking intensity (Fci) to characterize frost cracking and evaluate six commonly used variables together with Fci as potential controls on erosion. We then construct empirical statistical equations calibrated against compiled 10Be-derived millennial-scale basin-wide erosion fluxes. Results show that local relief (LRF) provides the most robust prediction across the full dataset, whereas Fci provides additional explanatory power mainly in catchments experiencing intense frost cracking. A cutoff-sensitivity analysis identifies Fci = 5 °C/cm as an empirical grouping criterion, and a model combining LRF, Fci, and MAP provides the best empirical performance for catchments with mean Fci > 5 °C/cm. We interpret this regime-dependent behavior to indicate that frost cracking may modulate near-surface rock-mass weakening, sediment availability, and the grain-size distribution of material supplied to channels in cold, weathering-limited basins. An exploratory comparison between published 10Be-derived millennial-scale and gauging-derived decadal fluxes further suggests that post-LGM warming and reduced Fci may have contributed to lower modern fluxes in some frost-cracking-sensitive basins, although this response is spatially heterogeneous and catchment-dependent.
The progradation of the deltaic sequences within the Pangong Lake (Bangong Co), Ladakh Himalaya, has been studied using sedimentological, geomorphological, optically stimulated luminescence (OSL) chronology and geophysical investigations. This study advances the understanding of lake-delta processes in high-altitude arid regions by documenting how deltaic systems in small basins encode millennial to sub-millennial scale hydro-climate variability. The deltaic evolution of Pangong Tso reflects temporal variations in accommodation and fluvial sediment supply, consistent with independently inferred lake-level fluctuations. Five facies associations (FA) representing bottomset, foreset, topset, beach, and distributary channel environments are identified, defining a typical Gilbert-type delta architecture. Spatio-temporal correlation among these facies indicates that deltaic depocenters migrated in response to changes in accommodation space, expressed through shifts in shoreline position. The phases of relatively reduced accommodation are consistent with relatively lower lake levels in the late Pleistocene–early Holocene. During these phases, the subaerial exposure of the basin margins promoted fluvial incision and delta progradation. Subsequent increases in accommodation, possibly associated with relatively higher lake levels during the mid-Holocene, led to aggradational to retrogradational stacking and the development of delta-front deposits. Episodes of lake-level fall were associated with the reactivation of distributary channels and renewed progradation. The lake level has declined rapidly since 1.1 ka, resulting in an abrupt shift in sediment accommodation space lakeward. Overall, the Pangong Tso system demonstrates strong source-to-sink connectivity, where sediment supply and accommodation space respond rapidly to hydroclimatic forcing with minimal temporal lag. Our findings contribute to broader models of delta evolution and stratigraphic response in closed-basin settings and provide insights into the interaction between hydroclimatic variability, accommodation space, and sediment supply in short-routing fluvial systems.
