Limited by the current monitoring and collection methods of subsurface flow data, the characteristics of phosphorus (P) migration loss in different soil layers on sloping cropland under natural rainfall conditions need to be clarified. In this study, a reliable U-trough collection method was used to explore P migration losses in different purple soil layers (L₀: surface, L₁: 0–20 cm, L₂: 20–40 cm, and L₃: 40–60 cm) for purple soil sloping cropland during a field monitoring campaign spanning two calendar years under natural rainfall conditions. The results suggested that the annual surface P loss load was 1.08 kg*ha⁻¹*yr⁻¹, where 92.1% was particulate P. The annual leaching loss load of total P from the L₁ layer was 3.55 kg*ha⁻¹*yr⁻¹, where 74.2% and 10.5% were intercepted by the L₂ and L₃ layers, respectively, to increase the in situ soil P stocks. Only 15.3% of L₁’s annually leached P, and only half the surface P loss load (L₀), was lost to underground water. Heavy-rain events on the purple soils in the studied sloping cropland induced 43.9% of the annual surface P loss load. In contrast, 36.1%, 30.5%, and 30.3% of P’s eventual annual leached loss loads resulted from heavy, moderate, and torrential rain events, respectively. The maize season was the main period of surface and subsurface P migration loss. Due to the limitation of experimental conditions, the results of this experiment could not represent the soil phosphorus migration process in the natural purple slope farmland in nature, but it could partially represent the soil phosphorus migration in the newly cultivated purple sloping cropland in Chongqing.

Due to the difficulty in monitoring subsurface runoff and sediment migration, their loss loads are still not clear and need further study. This study monitored water and soil loss occurring within experimental field plots for two calendar years under natural rainfall events. The sediment loss load was quantified by considering the corresponding water flow flux and its sediment concentration. The results showed that 60.04% of the runoff and 2.83% of the sediment were lost underground. The annual underground sediment loss reached up to 54.6 kg*ha−1*yr−1. A total of 69.68% of the runoff yield and 67.25% of the sediment yield were produced during the corn planting stage (CPS: March–July). Heavy rain and torrential rain events produced 94.45%, 65.46% of the annual runoff and 94.45%, 76.21% of the sediment yields during the corn-planting stage and summer fallow period (SFP: August–September). The rain frequency, rainfall, and rainfall duration of each planting stage significantly affected the resulting runoff and sediment yield. Measures aimed at the prevention and control of water-soil loss from purple soil sloping land should focus heavily on torrential rain and heavy rain events during the CPS and SFP. This paper aims to provide a practical reference for quantifying the water and soil loss from purple soil sloping cropland.

Long-term quantification of the migration loads of subsurface runoff (SSR) and its collateral soil nutrients among different soil layers are still restricted by the runoff collection method. This study tested the reliability of the U-trough collection methods (UCM), compared with the seepage plate collection method (SPM), in monitoring the runoff, sediment and nutrient migration loads from different soil layers (L₁: 0–20 cm depth; L₂: 20–40 cm depth; L₃: 40–60 cm depth) for two calendar years under natural rainfall events. The results suggested that the U-trough could collect nearly 10 times the SSR sample volume of the seepage plate and keep the sampling probability more than 95% at each soil layer. The annual SSR flux from L₁ to L₃ was 403.4 mm, 271.9 mm, and 237.4 mm under the UCM, 14.35%, 10.56%, and 8.41% lower than those under the SPM, respectively. The annual net migration loads of sediment, TN, and TP from the L₁ layer under the UCM were 49.562 t/km², 19.113 t/km² and 0.291 t/km², and 86.62%, 41.21% and 81.78% of them were intercepted by the subsoil layers (L₂ and L₃), respectively. While their migration loads under the SPM were 48.708 t/km², 22.342 t/km² and 0.291 t/km², and 88.24%, 53.06% and 80.42% of them were intercepted, respectively. Under both methods, the average leached total n (TN), total p (TP) concentrations per rainfall event and their annual migrated loads at each soil layer showed no significant difference. In conclusion, the UCM was a reliable quantitative method for subsurface runoff, sediment, and soil nutrient migration loads from diverse soil layers of purple soil sloping cultivated lands. Further studies are needed to testify the availability in other lands.

The use of biodegradable chelating agents and fertilizer to improve phytoremediation is a cost-effective and environmental-friendly method for remediation of copper (Cu)- and cadmium (Cd)-polluted agricultural soil. A pot experiment was conducted to investigate the effects of phosphorus (P) fertilizer and the chelator ethylenediamine disuccinic acid (EDDS), both individually and in combination, on the phytoremediation efficiency of castor plants. The experiment included six treatments with three replicates, which were as follows: control (no P or EDDS), EDDS alone, P at 100 mg kg⁻¹, P at 300 mg kg⁻¹, P at 100 mg kg⁻¹ + EDDS, and P at 300 mg kg⁻¹ + EDDS. The results demonstrated that phosphorus significantly promoted the growth of castor plants. In the treatment in which 300 mg kg⁻¹ P₂O₅ and 5.0 mmol kg⁻¹ EDDS were added, the shoot dry weight and root dry weight increased by 42.0% and 67.6%, respectively, when compared to the treatment only applying EDDS, and this treatment significantly promoted the absorption of Cd by shoots of castor. In the absence of phosphorus application, EDDS significantly diminished the dry weight of castor roots by 27.3%. Nevertheless, it improved the concentrations of Cu in the shoots and roots of castor plants, which were 3.43 times and 3.27 times higher than those of the control, respectively. Furthermore, when combined with phosphorus fertilizers, EDDS further promoted the absorption of Cu and Cd in the shoots of castor, which significantly increased by 13.34 times and 0.47 times, respectively, with addition of 100 mg kg⁻¹ phosphorus and 5.0 mmol kg⁻¹ of EDDS compared with the control. Phosphorus and EDDS synergistically decreased the activity of POD enzymes in leaves and roots compared with those treated with only EDDS and alleviated the toxicity of EDDS and heavy metals to castor plants. These findings provide scientific evidence for the use of agronomic measures and chelators to optimize phytoremediation efficiency in Cu and Cd co-contaminated soils.

Screening for cultivars with low Cd in edible organs but high Cd accumulation in non-edible organs represents a sustainable strategy for “producing while remediating”. However, the potential of potato cultivars to achieve this dual goal in high-geogenic-Cd karst areas remains unclear. A field experiment was performed to investigate biomass, Cd concentration, bioconcentration factor (BCF), translocation factor (TF) and Cd removal amount (R) in various organs of 26 potato cultivars, which were then classified into different Cd risk types. The results showed that in soils with 2.05–2.98 mg·kg⁻¹ Cd, fresh tuber Cd concentrations ranged from 0.021 to 0.132 mg·kg⁻¹, with nearly 90% of tubers complying with FAO/WHO and Chinese food safety standards. Cd levels in stems, leaves and roots were 1–2 orders of magnitude higher than in tubers, supporting potato as a suitable staple for safe production in such regions. BCF followed the order stem and leaf > root > tuber, while TF followed the order root to stem and leaf > stem and leaf to tuber. Tuber Cd was jointly controlled by Cd uptake, translocation and biomass dilution, with TFstem and leaf–tuber as a key predictor. Annual Cd removal by whole plants ranged from 1082.99 to 7283.64 mg·hm⁻². Principal component analysis classified the cultivars into four types: edible-safe, safe remediation, risky remediation and high-risk types. Notably, cultivars V2, V6, V9, V13, and V18 had tuber Cd concentrations below 50% of the permissible limit, while V16, V8, and V25 achieved >5000 mg·hm⁻² Cd removal without exceeding safety limits. These findings support the dual goal of safe potato production and in situ Cd remediation in high-background karst regions by selecting cultivars with low Cd accumulation in tubers but high Cd accumulation in non-edible organs.

The vertical migration of soil nitrogen (N) losses in sloped farmlands under natural rainfall conditions remains inadequately understood. This study conducted a two-year (March 2023–February 2025) in situ runoff field monitoring experiment on purple loam slopes in Chongqing, China, systematically investigating the effects of different rainfall patterns (TR, HR, MR, LR) and planting stages (CPS, SFS, MPS, WFS) on the vertical migration of nitrogen at four depths (0, 20, 40, and 60 cm) under natural rainfall conditions. The results demonstrate that rainfall is the key driver of vertical nitrogen migration. The migration loads of total nitrogen (TN), total dissolved nitrogen (TDN), and nitrate nitrogen (NO₃⁻-N) all increased significantly with increasing rainfall intensity (p < 0.01), showing the strongest correlation with rainfall amount in the shallow soil layer (L₁). Nitrogen migration loads exhibited a clear decreasing trend with increasing soil depth, declining progressively from the surface (L₁) to deeper layers (L₃). However, higher loads of nitrate nitrogen were maintained in deeper layers, given its strong mobility. The study found that although extreme rainfall events (TR and HR) accounted for only 6.05% of total rainfall events, they contributed to more than 60% of the total nitrogen migration load, highlighting extreme rainfall as the primary driver of nutrient loss. Over 70% of nitrogen loss occurred during the corn planting stage (CPS) with high fertilizer demand, highlighting that this period is critical for nitrogen loss and represents a key window for risk management. The increased soil depth functions as a “sink”, exhibiting certain nitrogen retention and filtration effects. The total nitrogen content in deeper soil layers (L₂, L₃) shows cumulative accumulation, confirming the nitrogen migration pattern from sources (surface layers) to sinks (deep layers) within the soil profile. This study elucidates the core driving mechanisms and critical risk periods for vertical nitrogen migration in purple soil on sloped farmland, providing crucial scientific evidence for precise regional nitrogen fertilizer management and non-point source pollution control.