The changing climate raised more concerns about the durability of aged slopes and embankments due to the increased frequency of extreme rainfall events. Recently, there has been a growing interest in the utilization of biopolymer as a biomediated soil improvement method. However, challenges, such as, strength loss due to exposure to adverse environmental conditions and limitations on the suitability of soils for effective treatment, can be problematic in practice. Therefore, this study introduces an innovative approach by combining biopolymer with another eco-friendly material, biochar. The erodibility of the reinforced soil was examined through both wetting and drying tests and slope-rainfall simulation tests with the consideration of different rainfall intensities and slope inclinations. The findings suggest that cyclic wetting and drying conditions can lead to a progressive degradation (decrease in strength) of soils reinforced with biopolymer, starting from the initial cycle. Conversely, incorporating biochar into the biopolymer-reinforced soils successfully postponed this decline in both compressive and shear strength, prolonging the soil's resilience by two to three cycles. In addition, soil slopes reinforced with the combined treatment exhibited reduced soil runoff and increased durability under both light and heavy rainfall compared to slopes reinforced with either biopolymer or biochar alone. The findings of this study provide an innovative method for controlling soil erosion on sandy soil, suggesting its potential application in slope stabilization and restoration.

A novel experimental program was undertaken to explore crack development inside an expansive soil column under cyclic Wetting and Drying (W-D) and its influences on the Soil-Water Characteristic Curve (SWCC). Experimental investigations revealed that in the first drying cycle, fine and evenly distributed cracks developed from initial defects in the upper surface. In the subsequent drying cycles, the unhealed cracks during the wetting cycle further grow in width, length and depth while the initiation zone of new cracks was limited. The crack development characteristics within the soil column were described using fractal dimensions. The cracking degree of soil cross-sections gradually reduced with increasing depth. Also, each W-D cycle process contributed to a similar increment in the crack fractal dimension of soil at the same depth. In addition, the saturated water content, air-entry value and residual suction of cracked expansive soils gradually reduced with an increase in the number of W-D cycles. An obvious bimodal phenomenon was only observed in the SWCC of soil samples after the third W-D cycle. In order to quantify the damage induced by crack development to the soils, the damage variable was proposed based on the crack fractal dimensions. Furthermore, the Damage Impact Factor (DIF), as a function of the volume damage variable and soil plasticity index, was introduced into the traditional Fredlund and Xing (F-X) model to estimate the SWCC of cracked expansive soil. The form of modified F-X model is relatively simple and can be conveniently extended into geotechnical engineering practice applications.

In continuously-flooded paddies, the small, fast-growing aquatic plant duckweed ( Lemna minor L.) considered to compete with rice for nitrogen, thereby having a negative impact on early rice growth. While duckweed overpopulation was known can be effectively overcome through field water management, the influence of such management on the N fate and its use efficiency in rice-duckweed systems is poorly documented. Accordingly, a three-year (2020-2022) field experiment was conducted to examine the combined impact on rice yield, as well as N loss and utilization, of two water management approaches (flood irrigation, FI vs . alternate wetting and drying irrigation, AWD), factorially combined with two rice production systems (rice-duckweed, +D, vs. duckweed-free rice, -D). In AWD+D fields, the density of duckweed generally remained below 250 g m( - 2) (about 85 % coverage), whereas in FI+D fields it reached 100 % coverage (300 g m - 2 ) within 5 days after transplanting, with individual duckweeds overlapping one another. Following AWD irrigation, duckweed performed as a nitrogen cache, akin to a split fertilizer application, with the first of several splits occurring at the rice crop's early tillering stage. Within the first 2 days of a specific wet -dry cycle, duckweed can store 0.5-1.5 g N m( - 2) , and then, within a further 3 days, release 0.3-1.0 g N m( - 2) . In contrast, in FI+D paddies, this caching function occurred once under midseason drainage, with further N being stored in the duckweeds during the remaining rice production season. As a result, at harvest the 0-0.10 m soil layer's N level increased significantly ( p <0.05) in both FI+D (8.5-16.8 %) and AWD+D (14.9-20.8 %) compared to FI-D and AWD-D, respectively. Due to the coverage and storage -release function of duckweed, apparent N loss decreased in rice-duckweed system by 1.4-12.5 % in the FI field and 22.1-31.3 % in the AWD field compared to their respective duckweed-free systems. In FI fields, except for a 10 % relative reduction in nitrogen recovery efficiency (NRE) in 2020, duckweed didn't significantly affect rice yield or NRE. The yield reduction (3.5-6.7 %) and the NRE increase (0.8-7.4 %) under AWD-D ( vs . FI-D) was, in the presence of duckweed, compensated for and overrun, resulting in a greater yield (5.3-6.7 %) and NRE (5.4-28.9 %) in the AWD+D vs. AWD-D field. When duckweed was present, AWD irrigation improved the by-path nitrogen cycling through duckweed, making the AWD+D system more beneficial for rice cultivation and the agroecosystem's environment health. The AWD+D system offers a promising measure for building an efficient and sustainable rice-duckweed agroecosystem.

The interface between plants' roots and soil is strongly affected by rhizodeposits, especially mucilage, that change mechanical and hydrological behaviour. In addition to impacts to aggregation, water capture and root penetration, rhizodeposits may also affect the pull-out resistance of plant roots. Due to the complex architecture of plant roots and an inability to restrict rhizodeposit production, this study used a simplified system of wooden skewers to simulate roots and chia seed mucilage as a model to simulate rhizodeposit compounds. Pull-out tests were then carried out to measure the impacts of mucilage, and one (WD1) or two (WD2) cycles of wetting and drying of soils. Using a mechanical test frame, the maximum pull-out resistance (Fmax) and pull-out displacement (dL) were recorded, allowing for pull-out energy (E), average pull-out force (F over bar $$ \overline{F} $$) and bond strength (tau max) to be calculated. The results showed that all pull-out parameters of the samples with added rhizodeposit compounds tended to decrease between WD1 and WD2, but they were still significantly greater than without the added mucilage. The model rhizodeposit increased all pull-out parameters by a minimum of 30%. With an additional wet-dry cycle, the mucilage tended to cause a decline in pull-out parameters relative to a single wet-dry cycle. This suggests mucilages could enhance the mechanical resistance of roots to pull-out, but resistance decreases over time with cycles of wetting and drying. To conclude, an important role of mucilage is pull-out resistance, which has relevance to plant anchorage and root reinforcement of soils.

Context Plant roots can increase soil shear strength and reinforce soil. However, wetting and drying alternation (WD) could lead to soil structure destruction, soil erosion and slope instability.Aims This study tried to explore the effects of wetting and drying alternation on shear mechanical properties of loess reinforced with root system.Methods Direct shear testing was conducted on alfalfa (Medicago sativa L.) root system-loess composites with three soil bulk densities (1.2 gcm-3, 1.3 gcm-3 and 1.4 gcm-3) under 0, 1, 2 and 3 cycles of wetting and drying alternation (WD0, WD1, WD2 and WD3).Key results The morphological integrity of the root-loess composites was obviously better than the non-rooted loess after WD. Under the three soil bulk densities, negative power-law relationships were observed between the shear strength, cohesion and internal friction angle and the cycles of WD. WD deteriorated the soil shear strength. The most obvious decrease in soil shear strength occurred under WD1, which was 13.00-22.86% for the non-rooted loess and 17.33-25.09% for the root-loess composites. The cohesion was decreased more than the internal friction angle by WD.Conclusions The most obvious damage to the soil was under WD1. The roots inhibited the deterioration effect of WD on the shear property of loess, and the inhibition by the roots decreased with the cycles of WD.Implications The results could provide new insights into the mechanical relationship between plant roots and loess under WD, and provide a scientific basis for the ecological construction in the loess areas. Wetting and drying alternation (WD) on the mechanical properties of root-soil composites is not clear at present, or if roots can inhibit the deterioration of soil under WD. This paper investigated the effect of WD on the shear strength of root-loess composites. WD was found to deteriorate soil shear strength and cohesion, while roots inhibited the deterioration of WD on the shear property of loess. The results provide a scientific basis for ecological construction in loess areas.